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EARLY

DEVELOPMENTAL STAGES OF
MANIS JAVANICA DESM.

tî. J. VAN GORDT.

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EARLY

DEVELOPMENTAL STAGES OF
MANIS JAVANICA DESM.

PROEFSCHRIFT

TER VERKRIJGING VAN DEN GRAAD VAN DOCTOR
IN DE PI
^T- EN DIERKUNDE AAN DE RIJKS-
UNIVERSITEIT TE
UTRECHT, OP GEZAG VAN DEN
RECTOR MAGNIFICUS D
r. W. VOGELSANG, HOOG-
LEERAAR IN DE FACULTEIT DER LETTEREN
EN WIJSBEGEERTE, VOLGENS BESLUIT VAN DEN
SENAAT DER UNIVERSITEIT TEGEN DE BEDEN-
KINGEN VAN DE FACULTEIT DER WIS- EN NATUUR-
KUNDE TE VERDEDIGEN OP VRIJDA
G 18 MAART 1921,
DES NAMIDDAGS TE 4 UUR, DOOR

GREGORIUS JOHANNES VAN OORDT,
GEBOREN TE ARNHEM.

AMSTERDAM
JOHANNES MULLER
1921

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AAN MIJN VADER,

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Gaarne maak ik van de gelegenheid, welke een proefschrift biedt,
gebruik, om mijn oprechten dank te betuigen aan alle Hoogleeraren
in de faculteit der Wis- en Natuurkunde voor hun onderwijs en hunne
leiding bij mijne studie in de plant- en dierkunde.

Met veel genoegen denk ik, Hooggeleerde Pulle, terug aan uwe
colleges in de systematiek en geographische verspreiding der Planten.

U, Hooggeleerde Went, ben ik niet alleen zeer erkentelijk voor
uw onderwijs, doch ook voor de belangstelling, welke gij steeds in
mij getoond hebt.

Zeer aangename herinneringen, Hooggeleerde Wichmann, heb ik
aan uwe colleges in de geologie en palaeontologie, en aan de groote
welwillendheid, waarmede gij mij steeds te woord stondt.

Gaarne herdenk ik hier de aangename wijze, waarop door U,
Hooggeleerde Jordan, het onderwijs in de vergelijkende physiologie
gegeven wordt, en de vriendschappelijke verhouding, die er steeds
tusschen
U en uwe leerlingen bestaat.

U, Hooggeleerde Nierstrasz, Hooggeachte Promotor, ben ik vooral
dank verschuldigd voor de leiding, welke Gij mij gedurende mijn
geheele studie gegeven hebt, en waardoor ik mij nu zoozeer aange-
trokken gevoel tot morphologisch onderzoek. De critische geest, welke
op den voorgrond treedt bij uw onderwijs, is mij ook bij het vervaar-
digen van dit proefschrift tot grooten steun geweest.

De voortdurende belangstelling, welke Gij, Hooggeleerde Ihle, in
mij en mijn werk stelt, uw veelzijdige kennis en de groote vrijheid,
welke Gij mij steeds verleent bij de keuze van een onderwerp van
onderzoek, wil ik hier in de eerste plaats gedenken. Ik ben er zeker
van, dat Gij mij de vriendschap, welke ik steeds van
U ondervonden
heb, ook den verderen tijd, dien ik in uw laboratorium hoop werk-
zaam te zijn, niet onthouden zult.

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Een woord van groote erkentelijkheid geldt ook U, Zeergeleerde
de Lange. Met groote bereidwilligheid hebt Gij mij het materiaal
uit uw instituut, het
HuBRECHT-laboratorium, ten onderzoek afgestaan,
met raad en daad hebt Gij mij geholpen bij de bewerking van dit
proefschrift. Dankbaar herdenk ik hierbij uw groote kennis.

Een woord van hartelijken dank mag hier zeker niet ontbreken
aan Mej. H. W.
Schalkwijk, die met zoo groote nauwgezetheid
zorg gedragen heeft voor de vertaling van dit proefschrift.

Met veel genoegen denk ik hier terug aan den tijd, doorgebracht
in het Utrechtsch Studenten-Corps. Moge het Corps na de veran--
deringen der laatste jaren groeien en bloeien als nooit te voren!

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MEDEDEELINGEN UIT HET EMBRYOLOGISCH INSTITUUT
VAN HET HUBRECHT-FONDS\'

N°. III

EARLY DEVELOPMENTAL STAGES
OF MANIS JAVANICA DESM.

by

G. J. VAN OORDT

assistant zoological laboratory of the veterinary college

at utrecht

VERHANDELINGEN KONINKLIJKE AKADEMIE
VAN WETENSCHAPPEN TE AMSTERDAM
(TWEEDE SECTIE) DEEL XXI, N°. 3
(with 6 plates)

Amsterdam
JOHANNES MÜLLER
M
aart 1921

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EARLY DEVELOPMENTAL
STAGES OF MANIS JAVANICA DESM.

TABLE OF CONTENTS.

Introduction................................................................................page i

Material and methods..............................................................„ 2

Terminology..............................................................................„ 5

Descriptive part...................................................„

1. Early segmentation-stages..................................................„

2. Origin of the hypoblast....................................................„ 14

3. Description of surface-views of the embryonic shields „ 17

4. Development of the mesoblast........................................„ 18

5. Origin and development of the notochord....................„ 28

The so-called process of gastrulatioji in Mammals..................„ 50

Comparative pa7~t........................................................................55 69

1. Early segmentation-stages..................................................» 69

2. Trophoblast and embryonic knob . . . . !..........................„ 71

3. Mesoblast..........................................................................» 73

4. Notochord ........................................................................^

Literature cited...................................................................» 85

Description of plates.....................................,, 93

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INTRODUCTION.

In the comprehensive collections of embryological material, brought
together by the late Prof. Dr.
A. A. W. Hubrecht in the end of
the last and the beginning of this century, a great number of pregnant
uteri and embryos of
Manis javanica Desm. are to be found. These
Mzwj-specimens, not yet being investigated,
(Hubrecht himself
published only some short communications on
Manis) Dr. DaiN. de
Lange Jr.,
Director of the HuBRECHT-Laboratory at Utrecht, put a
part of the collection of Mz;//j--embryos at my disposal in order to
examine the early developmental stages.

There are very few publications on the early ontogenetic processes
in
Manis. Some contributions to the knowledge of placentation have
been published. Only a few communications of
Hubrecht, treating
early developmental stages, are known to me. A concise description
of the placenta was that of
Sharpey (1864) in Huxley\'s „Elements
of comparative Anatomy", which was elaborated by
Turner by means
of investigations on the same object. In 1878 follows the not quite
correct description of the placenta by
Anderson.

The development of certain organs (scales, mammary gland, hand-
skeleton) and the placentation of
Manis javanica and Manis triciispis,
were described in detail by M. Weber (1891).\'The youngest embryo
of
Manis javanica having a length of 8.25 mm., no observations could
be made on the first developmental stages.

Resink (1903) too mentions something about the placentation after
an examination of the preparations of
Weber.

First Hubrecht gave a short communication on Manis javanica
in his „Spolia n^oris" (1894), in which were reproduced (figs. 42—45)
two older embryos with foetal membranes.

In Hubrecht\'s paper entitled „Early ontogenetic phenomena in Mam-
mals etc." (1908) we also find some observations on
Manis. In the first

Vcrhand. Kon. Akad. v. Wetensch. (20 Sectie). Dl. XXI. C i

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2 EARLY DEVELOPMENTAL STAGES

place a reproduction of an embryonic knob with hypoblast is given
(plate C, fig. 18, erroneously attributed to
Gaieopithecus) and later on
Hubrecht mentions (pp. 32 and 34) that the „protochordal plate"
and the „annular zone of proliferation" are also well developed in
Manis. On p. 114 a few notes on the placentation follow.

In 1910 the above-mentioned, misnamed figure is reproduced and
also some sketches are given of young segmental stages (figs. 5«, 5
and 6) to explain his opinion, at that time, on the origin of the
trophoblast. This view is repeated in his posthumous work on the
early developmental stages and placentation of
Gaieopithecus (1919)
From the above it follows that the early development and the placen-
tation of the only genus known of the Pholidota have been insuf-
ficiently investigated.

Material and methods.

One hundred and eighty uteri of Manis Javanica, which Hubrecht
received between 1891 and 1902 from different regions of the Malayan
Archipelago, are to be found in the
HuBRECHT-Laboratory. -

They were collected by different persons. Not all uteri are pregnant.
Of about
106 this could be ascertained, most of the others being
virginal or puerperal. The uteri, with ovary and oviduct, were fixed
in toto in picro-sulphuric acid (
Kleinenberg\'s mixture)\'^) immediately
jifter death.

After having been transferred into alcohol 90 7o 3 the uteri were
forwarded to the Zoological Laboratory of the University of Utrecht
and examined here.

In the year 1907 (and earlier) those oviducts and uteri were sectioned,
which, judging from the state of the corpus luteum, probably contained
very early developmental stages. After staining and embedding in
paraffin, the whole oviduct and the uterus were sectioned and examined
till the ovum was found. Also some older embryos, visible macrosco-
pically, were sectioned.

In the beginning of this investigation (September 1918), only a few

\') In the catalogue this is not always mentioned, but in Hubrecht\'s „Spolia nemoris" (1894) it
is stated (p. 82).

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embryos (N^ 64 and 68) and early developmental stages (N°. 151,
and 164) needed to be stained and sectioned.

A number of other oviducts and uteri were examined, but nothing
was found in them. All sections of the Mz;?/>-embryos have a thickness
of 10 y.. The figures reproduced are drawn at a relatively high magni-
fication (the reconstructions excepted) with the aid of the camera lucida
or of the large projection-apparatus of
Zeiss, which is to be found
in the Anatomical Laboratory of the University of Utrecht i). Most of
the drawings are greatly reduced in reproduction.

As is known, Mam\'s brings forth only one young at a time. Mam\'s
has an uterus bicornis with a relatively short uterus-body. The ovaries
are lying free. Sometimes both horns of the pregnant uterus are swollen.
It then seems, as if we have to do with a case of twins, but this is
not so, the foetal membranes of the embryo extending in both horns
(to compare
M. Weber 1891, fig. 49).

As is mentioned the HuBRECHX-Laboratory (in which all the collections
of the late Prof
Hubrecht are to be found) possesses about 106 pregnant
Mam\'s-uten. Three uteri, however, contain no embryo, though there
are rests of the blastocyst. For this reason the collection of
Manis-
embryos is, so far as I can ascertain, composed of 103 specimens.
A relatively small part of them consists of early stages (to compare
table i).

It was to be expected a priori that this series would not be complete.
A few cleavage-stages are present. The formation of the hypoblast
can be traced in two specimens. Two embryonic areas are in the
prunitive streak-stage. Embryos, in which a relatively small number
of somites is formed, are well represented.

It is not my intention to describe these different ova one after the
other, according to age. On the contrary, after description of the
early segmentation-stages, the origin of the hypoblast will be treated

the above-mentioned ova. After a concise description of the surface-
views of the embryonic areas, a chapter will follow, in which the
formation of the mesoblast will be traced, and finally the origin and
development of the notochord will be described in detail.

) I wish to express my thanks to Prof. A. J. P. van den Broek, whose kindness enabled me to
use this apparatus.

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TABLE i.

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CO

Cat. N°.

Stage

Plane of
sectioning

Collected at

Year

Fixation

i

Stained in

51

unsegmented ovum in oviduct.

Benkajang (West-Borneo)

?

picro-sulphuric acid

haemalun.

16

J) )? n 7t

Muntok (Banka)

1900

5)

paracarmine.

171

3-cell stage „ „

1901

»

iron-carmalun.

4-4

4-cell stage „

Solok (Sumatra)

1891

\\

haemalun.

87

blastocyst with embryonic knob
in utero.................

Djember (Java)

1892

"

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113

blastocyst with embryonic knob
in utero.................

Benkajang (West-Borneo)

?

!

" i

iron-carmalun.

>51

primitive streak-stage........

transverse

Muntok (Banka)

1899

i

" i

iron-haematoxylin-eosin.

180

i

sagittal

n »

1900

1
!

iron-carmalun.

83

4 a 5 pairs of somites.......

sagittal

Moeara Laboe (Sumatra)

1891

)5

»

64

8 a 9 „ „ „ .......

transverse

Benkajang (West-Borneo)

?

»

haemalun.

32

1 10 a n » » » .......

PadangPandjang (Sumatra)

1891

J> »

iron-carmalun.

89

\'13 » « .......

«

Benkajang (West-Borneo)

?

35

31

; 13 a 14 » » , .......

w

Banka

1891

«

»

108

i

sagittal

Benkajang (West-Borneo)

?

n

68

16 „ „ „ .......

transverse

» »

?

»

iron-haematoxylin-eosin.

53

18 „ „ „ .......

Moreover:

«

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f

»

iron-carmalun.

38

109
179

1 rests of blastocyst present,
^ no embryo.

Buitenzorg (Java)
Benkajang (West-Borneo)
Muntok (Banka)

1891
?

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Then a theoretical part will follow, in which the so-called process
of gastrulation in Mammals (and in Vertebrates in general) will be treated
and after this a comparative part, in which the literature will be
discussed, so far as necessary in connection with the descriptive part.

I wish to express my sincere thanks to Prof. Dr. H. F. Nierstrasz
for his many critical remarks in reading the manuscript and to
Dr.
Dan. de Lange Jr., Director of the HuBRECHT-Laboratory, not
only for his kindness in putting the M^w^-embryos at my disposal, but
also for his usefull help and valuable advice during this investigation.

Terminology.

Before describing the different developmental stages, I will first give
a survey of the terms to be used in this investigation. In the literature
dealing with the early ontogenetic phenomena in Mammals, so many
different names are used, especially in connection with the formation
of the germinal layers and the so-called process of gastrulation, that
it is certainly recommendable to begin with inserting a list, arranged
in such a way that the reader will know at once, what standpoint
is taken by the author with regard to the different questions. In the
following list I will use the detailed survey of „Termini technici"
given by
C. Rabl in his paper on „Edouard van Beneden" (1915),
especially there, where synonyms are mentioned. Terms generally used
or self-evident will be omitted in order to be concise.

Gastrula, gastrulation. Strictly speaking, Haeckel\'s definition („Gas-
traeatheorie" 1874) is not applicable to Mammals, because I think
that in the didermic blastocyst of Mammals (consisting of epiblast
and hypoblast) no primitive mouth in
Haeckel\'s sense is formed. In
the other Vertebrates too, except in
Amphioxus, no gastrulation can be
distinguished, because in the didermic embryonic stages no aperture,
the blastopore, communicates with the exterior. The formation of the
gut has not come to an end after the formation of the hypoblast in
Vertebrates. The alimentary tract consists of archenteron, formed during
protogenesis arid of metenteron, formed during deuterogenesis (As-
sheton 1905, 1909).

Epiblast, the outer layer of the didermic blastocyst, not yet differ-
entiated. An embryonic and extra-embryonic epiblast (= trophoblast)
can be distinguished. Epiblast = ectoderm.

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Hypoblast, the inner layer of the didermic blastocyst, generally formed
by delamination (immigration in Marsupials,
Hill 1910, Hartman
1919,) out of the embryonic knob. It forms the lining of the arch-
enteron. Synonyms: „Dotterentoblast"
(Bonnet e. g. 1920), lecitho-
phore
(Van Beneden), paraderm (von Kupffer 1882), caenogenetisches
entoderm
(Wenckebach 1891), subgerminal layer. An extra-embryonic
hypoblast also occurs.

Mesoblast, the tissue lying between epiblast and hypoblast and
formed in several places of the germdisc.

Epi-, meso- and hypoblast are topographical conceptions.

Blastocyst, the generally bladder-shaped stage, consisting of trophoblast
and embryonic knob, in Mammals, excepting Monotremata. Synony-
mous with blastodermic vesicle.

Trophoblast (Hubrecht 1888), the part of the\'blastocyst, not par-
ticipating in the formation of the embryo. Except for the part lying
on the embryonic knob, (the so-called
rauber-layer (Rauber 1875))
it is an organ, which takes part in the nutrition of the embryo.
Therefore
Duval calls it ectoplacenta. Trophoblast = couche enve-
loppante
(Van Beneden 1899^).

Embryonic knob {Embryonalknoten), Hubrecht 1890, the inner
mass of cells in the wall of the blastocyst. The germdisc will derive
from it. It is generally covered by a part of the trophoblast, which
disappears later on and is called
rauber-layer (Rauber 1875), when
distinguishable from the rest of the trophoblast.

Germdisc, embryonic shield, e?nbryonic area, the part of the blastocyst,
derived from the embryonic knob, when the latter stretches, comes
to lie superficially and consequently is not entyped. After anlage of
the medullary groove and somites the embryo proper is formed.

The blastocyst-cavity (blastocoel (Huxley), subgerminal cavity) ori-
ginates from the compact morula by absorption of fluid from the
uterus-lumen. After formation of the hypoblast it becomes entirely
or partly the umbilical vesicle or yolksac, from which the archenteron
derives.

Archenteron, the anterior part of the gut of the Vertebrates, formed
during protogenesis, its lining being formed by the hypoblast (= Dot-
terentoblast
Bonnet). Jit is called „Ergänzungshöhle" by Bonnet (1920).

Metenteron, the posterior part of the gut, connecting itself in all

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Vertebrates with the archenteron, and formed by overgrowth and
ingrowth of cells at the dorsal deuteroporic lip (= blastoporic lip of
earlier authors) during deuterogenesis. It is the lumen in the head-
process of the Amniotes.

A blastopore occurs, in the sense, which Haeckel attached to it
(„Urmund" in „Gastraeatheorie" 1874), only in
Amphioxus amongst
Vertebrates. It is not developed in Craniota (= „blastopore virtuel",
Brächet 1902), because here the metenteron, not the archenteron,
communicates with the exterior by the
deuteropore or somatopo?r,
the latter being still often called blastopore.

Protogenesis (Assheton 1905, 1909) the first developmental process
of the\' embryo in which, among others, the archenteron is formed
(„the production of a radial symmetry, due to growth from one centre
involving gastrulation",
Assheton 1909, p. 240). The cephalogenesis
of Hubrecht and Brachet\'s acrogenesis and cephalogenesis (1914)
taken together express about the same.

Deuterogenesis [h&^v.Y.To^ 1905, 1909), the following second develop-
mental phase of the embryo, in which, among others, the metenteron
is formed by overgrowth and ingrowth (invagination) of cells (=
notogenesis Hubrecht and cormogenesis Brächet (1914)). „Deutero-
genesis is growth in length, bringing about bilateral symmetry",
(Assheton 1909, p. 240).

Headprocess of the primitive streak (Koelliker i 879) = Kopflortsatz
of the German authors (also called
cliorda-anlage), the name used
in the descriptive part of this investigation for the homologue of the
wall of the metenteron in Mammals (Birds and Reptiles). It is a
prolongation of the primitive streak in the direction of the future
head-region of the embryo. The part of it, formed in later stages
in the region of the primitive streak and from it, I call, for simpli-
city\'s sake, headprocess too. Synonyms: Urdarmstrang
(Bonnet 1920),
Mesodermsackchen (O.
Hertwig 1903), wall of the archenteron,
Urdarm, etc.

Lieberkilhn-canaL (Lieberkühn 1882) the name used in the des-
criptive part of this investigation for the metenteron (= canal archen-
tdrique v.
Beneden 1912, Urdarmlumen, also chordal canal (the latter
not used by me in this sense), sometimes called neurenteric canal,
which is not correct, because this canal in Anamnia is found in

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early developmental stages

older stages, then forming the connection between medullary tube and
posterior part of the gut, and lying behind the anal plate.

Hensen-knob (Hensen 1876) = Gastrulaknoten or Primitivknoten
(Bonnet ,1889), protochordal wedge (Hubrecht 1890, 1902), the
thickened part of the headprocess, where it originates from the primi-\'
tive streak.

Primitive streak (Pander 1817, von Baer 1837), the term used in
the descriptive part for the proHferation of cells, lying in the median
line in the posterior region of the embryonic shield. The greater part
of the mesoblast originates from it; the hypoblast sometimes fuses
secondarily with it. From the primitive streak, during the growth in
length of the embryo, the organs are formed from before backwards.
It is the homologue of the changed deuteroporic lips of the Anammia
(blastoporic lips in the old sense) and is called by His „Achsenstrang",
by
Bonnet „Urmundleiste" or „Gastrulaleiste".

Primitive groove, the longitudinal groove in the middle of the
primitive streak (= „Gastrulagrube"
Bonnet).

Chordal plate, the stage of the chorda-anlage, which develops out
of the headprocess after incorporation of the latter in the hypoblast,
and which occupies the roof of the primitive gut as a broad, flat plate.
After separation from the hypoblast, the definite
mtochord is formed.

Prochordal plate (= protochordal plate, Hubrecht 1890 and later,
Erganzungsplatte,
Bonnet 1901, 1920, Endodermale Zwischenplatte,
v. Davidoff 1899), the thickened plate of the hypoblast, lying in
front of the anterior end of the headprocess, which fuses later on
with it. The cephalic mesoblast originates from it. According to
Bonnet the anterior part of the notochord, the hypoblastic layer of
the primary pharyngeal membrane and the cephalic mesoblast are
derived from it. According to
Hubrecht and Heape (1883) only
cephalic mesoblast and the anterior part of the notochord are derived
from it.

Annular zone of proliferation (Hubrecht 1890), the peripherally
situated, ring-shaped part of the hypoblast, from which the peripheral
mesoblast originates. It is called by
Bonnet „Mesoblasthof" (sheep
1884), by
Kollmann 1884: „Randkeim" or „Akroblast".

Cephalic mesoblast and peripheral mesoblast are two of the four parts
composing the mesoblast, both originating from the hypoblast, respec-

8

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tively from the prochordal plate and the annular zone of proliferation
Both are protogenetic and are called by
Schlater primary meso-
blast (1907).

Peristomal and gastrai mesoblast, the two other, deuterogenetic
mesoblast-parts, respectively formed out of the deuteroporic lips ( =
blastoporic lips in the old sense, primitive streak of the Amniotes)
and metenteron-wall (= headprocess). They are also called axial
mesoblast, and by
Schlater (1907) secondary mesoblast.

Chordal canal, the canal situated in the part of the headprocess,
which is formed from the primitive streak, when the embryonic shield
becomes longer (the „Primitiefstreifenteil des Chordas" of the German
authors). It moves caudal ward during the growth in length of the
embryo. It is probably to be homologized with the neurenteric canal
of the Anamnia.

Archamnion-cavity, the cavity occurring in some Mammals, from which
the amnion-cavity is derived directly. This is called by
Bonnet a
„Schizamnion", contrary to „Faltenamnion", formed by means of
folding.

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DESCRIPTIVE PART.

I. \'Early segmentation-stages.

In the collection of Mams-emhvjos, there are, besides the cleavage
stages, two unfertilized or unsegmented eggs, indicated in the catalogue
by N°. 51 and N°. 164, both situated in the oviduct.

The one N°. 51 (fig. i, plate I) occupies the centre of the right
oviduct. It has a diameter of ± 70 and is surrounded by a great
number of dark-stained nuclei. These nuclei are also to be seen on
the egg-membrane. I have not been able to find the nucleus of the
ovum. The large nucleus (;/) in the centre of the egg cannot be
considered the nucleus of this cell, but is, like all the other nuclei
reproduced in fig. i, one of the folHcle-cells, which pass into the
oviduct with the ovum, when the follicle bursts. In describing the
cleavage stages, I will speak at greater length about the signification
of this covering. It is not at all remarkable that the nucleus of the
egg-cell is not visible. In most of the early stages of Mammals the
nucleus is scantily provided with chromatin, due to the chromatin-
emission; gradually the nuclei of the blastomeres become richer in
chromatin and owing to this, can be stained more deeply. The nuclei of
the follicle-epithelium are well stained. Only a few of them lie against
the egg-membrane, most of them at some distance of the ovum.

The ovum N°. 164 being fixed badly, I om\'it a further description.

The cleavage stage is represented by two specimens. The younger
of the two is reproduced in fig. 2, plate I. This ovum, lying in the
left oviduct, is probably a three-cell stage: in the successive sections
only three nuclei are visible. The lower part of the ovum shows one
nucleus in each of the two sections, at a distance of about 30 from
each other, which points to two cells. Between both nuclei a cell-
membrane is not to be found, probably owing to the membrane running
parallel with the sectional plane.

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early developmental stages of manis javanica desm. i i

The third nucleus is present in the upper part of the ovum. This
one is not oval in shape like the other nuclei, but oblong and contains
two dark-stained bodies. The suppositon is obvious that this cell will
soon divide, and that this ovum is passing from the three- to the four-cell
stage. The nuclei of the corona radiata, lying close round the ovum,
are not very deeply stained. Fig,
2 shows about 13 of these nuclei.
The small nucleus, apparently lying in the upper part of the ovum,
also belongs - to the corona radiata as the surface of the cell has
been cut. The largest diameter of this ovum is about
72 ,a measured
without the corona radiata.

Besides the ovum and the covering enveloping it, fig. 2 also shows
a fold of the oviduct-wall to indicate that the egg is lying close
against this wall in a state of preservation. Naturally this does not
imply that the ovum lies here in living condition too.

The four-cell stage is represented in the collection by one specimen
(N"\'. 44), of which two sections are reproduced (figs. 3 and 4). All
four blastomeres are visible in both figures, but of the four nuclei
only three are to be found in the difeent sections. However, the
cell-membranes of these four cells are so clearly visible that it is certain
that we have to do with a four-cell stage. In- contradiction to the
preceding ovum, this is preserved and stained much better.

The nuclei of the corona radiata, surrounding the egg in great
numbers, are clearly visible. From the figures it is obvious that the
ovum is not spherical, which is most probably the case in living
state. This egg too lies in the left oviduct, close to the wall. The
oviduct-epithelium of this
Mafn\'s-tmh\\-jo N°. 44 is as distinctly ciliated
as in the embryo first described. Whether this ciliated epithelium
moves the ovum in the direction of the uterus or whether this is caused
by a fluid-mass, propelled by the peristalsis of the oviduct-wall
(SOBOTTA 1914), cannot be traced.

The largest diameter of this ovum is somewhat more than that
of the preceding one and is 90 a.

The sections, reproduced in figs. 2, 3 and 4, are the same
Hubrecht (1910) sketched. He intended to prove that the nuclei,
to be found at the periphery of the ovum, belong to the egg-cell it-
self They were supposed to represent the nuclei of the future tropho-
blastic covering, which in
Manis and also in Gaieopithecus was

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12 early developmental stages

supposed to originate very prematurely out of the nuclei of the ovum.
As
de Lange remarks in a note in the posthumous publication of
Hubrecht ( i 9 i 9), the number of trophoblastic nuclei decreases very
considerably in the successive developmental stages. This could not
be the case if they were future trophoblastic nuclei, as the number
would then increase.

I have re-examined these Galeopithecus-^Yt^iSL\\:2L\\.\\om in the Hubrecht-
Laboratory and quite agree with this opinion. Moreover the great
difference in the structure of the nuclei of the circumference and
centre points to the fact that these bodies cannot be of the same
origin. In
Manis too, the same was supposed to be the case.

Next to the ovum, represented in fig. i, there is a great number
of nuclei, which cannot have originated out of the nucleus of the
egg-cell, the latter not having divided as yet. They cannot belong to
the ovum and are undoubtedly discus-nuclei, which have been expelled
from the ovary together with the ovum and which have both passed
into the oviduct. The greater part of these nuclei probably belongs
to the discus proligerus; a small part is situated against the circum-
ference of the ovum and must be considered the corona radiata.

In the three-cell stage (fig. 2) the number of nuclei of the corona
radiata in each section has been reduced to twelve or thirteen,
whilst the discus-cells have disappeared here. In the four-cell stage
(figs. 3 and 4) the nuclei of the corona radiata or cumulus proligerus,
form a distinct layer around the egg-cell in the same manner. Their
number is greater\' here and amounts to about 40 a 50 in each section.

From the following considerations it is evident that I am entitled
to the opinion that these nuclei really belong to the follicle-
epithelium.

In the first place, I have measured the different sizes of the nuclei
of the ovum, and of the covering of the ovum in the oviduct of
Manis and Galeopithecus and compared them with the sizes of the
nuclei of the follicle-epithelium in the ovary of both species (table
2).

From this list, in which average numbers are indicated, it follows
that the nuclei of the follicle-epithelium of the ovary are\' about as
large as the nuclei of the covering around the ova, situated in the
oviduct. This is the case in
Manis as well as in Galeopithecus. In
both species the nuclei of the cleavage cells having, as indicated, a

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table 2.

Catalogue N°.

Size of nuclei
of follicle-
epithelium in
ovary.

Size of nuclei
of covering

of ovum
In oviduct.

Size of nuclei
of

cleavage-cell.

Size of
trophoblastic
nuclei.

Size of nuclei
of embryo-
nic knob.

Manis N°. 51.....

± 6 [x

6—7 u

nucleus lacks

„ N°. 171....

5—6 y.

5

16 fi

„ N°. 44.....

6 y.

6 u.

15 a and 16 a

Galeopithecus N°. 5

6—7 a

6 u

nuclei not visible

N°. 56

6- 8 a

6— 7 u

14 [J., 16 u, 18 y.

Manis N°. 87

i

8—9 y.

9—10 y.

diameter three times as large as that of the nuclei of the follicle-
epithelium, are of about the same size.

In the second place, there is a distinct limit between outer layer
and ovum, from which it follows that these are different things.
Moreover there is a marked difference in staining and appearance
of both nuclei. The nuclei of the covering are well provided with
chromatin and consequently much darker than the nuclei of the
blastomeres, which are larger, more inflated and less stained. The
follicle being composed principally of a discus and much liquor fol-
liculi, it is easy to understand that at the moment of the bursting
of the follicle, both ovum and a part of the discus are expelled from
the ovary and pass into the oviduct. Here these cells are probably
used for the nutrition of the developing ovum. The smaller number
of the nuclei in the younger egg (N°. 171), compared with that in
the slightly older four-cell stage (N°. 44), need not be a proof of
Hubrecht\'s supposition, as the consumption of the follicle-cells naturally
shows individual differences in different ova.

Finally I must point to the fact that, if Hubrecht\'s supposition
were true, it would not be easy to explain what follows. In early
stages the nuclei, which
Hubrecht supposed to be future trophoblastic
nuclei, are much smaller, than those of the ova. Later on, however,
when the trophoblast has been formed as a distinct layer and represents
the wall of the blastocyst, the nuclei of the embryonic knob and
trophoblast are of about the same size. In the above list the sizes

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14 early developmental stages

of the trophoblastic nuclei and of the nuclei of the embryonic knob of
Manis N°. 87 have been indicated. The difference is apparent and
v^ants an explanation, I cannot give.

We can conclude by saying that the nuclei, which Hubrecht
supposed to be future trophoblastic nuclei in Mafiis and Gaieopithecus,
are nuclei of the follicle-epithelium, which, in the stadia above des-
cribed, still remain around the ova. What led
Hubrecht to this
opinion on the origin of the trophoblast is not easy to understand,
especially as he has described and reproduced under this name in
1895 and 1912 young ova with a follicle-epithelium.

The material lacking in the collection, 1 cannot trace the origin
of the trophoblast.

On comparing the figures in Hubrecht 1910 and the above sections
(figs.
2, 3 and 4) one will observe that the ovum, reproduced by
me in figs.
3 and 4, is considered a two-cell stage by Hubrecht
(Hubrecht 1910,
figs. and ^b) and that the ovum, sketched in
fig.
2 (Hubrecht 1910, fig. 6, p. 593), is taken to be a four-cell
stage by him. On comparing carefully the successive sections of both
ova, I concluded from the above-mentioned data that the former
represents a four-cell and the latter a three-cell stage.

11. Origin of the hypoblast.

The origin of .the trophoblast, the layer forming the outer covering
of the blastocyst and taking an important part in the nutrition of the
embryo later on, cannot be traced, the material lacking in the collection.

Both ova, succeeding those described in the preceding chapter in
age, possess a well-developed embryonic knob. Moreover the formation
of the hypoblast is distinctly visible in these eggs. (N°.
87 and 113).

Figs. 5 and 6, plate I represent sections of the younger stage (N°. 87).
The blastocyst, lying free in the uterine lumen, is strongly compressed
in preservation. Both embryonic knob and trophoblast are cleariy distin-
guishable. The trophoblast, forming a rather thick layer, covers the
embryonic knob. The nuclei of the trophoblast are more deeply
stained! and more compactly arranged. Hence it is easy to distinguish
this inner
mass of cells and its covering. The same was observed by
Bonnet (1897) in the dog and Baumeister (1913) in Erinaceus.

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Fig. 5 gives a section through the centre of the embryonic knob.
In this section no hypoblast is visible but in the next one (having
lo jx thickness) this is the case. Here the embryonic knob is divided
(fig. 6) distinctly into two layers and has become the embryonic
epiblast and formed a very thin lower layer, the hypoblast, by
delamination. Between epiblast and hypoblast a distinct slit is visible.
The thickness of these two layers taken together is about the same,
as that of the embryonic knob. From this it follows that the delami-
nation has begun on one side of the embryonic knob and is not
quite finished as yet. In this stage, the hypoblast as such covers
only a part of the embryonic knob and is visible as a very much
flattened layer. The length of the embryonic anlage is 75 a, the
thickness (without the trophoblastic covering) 28 a.

In the blastocyst, reproduced in the figs. 7 and 8, plate I, the
hypoblast has grown round the inside of the trophoblast and formed
a completely closed sac, its lining consisting "of flattened epithelium-
cells. The hypoblast, lying against the embryonic knob, consists of
much taller Cells. Fig. 7 shows a schema of this stage, the situation
in the uterine lumen also being indicated. In fig. 8 is reproduced the
middle part of fig. 7, much more enlarged and more accurately drawn.

The large blastocyst with its wide lumen is lying free in the
uterus. It has been compressed in preservation, but not as strongly
as the preceding one. In one place it has burst, probably during
fixation. The outer layer of the blastocyst is formed by the tropho-
blast, which also covers the embryonic knob. The. trophoblast has
about the same thickness as that in N°. 87; some small diff^erences
can be distinguished: around the embryonic disc it is slightly thinner.
The hypoblast, which lacks nowhere and is very thin in the extra-
embryonic part of the vesicle, becomes much thicker with tall and
cubical cells beneath the embryonic knob i). The epiblast is found in
the embryonic disc between trophoblast and hypoblast. The nuclei
in this layer are more numerous than in the other parts of the blastocyst,
the protoplasm showing a somewhat vacuolized structure. The nuclei
of the trophoblast and hypoblast, which are more clearly visible, are
more numerous.

\') This Bonnkt also observed In the dog (1897).

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A „schizamnion"-cavity cannot be distinguished in either of these
blastocysts. Neither are there any indications of this blastocyst having
been in contact with the uterine wall.
Manis possesses a makromphalic
blastocyst as can be seen in fig. 7 and fig. 20, and it is believed
that in this stage contact with the uterine epithelium is out of the
question as yet, the blastocyst on the contrary still floating free in
the uterine lumen.

The largest diameter of the embryonic disc of N°. 113 is ± 125
the thickness (without the trophoblast) ±: 70
[x. The blastocyst des-
cribed is the same of which
Hubrecht 1908, plate C, fig. 18 reproduced
the embryonic part. In the plate-description the name
Galeopithecus
was erroneously attributed to this stage, and this was corrected in a
following communication
(Hubrecht 1910, p. 592).

In both blastocysts described the trophoblast covers the embryonic
knob. In the germdisc, succeeding in age, a primitive streak has been
formed. Then the embryonic epiblast passes peripherally into the tropho-
blast, which consequently no longer covers it. How the trophoblast
disappears from the embryonic disc, I cannot trace, the s\'tages lacking.
This is a pity as opinions difirer on this point.

In the primitive streak-stages, to be described in subsequent pages,
the hypoblast lies as an uninterrupted layer under the primitive streak.
Secondarily in older embryos, possessing 8—18 pairs of somites epi-,
meso- and hypoblast are intimately fused in the cranial part of the
primitive streakj in the mid-axis of the embryo.

Heape has figured this as early as 1883. The description of the
development of the notochord, however, must be compared to see this.

Here we must still mention that the thickness of the hypoblast,
extending as a continuous layer along the inside of the primitive streak-
stages, can be very different in the same embryo. This is apparent in
the description of N°. 180 (p. 22).

Beneath the posterior part of the primitive streak (to compare fig. 29
plate III) the hypoblast forms a distinct epithelium, composed of large
cubical cells. Gradually it decreases in thickness and becomes a very flat
layer beneath the headprocess.
Huber also figured this in his recent paper
on the development of the notochord in
Cavia (1918, fig. 4). Before
the anterior end of the headprocess the hypoblast suddenly becomes
a thickened plate, composed of two or three strata of cells. This plate,

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the prochordal plate, will be discussed more fully later on. To sum-
marize, we have seen that in
Manis the hypoblast is derived from
the embryonic knob by delamination. This is in agreement with the
observations in almost all other Mammals. Only in Marsupials it was
observed lately by
Hill (1910) and Hartman (1919) that the anlage
of the hypoblast is formed by immigration of some few „entoderm-
mothercells" out of the embryonic knob.

After delamination the hypoblast grows round the inside of the
whole blastocyst and here forms against the trophoblast a closed sac,
the umbilical vesicle, out of a part of which the archenteron is formed.

III. Description of surface-views of the embryonic shields.

Fig. 9, plate II (embryo 180) represents the ventral side of one
of the two primitive streak-stages. From the sections (to compare next
chapter) it follows that this embryonic area is slightly older than
151, which strongly resembles it in shape (fig. 21, plate III).
The shield is pear-shaped; the white spot indicated, is the place,
where the headprocess originates from the primitive streak. The
dorsal side of this embryo could not be reproduced, for it is covered by
the uterine wall, with which the trophoblast has fused in some places.

Embryo 83 is shown in fig. 10, plate II. It is a sole-shaped
embryonic shield, possessing 4 pairs of somites with a pair forming.
The anterior pair of somites is not sharply outlined in front, as it is in
connection with the mesoblast here, which appears from sagittal sec-
tions through the embryo. The medullary groove is still quite open.
Neither the brain nor the pleuro-pericardial space is to be seen. The
gastral mesoblast (to compare the chapter on mesoblast-formation),
from which the somites are formed, is traceable cranialward, where
It fuses with the so-called cephalic mesoblast; this is visible in the
sections only. Around the anlage of the embryo proper a well-deve-
loped area vasculosa is to be seen.

Figs. 11 and 12, plate I represent a sole-shaped embryonic shield
(embryo N°. 64), possessing eight pairs of somites with a pair
forming. The anlage of the brain is visible. The pleuro-pericardial
space i§ to be seen in sections only. The medullary groove is still
open. A pro-amnion is not formed. (Fig. 11 dorsal, fig. 12 ventral view).

Verhand. Kon. Akad. v. Wetensch. (ze Sectle). Dl. XXL C 2

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Fig. 13, plate II (embryo N°. 32) shows an embryonic area with
.10 pairs of somites and an forming. The pro-amnion is clearly
visible, the pleuro-pericardial space is not to be seen. The brain-
anlage and a beginning of the head-bend are distinct. The medullary

groove is partly closed.

Embryo N°. 89 possesses 13 pairs of somites and is reproduced in
fig. 14, plate II. The pleuro-pericardial space and the brain-anlage are
clearly visible. The medullary groove is closed for the greater part.

About the same can be observed in the slightly older embryo N°. 31,
in which a pair of somites is forming (fig. 15, plate II). In

neither of the embryos a pro-amnion is visible (figs. 14 and 15), but
this is the case in the younger embryo N°. 32 and is also to be seen
in the transverse sections of 31. The sketches having been drawn
before 1907, I cannot ascertain whether the pro-amnion is pushed
aside by the artist, in order to set off the embryo.

Fig. 16, plate II (embryo N^ 108) shows a distinct head-bend. The
embryo, in which 13 pairs of somites have been formed, is sketched

from the ventral side.

Embryo N°. 68 possesses a well-developed head (fig. 18, plate I).
The heart and the brain are clearly visible in fig. 17 (dorsal view)
and fig. 18 (ventral view); 16 pairs of somites have been formed.
The medullary tube is still open posteriorly. A pro-amnion is

distinctly perceptible.

Embryo N^ 53 with 18 pairs of somites (fig. 19, plate II) shows a
distinct brain. Pro-amnion and head-bend are easily distinguishable.

IV. Development of the 7nesoblast.

The origin and development of the mesoblast can be well traced
in two Mafiis-tmhryo^, both in the primitive streak-stage. The one
(N°. 151) is almost cross-, the other (N°. 180) sagitally sectioned. The
former is slightly younger than the latter. A schema of this younger
M7/7/j--stage presents fig. 21, plate III. This reconstruction has been
made in such that, at a magnification of 100, the breadths of

different organs of each section of the germdisc were set out on
millimetre-paper. The thickness of the sections being 10 every
millimetre agrees with one section. As a fixed point the middle of each

-ocr page 35-

section of the embryonic area was taken. To obtain a survey of the
different organs and of the germdisc itself in situ, this schema suffices.
The sections, which will be discussed in subsequent pages, are
indicated.

The length of this embryonic shield\'is about 720 (72 sections
of 10 a each), the largest breadth 800 y,. As is shown in the repro-
duction, the embryonic area is pear-shaped, the primitive streak
{p.s.) occupying the hinder part. Somewhat more in the direction
of the future head-region a tongue-shaped organ originates out
of the primitive streak in cranial direction. It is the headprocess of
the primitive streak, which will be called headprocess for the present.
This name means that the primitive streak forms a process in the
direction of the future head-region. As will soon be apparent in the
description of the different sections, the hypoblast covers the whole inner
face of the blastocyst just as in the younger M^^;;/>-stage N°. 113. Fig. 20,
plate I is a reproduction of a section through the whole, strongly
folded blastocyst, partly lying against the uterine wall. The embryonic
anläge is. very small, compared with the large extra-embryonic part.

The hypoblast forms two thickened parts, both indicated in fig. 21,
plate III. One of them, lying just before the anterior end of the head-
process has been called protochordal plate by
Hubrecht. This name
indicates that a part of the notochord develops out of this plate. Another
part of this organ derives from the protochordal wedge
(Hubrecht
1890 = hensen-knob, Hensen 1876). This thickened part of the
hypoblast must be called prochordal plate, if we suppose that the
notochord reaches to this point and fuses with it, but that no elements
originate from it to form the anterior part of the notochord.

Whether this opinion is right or wrong, I will not decide here;
I will treat this question later on (pp. 42 and 43), when discussing the
development of the notochord. The second thickened part of the hypoblast

to be found in the periphery of the embryonic shield and is called
annular zone of proliferation by
Hubrecht (1890). Out of it the
peripheral mesoblast will develop.

On studying the cross-sections, (figs. 22—28, plate III) I can
observe the following facts.

The section, reproduced in fig. 22, passes through the posterior part of
the primitive streak. In the mid-axis, on the upper side, a small shallow

c 2*

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pit is found. (Fig. 2i must be compared for this and following sections).

This is taken to be a remnant of the primitive groove, which has
now disappeared for the greater part. Out of the primitive streak
mesoblast-cells proliferate. This part of the mesoblast is called peri-
stomal mesoblast in connectign with the theoretical signification attached
to the primitive streak. On the right and on the left the primitive streak
passes into the trophoblast, which has disappeared from the upper side
of the embryonic area. The inner face of the germdisc is formed by
a layer of hypoblast, equally thick everywhere, with round nuclei.

Somewhat more cranialward the epiblast rises above the level of the
trophoblast (fig. 23). In this
and following figures the epiblastic tissue
has been drawn but partly: for simpHcity\'s sake it has been dotted.
This section too passes through the primitive streak, which also forms
peristomal mesoblast here. The hypoblast also lies as an uninterrupted
layer at the inner side of the germdisc, and is slightly thinner in the
middle only.

The following section sketched (the from the first section

described) passes through the front part of the primitive streak (fig. 24).
The anterior part of the primitive streak and the posterior. part
of the headprocess are strongly thickened in this region of the
embryo. This place has been called
hensen-khob (Hensen 1876) or
protochordal wedge
(Hubrecht 1890) or „Primitivknoten" (Bonnet
1889). The hensen-knob is also covered by the hypoblast. Caudad
from this place, over the whole length of the primitive streak, the
peristomal mesoblast is formed. It spreads laterally from the primitive
streak between epiblast and hypoblast.

Fig. 25 represents a section through the headprocess, which is
developed slightly in this embryo. In this region the epiblast of the
embryonic area forms no mesoblast; it is a continuous layer, composed
of tall columnar cells. Between headprocess and epiblast, there is a
distinct Hmiting membrane. The headprocess extends cephalad as a
wedge-shaped or tongue-shaped organ between epiblast and hypoblast.
On the left side of the section, just against the border of the germdisc,
the hypoblast shows a tickened place

\') On comparing the figs. 22—28 and 21 it will have been noticed that what is on the left in
the cross-sections is on the right in the schema (fig. 21). This comes out clearly in the asymmetric

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In fig. 26 too, which is a section through the cranial end of
the headprocess, this thickening lies only on the left side of the
reproduction. This is a part of the annular zone of proliferation
(Hubrecht 1890), which, however, does not proliferate as yet in this
part of the embryonic area.

This is the case in the section, represented in fig. 27, where the
annular zone is found on the left as well as on the right. Here on
the left, we see the very first mesoblast-formation, on the right this
process has^ not yet begun. It is called the peripheral mesoblast
(Bonnet 1920, p. 111), because it is formed at the periphery of the
germdisc. In the centre of the embryonic shield the hypoblast still
shows another thickened part, situated exactly in front of the cranial
border of the headprocess and called /rochordal plate for that reason.
In
Manis this plate has an oval shape and is of about a breadth of
330 and 220 p. length. The annular zone is still better visible in
the anterior part of the area. Here it forms a ring around the anterior
part of the embryonic shield (figs. 28 and 21) and in different places
distinctly produces the peripheral mesoblast. In the same section the
anterior part of the prochordal plate is shown. Here the prochordal
plate is connected with the annular zone (fig. 21) and so, in the
mid-axis of the embryo, the hypoblast is thickened from the cranial
point of the headprocess to the anterior margin of the embryonic
area. In this younger primitive streak-stage the hypoblast is princi-
pally thickened in the anterior part of the shield. In some places the
mesoblast is formed from the hypoblast, viz. the annular zone of
proliferation. This is not yet the case in the prochordal plate,

Bonnet has also described Hubrecht\'s protochordal plate in other
Mammals and calls it „Erganzungsplatte", because, according to him,
the anterior part of the notochord, the mesoblast in the future front-
part of the head (the so-called cephalic mesoblast), a part of the
tore-gut and the inner layer of the primary pharyngeal membrane are
derived from it
(Bonnet 1920, p. 100).

In the older embryo, N°. 180, also a primitive streak-stage, the

^gs- 25 and 26, where this proliferating place {a.K..) is shown on the left side of the sections, whilst
^ cording to
fig. 21 this is the case on the right. This is easy to explain, however, because the em-
th has been cut and fixed to slides from before backwards, so that what is on thp left in

figures, is on the right in reality.

-ocr page 38-

mesoblast-formation has been going on for some time. This is visible
in fig. 29, plate III giving a survey of this embryo, which is very well
sectioned in the sagittal plane. The section passes through the mid-axis
of the embryo, of which the convex side hes closest to the uterine wall.
The primitive streak extends from the left to the point marked by an
the embryonic epiblast, clearly separated from the tissue lying beneath,
occupies the region between the primitive streak and the trophoblast
(on the right). The wall of the blastocyst is torn off just in front of
the embryonic area, probably in the preparing of the embryo. The
place of attachment, however, is evident._ Along the whole length of
the primitive streak the peristomal mesoblast is formed. Near the the
headprocess develops out of the primitive streak in the direction of
the future head-region and extends between epiblast and hypoblast,
gradually becoming thinner. The hypoblast is visible as a very thin
layer beneath this organ. Under the cranial part of the headprocess
it is composed of a few flattened cells. Exactly in front of the anterior
end of the headprocess, it suddenly passes into a thickened plate, the
prochordal plate, with nuclei in two or three strata. Before the
germdisc the prochordal plate passes into the extra-embryonic hypoblast,
present in a single layer of cubical cells. In this stage the prochordal plate
reaches to the anterior margin of the germdisc. Behind the embryo
the extra-embryonic hypoblast is nearly as thick as in front of the
embryonic area. However, beneath the posterior part of the primitive
streak, it gets much thicker, generally remaining a single layer of
cells, gradually becoming thinner more in front. In this embryo too
the prochordal plate produces no cephalic mesoblast as yet.

Fig. 30, plate III shows the middle part of fig. 29 greatly enlarged.
It represents the anterior part of the primitive streak, the part, where
the headprocess originates. A thickened
„hensen-Knob" is not present.
The headproccss extends in a cranial direction, the peristomal mesoblast
spreads laterally from the primitive streak. The hypoblast is a thin
flat layer of cells with few nuclei.

Figs. 31 and 32, plate III represent parts of sagittal sections through
the same germdisc. Fig. 31 shows a section of the prochordal plate
and the anterior part of the headprocess. This is not a section exactly
through the mid-axis of the embryonic area. The epiblast forms a
layer with three or four strata of nuclei, distinctly separated from the

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underlying tissue. Here the thin cranial part of the headprocess can
be observed, extending between epiblast and hypoblast. The hypoblast
under this part of the headprocess forms a very thin layer with few
nuclei and passes suddenly into the thickened prochordal plate, just
in front of the headprocess. Beneath the anterior margin of the
embryonic area the prochordal plate passes into the extra-embryonic
hypoblast, consisting of a single layer of cubical cells.

The formation of the mesoblast out of the prochordal plate has
started nowhere.

The peripheral mesoblast develops in this embryo. Fig. 32, plate III
shows the origin of this mesoblast behind the caudal region of the
germdisc. In the embryo, described in preceding pages (N°. 151, also a
primitive streak-stage), we have found the early development of the
peristomal mesoblast in the anterior part of the germdisc. Here, howe-
ver, the annular zone of proliferation has extended along thep eriphery
of the embryonic area, meeting behind the primitive streak. Here we
see a distinct mesoblast-formation, just outside the border of the
germdisc. In fig. 32 the mesoblast-cells proliferate out of the hypoblast.
Moreover the mitotic figure (m) indicates the direction, in which
the cleavage takes place. In other sections these figures are more
numerous. The direction of cleavage is the same in all. More anterior
(under the posterior border of the germdisc) the peripheral mesoblast
gradually passes into the embryonic peristomal mesoblast; a limit
c^annot be observed. The peripheral mesoblast, originated from the
hypoblast and the peristomal mesoblast, originated from the primitive
streak, form one sheet of cells. The peripheral mesoblast is also found
at the sides of the germdisc, but here the formation has stopped for
the greater part.

Out of the sides of the headprocess mesoblast also develops, spreading
peripherally between epiblast and hypoblast. This is the mesoblast
from which the somites, pronephros, wall of the coelomic cavity etc.
derive in the anterior part of the embryo later on. It is called gastrai
mesoblast, in connection with the theoretical signification of the
headprocess.

This also fuses \' with the peripheral mesoblast, developed at the
borders of the germdisc, and passes into the peristomal mesoblast cau-
dalward. So we see that a continuous sheet of mesoblast-cells is

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extending in this primitive streak-stage between epiblast and hypoblast.
On comparing the germdiscs N°. 151 and 180, we see that, taking
into consideration that N°. 180 is somewhat older than N°. 151, the
mesoblast in the former is much more developed. In the younger the
peripheral mesoblast has been formed but for a small part, the annular
zone of proliferation extending in the future cranial part of the disc
only. In the older germdisc, however, the formation of the peripheral
mesoblast has progressed further and in some places, especially at the
anterior borders of the embryonic area, it has come to an end. Behind
the posterior part of the embryo, outside the germdisc, this develop-
ment still continues in a place, where in the younger embryo there
is as yet no trace of the annular zone. The headprocess is more
developed in the older embryo than in the younger, and is tongue-
shaped in both shields, extending between epiblast and hypoblast. The
hypoblast in N°. 151, as yet a rather thick layer of epithelium-cells,
has become in N°. 180 a very flat layer under the headprocess.
It seems that the hypoblast, pushed away by the developing head-
process, has become a flat epithelium with few nuclei. In both embryos
the prochordal plate lies just in front of the anterior end of the head-
process. In the younger germdisc this plate is connected with the part
of the annular zone, lying in front. In this case the hypoblast is
thickened from the cranial point of the headprocess to the anterior
border of the germdisc. In N°. 180 the same is the case, the annular
zone having fused intimately with the prochordal plate. No hmit is
visible between these parts. For convenience sake I have called
prochordal plate the whole thickened part of the hypoblast, lying in
front of the headprocess in the second primitive streak-stage described.
The anterior part of the prochordal plate passes directly into the other
parts of the zone. The embryo being cut in the sagittal plane, it is
not easy to see this connection.

In tracing the further development of the mesoblast in Manis, we
lack several stages, for the embryo succeeding in age is much more
developed, possessing four pairs of somites with a forming and a
medullary groove. However, this stage is of great importance, because
it shows a very distinct formation of cephalic mesoblast. This embryonic
shield has been cut very well in the sagittal plane. Fig. 33, plate IV
shows a schema of a section through the mid-axis of it. We see

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that the peristomal mesoblast still originates from the primitive streak.
At the cranial side of the embryo the cephalic mesoblast develops
out of a very distinctly thickened part of the hypoblast, the prochordal
plate. The chorda-anlage originates from the headprocess, which is
strongly thickened in its caudal part, the
hensen-Knob. The chorda-
anlage is incorporated in the hypoblast and the so-called chordal plate
occupies the roof of the future gut-lining. This chordal plate passes
into the prochordal plate, composed of two or three layers of cells.
The cephalic mesoblast clearly develops out of this plate (to compare
fig. 34, plate IV, in which the cranial part of the chordal plate and the
prochordal plate are greatly enlarged). It spreads as a broad sheet of
cells between epiblast and hypoblast to the anterior border of the
embryonic shield, and has reached a considerable extension. This
marked formation of cephalic mesoblast takes place, when at least
four pairs of somites have been formed. It develops remarkably late
in
Manis \\ Bonnet e.g. finds (1901) that a distinct cephalic mesoblast-
formation takes place in young primitive streak-stages of the dog.

The chordal plate originates from the hensen-Knob. Along the whole
length of the chordal plate the mesoblast is connected with it at both
sides. Near the primitive streak this connection is clearly visible; more
cranially it is difiicult to trace, the embryo being cut in the sagittal
plane and the mesoblast being connected with the sides of the chordal
plate. In some places it is easy to see it, when we carefully compare
the consecutive sections.

This gastral mesoblast, out of which 4 pairs of somites with a
pair forming have originated, extends to the anterior part and the
periphery of the embryonic shield. Caudalward it passes into the
peristomal mesoblast and from these two parts most organs of meso-
blastic origin are derived. Behind the anterior part of the head-region,
in which the cephalic mesoblast is formed, the gastral mesoblast is
connected with the cephalic mesoblast. The latter is also composed
partly of peripheral mesoblast, and in other regions of the embryonic
area the gastral mesoblast or the peristomal mesoblast is fused with
the peripheral mesoblast. So we see that a continuous layer of meso-
blast-cells, developed out of primitive streak and headprocess, as well
as out of the hypoblast, extends between epiblast and hypoblast. So
the mesoblast of
Manis develops in several regions of the embryo.

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With the name mesoblast is indicated only the fact that this layer
lies betw^een epiblast and hypoblast. It is a topographical term and
does not signify that all parts composing it, are of the same origin.

In the Ma>iu-e.mh\\:jo, possessing 4 pairs of somites with a
forming, (this is still connected with the undifferentiated mesoblast)
a fore-gut has not yet been formed. This is the case, however, in
embryo N°. 64, which possesses 8 pairs of somites with a forming,
and succeeds the foregoing in age. In an other embryo, N°. 32, with
10 a
ii pairs of somites, a well-developed fore-gut is present. A pri-
mary pharyngeal membrane, however, is still absent. This oral plate,
the place where the hypoblast lies close against the epiblast and where
afterwards the mouth is formed, is derived from the anterior part of
the prochordal plate, which follows from the investigations of
Bonnet
(1901). Behind this pharyngeal membrane we must look for a place,
where the hypoblast and mesoblast are connected and indeed in the
embryo N°. 64 with 8 a 9 pairs of somites there is still an intimate
connection between the cephalic mesoblast and the sides of the fore-
gut. In a
Mams-tmhvyo with 10 a 11 pairs of somites it is iinpossible
to ascertain this with certainty. It then seems that the connection
of cephalic mesoblast and hypoblast exists no longer, for in older
embryos with 13, 16 and 18 pairs of somites I cannot find it.

In embryos with 8 a 9 and 10 a 11 pairs of somites (N°. 64 and
32) the gastral mesoblast is connected just behind the fore-gut with the
sides of the chordal plate, which is incorporated in the hypoblast here.
Superficially seen (fig. 36, plate IV) the mesoblast seems to be connected
with the hypoblast. This is not the case, however. The broad, thin layer,
extending beneath the medullary groove, is the chordal plate, which
can be stated with certainty, when we trace this plate caudalward.
This will be apparent from the description of the development of
the notochord. At the borders this chordal plate passes into the hypo-
blast, the gastral mesoblast being connected with this plate here.

In the region of the somites this is no longer the case,\'but caudal-
ward, in the neighbourhood of the primitive streak, the mesoblast
has not yet separated from the headprocess, there to be found, and
so mesoblast and chorda-anlage are still connected with each other
here. As the primitive streak shortens caudalward (the embryonic
organs originating from it) the zone, where chorda-anlage (i. e. head-

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process) and mesoblast are connected, recedes still more. At the place,
where formerly the peristomal mesoblast was formed, now the chorda-
anlage, developed out of the primitive streak („Primitivstreifenteil
des Chordas") is connected with mesoblast, derived from it as well. We
must still call it peripheral mesoblast and so, in the caudal region
of older embryos, the chorda-anlage, derived from the primitive streak,
and peripheral mesoblast are connected. For this the chapter on the
development of the notochord must be compared.

Summary.

The mesoblast of Manis originates in the primitive streak and the
headprocess, as well as in the hypoblast. The primitive streak ventrally
and laterally forms over the whole length the
peristomal mesoblast.
This is principally to be found in the posterior region of the embryo.
Out of the sides of the headprocess the
gastrai mesoblast develops,
when the formation of the peristomal mesoblast has already begun.
Later on, when the chordal plate is formed out of the headprocess, this
mesoblast is still connected with the borders of this plate, separating
from it in a succeeding stage. Caudalward the gastrai mesoblast fuses
with the peristomal mesoblast. At about the same time that the
peristomal mesoblast formation starts, the
peripheral mesoblast originates
from a ring-shaped part of the hypoblast, the annular zone of pro-
liferation. This formation begins in the future head-region of the
embryo, gradually moving backward along the borders of the germdisc.
The formation of the peripheral mesoblast, coming to an end in the
cranial part, still takes place behind the caudal part of the primitive
streak, outside the embryonic area. In the anterior region of the
germdisc another thickening in tlie hypoblast develops, just in h\'ont
of the cranial end of the headprocess. This is the prochordal plate,
which fuses in the mid-axis of the embryo with the part of the
annular zone, lying still more in front. In the older primitive streak-
stage described, both parts form one intimate plate from the cranial
end of the headprocess to the anterior border of the germdisc. From
this plate the
cephalic mesoblast originates, which has absorbed tlVe
peripheral mesoblast formed out of the anterior part of the annular
^one. This cephalic mesoblast forms only tlie mesoblast of the anterior

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head-region. The rest of the mesoblast of the head must be considered
gastral mesoblast. Before we can speak of a head proper, this cephalic
mesoblast is already fused with the gastral mesoblast, lying posteriorly.
During the development of the cephalic mesoblast in
Manis, the
anterior somites and wall of the coelomic cavity are derived from
the gastral mesoblast. The rest of the peripheral mesoblast also fuses
in the different regions of the area with the cephalic mesoblast,
gastral mesoblast or peristomal mesoblast. So a continuous layer of
cells, the
mesoblast, originated from the primitive streak, from the
sides ot the headprocess and from the two parts of the hypoblast
(prochordal plate and annular zone of proliferation) is formed between
epiblast and hypoblast.

The formation of the peripheral mesoblast comes to an end first.
Then, in the primitive streak-stage, the cephalic mesoblast is not yet
formed. In an embryo, possessing 4 pairs of somites with a forming,
the development of it is in full progress and is to be observed in
an embryo with Sag pairs of somites. In embryos with 10 and more
pairs of -somites this is no longer possible. The mesoblast is for a long
time connected with the chorda-anlage in regions, where no somites
have as yet been formed out of the mesoblast. This is the case at
the cranial part of the chordal plate (gastral mesoblast, fig. 36, plate IV)
and at the posterior part of the chorda-anlage, near the primitive
streak (peristomal mesoblast and the so-called „Primitivstreifenteil des
Chordas"). The peristomal mesoblast is formed first of all mesoblast-
parts in the primitive streak and originates from it as long as the
primitive streak exists, the latter being the organ, from which the
embryonic organs develop from before backward.

V. Origin and development of the notochord.

In the above description of the development of the mesoblast, the
origin of the notochord has been frequently discussed. The youngest
germdisc, in which a headprocess, from which the chorda-anlage
.will develop, is to be found, is embryo 151. In this embryonic
area (cf the reconstruction fig. 21 and the cross-sections figs.
24, 25 and
26, plate III) the headprocess still has the appearance of a tongue-
shaped organ, which extends cranialward from the anterior part of the

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primitive streak between epi- and hypoblast. From the headprocess
originate the gastral mesoblast, the chorda-anlage and perhaps elements
of the future gut-lining as
Rabl (1915) supposes.

In connection with the theoretical signification attributed to this
organ it ought to be called metenteron, but for the present I will
keep the more usual and meaningless name headprocess.

The part of the primitive streak, from which the headprocess
originates, will be indicated by the name of
hensen-knob (Hensen
1876), when this part is strongly thickened. Hubrecht called it
protochordal wedge, but often erroneously included the headprocess.
The transverse sections (figs. 24, 25, and 26) pass respectively through
the anterior part of the primitive streak (the
hensen-knob), through
the middle part of the headprocess and through the most cephalic
end of the headprocess.

In fig. 24 there is no limiting membrane visible between the
epiblastic part of the primitive streak and the headprocess, originating
from it. The
hensen-knob causes the hypoblast to project ventrally.
In both of the other sections (figs. 25 and 26) the headprocess is
distinctly separated from epiblast and hypoblast, and extends between
these germlayers, as far as a thickened part of the hypoblast, the
prochordal plate. The headprocess has a length of about 150 fx and
a breadth of -h 190 at its base. It is remarkable that in this
young primitive streak-stage no mesoblast develops as yet out of the
sides of the headprocess. This is the case, however, in the slightly
older germdisc, also in the primitive streak-stage and sectioned in
the sagittal plane (N°. 180, figs. 29—32). Here the headprocess
js considerably longer ( 500 ,0). The origin of the gastral mesoblast
clearly visible, when we compare the successive sections. It spreads
the form of two wings from the sides of the caudal and middle
part of the headprocess. From the figs. 29 and 30 it follows that
the headprocess extends from the cranial part of the primitive streak,
between epiblast and hypoblast. Here a
hensen-knob is not as clearly
visible as in the younger embryo N°. 151 and in the older embryo N°. 83.

Besides the gastral mesoblast originating from the headprocess
in the embryonic shield, this area is also distinguishable by an other
peculiarity from.N°. 151. In this embryonic area the hypoblast
extends as a continuous layer beneath the headprocess and primitive

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Streak, and is equally thick everywhere. In N°. i8o, however, the
hypoblast becomes much thinner under the headprocess and is reduced
to a flat epithelium with few nuclei. It gradually becomes higher
under the primitive
streak. Just in front of the anterior end of the
headprocess, it suddenly forms a thickened plate with nuclei arranged
in two or three strata: the prochordal plate. The hypoblast lacks
nowhere at the ventral side of the embryonic shield. This is the case,
however, in older stages, where for some time the chordal plate forms
the roof of the future gut. In neither of the Mj/?/V-embryos discussed,
in which a free headprocess is found between epiblast and hypoblast,
a so-called
LiEBERKtiHN-canal is to be found. This canal Lieberkuhn
described for the first time in Mammals (Cavia) in 1882. It is the
homologue of the metenteron of the Anamnia and, when formed
completely, penetrates the whole length of the headprocess and opens
ventrally into the archenteron, the cavity enclosed by the hypoblast.

The embryo next in age to the two germdiscs described, is N°. 83
with 4 pairs of somites and a 5"\' pair forming. Fig. 33, plate IV shows
a schema of the mid-sagittal plane of this embryo. Here the head-
process also originates from the Cephalic part of the primitive streak,
forming a distinct
HENSEN-knob. The posterior part of the embryonic
shield is somewhat curved, hence the section does not pass through
the mid-axis here. This is the case, however, in the greatly enlarged
section, reproduced in fig. 35. As is evident from figs. 33 and 35, the
ventral part of the embryo has been torn from the dorsal part.
Between primitive streak and headprocess, between the medullary
groove and the chordal plate, an artificial space is to be found.

The HENSEN-knob is clearly visible. Behind it, beneath the primitive
streak, lies the hypoblast, gradually becoming thinner and disappearing
completely under the
HENSEN-knob. In the section reproduced, two
lumina are found in the
HENSEN-knob. Neither is artificial, tor both
possess a regularly formed wall, and the numerous cell-membranes
of the larger of the two are radially arranged. Both occupy the
center of the
HENSEN-knob, but they are not connected. An opening
into the primitive streak or into the archenteron is not to be found.
In some adjoining sections a few similar lumina are to be seen,
but most of them are narrower than the largest of the lumina
reproduced. These lumina, also to be observed in older stages,

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are no rests of the LiEBERKiiHN-canal, according to me, but more
homologous with the rudimentary neurenteric passage of the Anamnia.
The
LiEBERKUHN-canal indicates only the canal, which originally is
to be found in the headprocess, the homologue of the metenteron
of the Anamnia. When the embryo grows in length, a new part of
the headprocess is derived from the primitive streak, the original part
having incorporated itself in the hypoblast. The older the embryo
becomes, the longer the chorda-anlage grows. The part of the head-
process, formed out of the primitive streak, moves more and more
caudalward, and the lumina to be found here do the same. The
neurenteric passage of the Anamnia connects the posterior part of
the medullary tube with the primitive gut. It also moves caudalward,
during the growth in length of the embryo, whilst the headprocess,
homologous with the metenteron, in a younger stage grows through
invagination in a cranial direction. For this reason the lumina, just
described, and others, still to be described in older stages, must be
homologized with the neurenteric passage of the Anamnia and not
with the
LiEBERKtiHN-canal or metenteron. I call it chordal canal,
because in Mammals the notochord principally originates out of the
headprocess, in which these lumina are to be found.

The HENSEN-knob passes cranialward into the chorda-anlage, which
is not covered by hypoblast ventrally. Here the chorda-anlage forms
the roof of the future alimentary tract. So the chorda-anlage is in-
corporated in the hypoblast. In this study this developmental stage
of the notochord will be called chordal plate. Later on, when the
chordal plate has separated from the hypoblast, the notochord proper

is formed.

The chordal plate continues caudalward in a layer, in which the
nuclei are more numerous than in the other part of the
Hf^nsfn-
l^nob. During the longitudinal growth of the embryo, the chorda
plate is derived from the dorsal part of the headprocess, the pstra
"mesoblast from the sides. In front of the
hensen-knob, the chordal
plate is still rather thick, consisting of tall columnar cells. It becomes
»nuch more flattened cranialward (fig. 34)- I" ^he cephalic part ot
the chordal plate it is difhcult or impossible to trace a difference
with the hypoblast, when we compare the sagittal sections, btil
ii^ore in front the chordal plate suddenly passes into the thickened

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prochordal plate. A limiting membrane between these parts is not
traceable; the anterior part of the chordal plate is directly continuous
with this part of the hypoblast. If we did not know from younger
stages that the headprocess reaches as far as the prochordal plate there,
we could not know that in this stage the chordal plate passes into
the prochordal plate. In older cross-sectioned stages we can also trace
the chordal plate, incorporated in the hypoblast, up to a region far
advanced in the head of the embryo.

In this embryo the gastral mesoblast is still connected with the
chordal plate (cf.
p. 25). There is no question of the edges of
the split hypoblast trying to fuse with each other, neither in this
nor
in other embryos (with about 8, 10 and in a few cases 17 pairs
of somites).

The embryo N°. 64 has 8 pairs of somites with a pair forming.
A fore-gut, having a length of 80 ix, is formed; the head-bend has
not taken place and consequently no pharyngeal membrane is to be
found. The sides of the fore-gut are still connected with the cephalic
mesoblast in some places. Here we still find a remnant of the pro-
chordal plate in the wall of the fore-gut.

Behind the fore-gut we see that the roof of the future alimentary
tract is distinctly thinner than the adjoining hypoblast. With the
borders of this plate the mesoblast is connected. It is evident that
here we see the cranial part of the chordal plate, incorporated in the
hypoblast and that the front part of the gastral mesoblast is connected
with It (fig. 36, plate IV). From this i-egion we can trace the chorda-
anlage backward, but it must be observed that the chordal plate as such
is not distmctly visible everywhere. In the anterior part there is a con-
nection between gastral mesoblast and chordal plate in several places.
More caudalward, at the level of the first somites, hardly any difference
IS to be seen between chordal plate and hypoblast. Here (he chordal
plate is but slightly thinner than the hypoblast. Gradually the differ-
ence betweai both parts becomes more marked (fig. 37). Under
the medullary groove a broad plate extends, which is a little thicker
than the hypoblast and easily distinguishable from it. On the right
the hypoblast is torn from the chordal plate, the connection, however,
IS clear A similar view shows fig. 38, drawn after a section through
the eighth pair of somites. The chordal plate being higher the dif-

-ocr page 49-

ference with the hypoblast is more distinct. The borders of the
chordal plate are still directly continuous with the hypoblast. The
chordal plate possesses numerous cells and is clearly distinguishable
from the hypoblast, which contains less nuclei. In the preceding
pages (embryonic shields N°. 151 and 180) we have seen that the
hypoblast in pripnitive streak-stages still extends under the headprocess.
This is no longer the case here, the headprocess having fused with
the underlying hypoblast: the ventral side of the headprocess cleaves
and spreads in a flat plate. The underlying hypoblast is split and
is pushed aside by the borders of the chordal plate, which is now
developed out of the headprocess. In the sections described the
borders of the chordal plate are directly continuous with the cleft
hypoblast: the incorporation of the hypoblast is quite finished here.
More caudalward, however, all possible stages of incorporation can
be traced in the same embryo.

The chordal plate in fig. 39 is still higher than is the case in an
anterior part. Here it does not pass directly into the hypoblast: the
latter extends slightly under the borders of the chordal plate. The
latter is not as broad as in more cephalic parts: the incorporation is not
quite finished here. A little more caudalward the hypoblast extends
beneath a considerable part of the chorda-anlage and has just been
cleft. The chorda-anlage is both higher and broader than in the
preceding sections. The undifferentiated mesoblast is not connected
)vith the chorda-anlage, which is the case in the section reproduced

%. 41. Here the connection between both parts is distinct, though
they were torn from each other, when the embryo was sectioned.
An

incorporation of the chorda-anlage in the hypoblast has not yet
taken place. The connection of the mesoblast and the headprocess
gradually becomes more intimate, the headprocess being distinguish-
able, because it has not fused with the epiblast of the primitive streak
and because the nuclei in the headprocess are more numerous than
in the mesoblast-wings. The place, where the headprocess originates
from the primitive streak, lies somewhat more caudalward and here
^e also see, how the hypoblast "in the mid-axis of the embryo is
secondarily fused with the primitive streak-tissue. To obtain a general
view of the organs in this posterior part of the shield in the embryos
jwst described and still to be described, I have made a schema of

Verhand. Kon. Akad. v. Wetensch. (ze Sectie). DI. XXI. ^ ^

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the mid-sagittal section of the medulla, chorda-anlage and primitive
streak of each embryo. To this end the heights of hypoblast; chorda-
anlage, and medulla were measured in the center of each cross
section at a magnification of
icq. As a fixed horizontal line the ventral
side ot the embryo was taken and above it the measures, found in
each section, were set out on millimetre-paper. The ventral part of
this region of the embryo being almost flat, we may use this method
here 1-or the head-region, however, it will not do; therefore" the
method was used only for the posterior part of the embryo. Figs

^^ the mid-sagittal plane

of the different embryos. Moreover, to indicate the situation of the
somites, their circumferences and also the upper face of the medullary

groove are projected on this sagittal plane. Finally the sections des-
cribed are indicated.

Fig. 62 presents a schema of the posterior part of embryo N° 64
On the right we see the primitive streak, out of which the medullary
groove and the headprocess originate cranialward. Like the primitive
s reak the headprocess is still covered by hypoblast on its ventral
s de. more in front the chordal plate, incorporated in the hypoblast,
« formed out of the headprocess. The hypobLt has secondarily fus d

wW ^uZ \'yi"g ^bove it just behind the place,

where the headprocess develops out of the primitive streak. Laterally

n oW . ^^^ distinguish

In older stages the same occurs and on comparing the different stages

we see that this zone moves caudalward, at theSame time tha die

primmve streak shortens in order to form the embryonic organ .

The M»«.-embryo 32 has 10 pairs of somites with an u"

forming. The medullary groove is closed along a great distance bu is

r nTof -l ^ along

as vef The ï Ù^V\'l Pharyngeal membrane is not found

I cfn nlwhe fi H \' \'\'^\'■^kness everywhere,

fn T r K \'"dications of the chorda being incorporated

with the\'C.:!,;"- "^M ^ -"-«el

Ite Just behl LrT "fprochordal

we t^ace helê ? hut, when

we trace the sections backward, it soon appears. Only in a verv few

places a connection of the gastral mesoblasf with the\\orde" of th"

-ocr page 51-

chordal plate is to be found. In most cases a distinct limiting mem-
brane is noted and consequently these parts are connected no longer.
The chordal plate is clearly distinguishable from the hypoblast and
is very broad under the undifferentiated mesoblast of the head-region.
More backward it becomes less broad, but in the region of the first
somites it has the shape, figured in fig. 42, a section through the
pair of somites. Here the chordal plate is much thinner than the
hypoblast lying on both sides. On the right the hypoblast is not quite
pushed away by the incorporating chordal plate. On the left, however,
the chordal plate, without being covered by the hypoblast, is quite
incorporated and passes into the hypoblast. It seems, as if the edges
of the cleft hypoblast are compressed by the incorporating chordal
plate. On the left and on the right the primitive aortae are to be found.

In the region of the somites VI—XI the very broad and flat
chordal plate becomes less broad and taller. Moreover, the edges of
the hypoblast are extending distinctly beneath the borders of the
chordal plate: the chordal plate is in the very act of incorporating in
the hypoblast. Here the chordal plate becomes taller than the hypo-
blast. In the region of the undifferentiated mesoblast, from which
the somites will develop, the situation is still about the same. Fig. 43
shows a rather broad and high chordal plate, the hypoblast extending
heneath the borders of it. A little more caudalward the hypoblast
is present as an uninterrupted layer under the headprocess. Fig. 44
represents the transitional stage: only a small part of the ventral side
of the headprocess is incorporated in the hypoblast in this place.
Moreover, the mesoblast is connected with the headprocess here,
^hich was not yet the case in the preceding section described (fig. 43)-
In the centre of The headprocess a lumen is visible. This is not only
present in one section, but traceable in 13 consecutive sections (± 130.a).
As will be evident from the description of the other embryos, one
or more similar canals occur in the headprocess. Sometimes two
lumina are present in the same seclion of the headprocess. This is
ne case in front of the canal just described. The nuclei and ceil-
niembranes are arranged radially around the lumen; this being also
^h^Icase, when the iLen appears double. Though I intend to discuss
t ie signification of these lumina in subsequent pages,
I remark heie

th^t the chordal canal must be considered the homologue ot the

c 3*

-ocr page 52-

canalis neurentericus of the Anamnia. In Manis it is present in
embryos with 4—18 pairs of somites.

On tracing the headprocess of N°. 32 still more caudalward, we
find about the same as in the embryo just described. Here the hypo-
blast is forming an uninterrupted layer beneath the headprocess. A
schema of this part of the embryo is to be found in fig. 63, recon-
structed in the same manner as fig. 62
[Manis N°. 64). In the region,
where the headprocess originates from the primitive streak, the hypo-
blast is secondarily fused with the tissue of the primitive streak
(laterally the three layers are distinguishable). More caudalward the
hypoblast is visible as a distinct continuous layer at the ventral side
of the primitive streak.

The fore-gut of embryo N°. 89 with 13 pairs of somites has a
length of 470 The chorda-anlage cannot be found in the most
cranial part of it along a distance of 18 sections ( 180 p.). From
this point, however, we can trace the chordal plate, incorporated in
the hypoblast, caudalward. A primary pharyngeal membrane is present,
traceable along a distance of 160 k 170 [x.

One of the first sections, in which the cranial part of the chordal plate
is to be found, is reproduced in fig.
45, plate V. Beneath the brain-wall
lies a flat plate, which rises distinctly above the level of the hypo-
blast, into which it passes. The wall of the fore-gut is rather thick
here, partly due to the embryo being cut tangentially. A separation
of the notochord from the hypoblast has not yet taken place in this
region, but in the slightly older embryo 31 the first indications
of this process are to be found.

As follows from fig. 45 the difference between the chordal plate
and the hypoblast, in which the former is incorporated, is not very
marked as yet. Gradually this becomes more distinct, however, and
in the region of the first somites we see that the chordal plate,
incorporating in the hypoblast, is a rather thin plate. (Fig.* 46 a section
through the 4"^ and
fig. 47 a section through the pair of somites).
Here we see that the hypoblast is compressed, evidently by pressure
of the borders of the incorporating chordal plate, for more peripherally
the hypoblast has its usual thickness. In N°. 32 (fig. 42) the hypo-
blast was also thickened in a slight degree. More caudally we see
that the chordal plate becomes taller than the hypoblast.

-ocr page 53-

Slightly further caudalward of the 13\'\' and last pair of somites we
find a region, where the headprocess has not yet incorporated in the
hypoblast and here, in the cranial part of the headprocess, a distinct
chordal canal is to be found, which opens ventrally into the cavity,
enclosed by the hypoblast (archenteron). Figs.
48 and 49 portray a
distinct view of this chordal canal, traceable along a distance of
± 140 IX (= 14 sections). It has a diameter of 17 .a. Fig. 49, plate V
shows the cranial part of this canal, where the headprocess is about
to spread, in order to form the chordal plate, pushing aside the cleft
hypoblast. When the embryo was sectioned, these edges were torn
from the chordal plate-borders. Two sections further cranialward the
chordal canal has opened into the archenteron and here we find a
distinct chordal plate (fig.
48). On tracing the headprocess caudalward
(cf. figs.
50, plate V and 64, plate VI), we see that the mesoblast is not
connected with the headprocess, this being the case a little further on.
In most sections caudalward of the chordal canal just described, a narrow
excentric lumen is to be found. This is not necessarily the case, however;
often no lumen can be observed in the sections through the head-
process. Sometimes the lumen appears double, and around it the cell-
membranes and nuclei are also radially arranged (fig. 5°)- I"
sections the headprocess is broader and less high than in a more cranial
part. Both lumina are traceable caudalward in
8 sections (=80 /7.),
then again fusing to a single lumen, which is to be found along a dis-
tance of 1
10 a in the center of the headprocess, and then again appearing
double along a distance of
40 (4 sections). Further caudalward a
single lumen occurs either to the left or to the right, torming short
canals in two or three sections. In this region the mesoblast is still
connected with the headprocess and a little further caudalward the
transition of the headprocess to the primitive streak takes place. Here
f^o in the mid-axis a region is to be found, where the hypoblast fuses
intimately with the primitive streak-tissue. This zone lies slightly
ii^ore backward (fig.
64). . ^ .. ^

31 is a U;;;>-embryo possessing 13 pairs ^

pair forming. Consequently it is slightly older than N . 89 The
f^^^e-gut has a greater length, being ± 560 long. The roof of 1
^^ formed for the greater part by the chorda-anlage which is not
^^ he found in the cranial part along a distance of ± 130 ^

-ocr page 54-

primary pharyngeal membrane is visible in lo sections. This represents
a length of at least loo [x (cf
p. 42). A connection between the
wall of the fore-gut and the cephalic mesoblast is no more to be
found. Fig. 51, plate
V shows one of the very first sections, in which
the chorda-anlage is visible. Beneath the medulla in the hypoblast we
find a clearly perceptible thickening, which undoubtedly belongs to
the cephalic part of the chorda-anlage. The hypoblast is rather thick
here, partly due to the lining of the fore-gut being cut tangentially.
The chordal plate passes into the hypoblast, no distinct limit being
found. It makes the impression, as if the notochord is in the very
first act of separating from the hypoblast. This process is much
clearer a few sections further caudalward (fig. 52). Here the notochord
(a chordal plate exists no longer, as it has become notochord) has a
markedly cylindrical shape, both hypoblast-edges being almost fused.
The separation of the notochord from the hypoblast has nearly
come to an end. In each section of the notochord very few nuclei
are visible. Mesoblast and notochord are separated by a distinct
membrane.

Further caudalward the notochord loses its cylindrical shape. In the
region of the first somites a real chordal plate is visible. At first it has
a small size, but gradually it gets the shape as is shown in fig.
53,
drawn after a. section through the pair of somites. On the left
the chordal plate is no longer covered by the underlying hypoblast,
on the right, however, this is still the case. The edges of the cleft
hypoblast are compressed by the spreading chordal plate, but in a less
degree than in embryo 89.

Caudalward the chordal plate increases in height and breadth. A
little behind the pair of somites the chordal plate is arched
[hg. 54 plate V). One section further the hypoblast extends beneath
the whole headprocess. From this point to the primitive streak the
headprocess shows cell-membranes and nuclei radially arranged around
one or two lumina. Even virtual openings are to be found. Sometimes
such a lumen suddenly becomes larger, and disappears in the following
section. The lumen, reproduced in fig.
55, is one of the larger lumina.
It IS traceable m three sections (= 30 ix). The whole headprocess shows
several parts of this chordal canal. A first indication of fhe formation
of this canal we find in the cells, which are arranged around a virtual

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lumen. Then a real lumen appears, which is to be found in the centre
or to the left or to the right in the headprocess. Out of this chordal
canal the ventral dehiscence of the headprocess takes place, in order
to form the chordal plate. A schema of the caudal part of embryo
31 is shown in fig. 65, plate VI. Two parts of the chordal canal
are indicated, only these occupying the centre of the sections. To the
left and to the right of the section figured, many lumina are to be
found. The hypoblast in this embryo has also fused with the primi-
tive streak-tissue in the mid-axis.

A third AfWj--embryo, also with 13 pairs of somites, is n\'\'. 108,
which is cut" nearly in the sagittal plane, the chorda-anlage thus being
sectioned obliquely. The cranial part of the notochord is reproduced in
%• 56, plate VI. The chordal plate occupies the roof of the fore-gut, and
ends, covered by the hypoblast, in the mesoblast of the head. As is
evident from fig. 56 the primary pharyngeal membrane is cut tan-
gentially and has a length of about 160 The cephalic part of
the lining of the fore-gut is distinctly thickened, a connection of the
^^^esoblast with this part is not to be found here. In the caudal region,
jve also find a headprocess, the hypoblast extending beneath it, and
here also in the mid-axis the hypoblast fuses intimately with the
primitive streak.

Embryo N°. 68 difi\'ers little from the other embryos, as to the
chorda-anlage. I portray only a sagittal schema and a transverse section
(%s. 66 and 57, plate VI). The sagittal schema shows a very short head-
Process in comparison with others. A very distinct chordal canal with
lumen is to be found (fig. 66). Here some other peculiarities fol-
a primary pharyngeal membrane is present, as well as a tore-gut,
f/aceable in 57 sections (at least 570 .a). The chorda-anlage can be
followed up cranialward along a great distance and is not present in
5 sections of the roof of the fore-gut only. It must be taken into
consideration that the fore-gut has been sectioned tangentially; so
part, in which no chorda-anlage is to be found, has a length of
J^ore than 50 For the same reason the primary pharyngeal mem-
hrane visible in 8 sections, has a length of more than 80 a

\'The notochord has distinctly separated from the wall of the fore-
Sut further caudalward. No figures of this are given, because in he
^«^bryo with 18 pairs of somites (N°. 53) this process, which has

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started in an embryo with 13 a 14 pairs of somites (N° 31) has
progressed further and is more distinct here. The hypoblast is com-
pressed by the chordal plate, incorporating in it (cf N"^ 89) Caudal-
ward we see a clearly perceptible chordal canal in the headprocess
The largest diameter of the wide canal is 24 [x (fig cy)- the
cell-membraries and nuclei are arranged radially. The canal is very
long, being traceable in 26 sections (± 260 .a) and opens ventrally
into the future alimentary tract. On the left and on the right a few
lumma are visible in the headprocess. Close behind the chordal canal
reproduced, the headprocess originates from the primitive streak, and

m this region the hypoblast is again intimately fused with the
primitive streak-tissue.

The last embryo to be described for the development of the notochord
IS JN . 53 with 18 pairs of somites. It is an important stage, because
the notochord has separated from the hypoblast in this embryo along
a great distance. \' ®

Fore-gut and primary pharyngeal membrane are well developed
and respectively traceable in g6 and 14 sections (= at the least 060
and 140 /J,). The chorda-anlage extends far cranialward, the wall of
the fore-gut being free of it in 6 sections. It is remarkable that the
cranial part of the chorda-anlage, represented in fig. 58, plate VI is still
incorporated in. the hypoblast, whilst parts of i? further caudalward
have already quite separated from the hypoblast. This figure (c8) is

f " chorda-anlage is traceable

smrTed ht\'e lif^\'^f-The separation of the notochord has already

fbo ,t tw ; T ? here, consisting of

Lctr renZ" 1 jhows the ,4-section caudad of the

ection, reproduced ,n fig. 58. Here the notochord has completely

fsh^Mv \' it. The noto^chord^

IS dightly compressed here, the hypoblast a little thickened.

but at th "»\'o^hord becomes more high than broad.

This i, sh ■ P^« of the somites it is cylindrical.

Imchld \' u \'\' \'hat the diameter of the

in the ni > . ^^^ be found than

in the parts situated further cranially.

r. u^"?, \'he chordal plate was incorpo-

rated in the hypoblast, can still be indicated, for Lre, beneath the

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notochord, the hypoblast is thinner than quite near it. In younger
stages v^^e have met with the compressed edges of the hypoblast; it
seems that the hypoblast has coalesced from these parts.

From the 11 pair of somites caudalward the chordal plate is still
slightly incorporated in the hypoblast, as is evident from fig. 61, a
section through the 13"^ pair of somites. The chordal plate is still a flat
organ. This we can see only in the region of the somites XI—XV
(cr. fig. fQj. beneath the last somites the chorda-anlage is com-
pletely covered by the hypoblast. On comparing figs. 66 and 67, we
see how the part in front of the primitive streak, covered by the
ypoblast, has become much larger: in an embryo with 16 pairs of
somites (N°. 68) the incorporation of the headprocess comes to and
end, for in the slightly older embryo N". 53 with 18 pairs of somites
hypoblast extends as an uninterrupted layer along a great distance
under the chorda-anlage. From the head-region of the embryo the
closure of the hypoblast beneath the notochord takes place caudal-
Ward. So we see that only along a short distance (the region of 5
pairs of somites) the chordal plate is still partly incorporated in the
hypoblast (the most anterior part of the chorda-anlage excepted).

ne incorporation of the head-process in the hypoblast takes place
no longer at a certain moment (probably when the stage of 16 pairs
° is reached); then the hypoblast remains intact.

Though the incorporation of the headprocess in the hypoblast is
out of the question in this part, yet lumina are formed in the head-
Process here. Properly speaking, we cannot use the name headprocess
i-e. the prolongation of the primitive streak in the direction of the
nture head-region. The organ we are discussing now, has formed out
Or the primitive streak in situ, which shortens during the longitudinal
growth of the embryo. Consequently the headprocess in this part of
ne embryo is no outgrowth of the original primitive streak. Three of
^^"i^ina, above mentioned, are indicated in the sagittal section (fig. 67).

^ith the description of the development of the notochord in this
^i^bryo we will end. In still older embryos the notochord has entirely
separated from the hypoblast and lies free between medullary tube
alimentary tract. Cranially it is always much thinner than
caudally, where it remains broad and tall with compactly arranged
for a long time.

-ocr page 58-

Two important questions must be treated now:

1°. Does the anterior region of the notochord originate from the
prochordal plate ?

2°. Do elements out of the metenteron-wall, i.e. headprocess, pass
into the definite lining of the gut or is the latter formed only by
the hypoblast, which, as we have seen, develops out of the embryonic
knob by delamination?

Bonnet answers the first question in the affirmative (like Hubrecht
1890) and concludes this (1901) amongst others from the following
facts. In dog-embryos of different stages he traced the place of the
anterior limit of the chorda-anlage and found that in embryos with
10 pairs of somites the chordal plate passes into the thickened part
of the hypoblast, which he calls „Erganzungsplatte" (= protochordal
plate
Hubrecht). In an embryo with 16 pairs of somites the most
cranial part of the notochord reaches to quite near the anterior point
of the fore-gut. He points out that the notochord gradually grows
cranialward and concludes that this added part is formed by the
„Erganzungsplatte" (to compare also the comparative part, p. 79).
Of course he can trace this best in embryos sagitally sectioned.

I can ascertain the following data in the M7;//j--embryos, which
are cross-sectioned for the greater part. In primitive streak-stages the
headprocess extends exactly as far as the posterior part of the pro-
chordal plate. After incorporation in the hypoblast the chordal plate
passes directly into this plate (cf. the sagittal sections of N°. 83,
33, 34, and the description of embryo N°. 64). The
Manis-
embryo with 10 completely formed pairs of somites (N^ 32) possesses
a gut-lining free from chorda-anlage along a distance of 280
(measured from the cranial point of the fore-gut). From this point
the chordal plate is traceable caudalward. The older the embryo,
the further the cranial limit of the chorda-anlage reaches, which
is apparent from table 3.

It must be noted that the cranial part of the fore-gut is curved
ventrally and is consequently sectioned in a more or less tangential
plane. Hence the measures, which are calculated from the number
of sections, are too small, especially in the older embryos, in which
the head-bend is more distinct than in younger ones. This is marked
by a >.

-ocr page 59-

Cat. N°.

- 1

32

89

31

108
sagittally
sectioned.

1

68

S3

Number of pairs of somites..

10 a 11

13

1
i

13 a 14

13

16

18

Lengtli of the part of the
fore-gut without chorda-anlage

300 iJ.

± 180 [1

> 130

200 fi

> 50 /X

> 60 //

The measure of the embryo sagittally sectioned (N°. 108) is correct
(ng- 56, plate VI), which corresponds to the measure calculated for
the embryo N°. 89.

These data are indications for the opinion that the notochord in
older stages reaches further cranialward than in younger ones. They
do not prove that the prochordal plate participates in the formation
of the anterior part of the notochord. Moreover, it is possible that
he anterior part of the notochord grows cranialward out of the chor-
dal plate itself, without being formed out of the prochordal plate.
In the ikr^;;/>-embryos, however, something else is to be observed.
We start from the younger stages, where the chordal plate gradually
passes into the prochordal plate and is quite incorporated in the hypo-
olast. In the embryo N°. 31, the anterior part of the chorda-anlage
still distinctly incorporated in the lining of the fore-gut, a beginning
or separation from the hypoblast being visible, however, further caudal-
^^^^ (cf. figs. 51 and 52, plate V). The difference is not very marked
"^I\'e, but in the embryo N°. 68 with 16 pairs of somites and especially
the embryo 53 with 18 pairs of somites, the cranial part is
distinctly incorporated in the hypoblast, whilst the hypoblast-edges
^^ave coalesced under the notochord more caudalward (figs. 58, 59
60, plate VI). Consequently the anterior part of the notochord
separates later from the hypoblast.

^n connecting these two observations — the growth of the anterior
of the notochord in cranial direction and the later separation
^\'om the hypoblast of the same part — I must admit that they
^^PPort the opinion of
Bonnet and Hubrecht, but they do not prove

-ocr page 60-

it. Both observations can just as well confirm the view that the ante-
rior part of the chorda-anlage grows out of the chordal plate. On
considering them in mutual relation, I come to the conclusion that
they certainly are a support for the view of
Bonnet. I do not use
the name protochordal plate to indicate this part of the hypoblast,
because it is not proved that the anterior part of the notochord arises
from it. I used the name
prochordal plate, because in young stages
it is lying just before the headprocess, out of which the chordal plate
originates. —

The second question, whether there is a transition of cells from the
headprocess to the future lining of the alimentary tract, is answered
in the affirmative by
Rabl (1915). Counting the number of cells in

a cross-section of a broad chordal plate at a fixed point _ the first

pair of somites — of a rabbit-embryo, and counting in the same
region later on the number of cells in a section of the notochord,
he found that in the former case each section of the chordal plate
possesses a much larger number of cells than in the latter. In some
cases only half the number of cells is found. From the chordal plate,
developing out of the headprocess (or archenteron-wall in
Rabl\'s ter-
minology), cells have disappeared and, according to
Rabl, these cells
have passed into the hypoblast, the lining of the future alimentary
tract.
Rabl even thinks that the greater part of the gut-lining de-
rives from cells of the headprocess. As will be discussed more fully in
the theoretical part, it is my opinion (agreeing with
Assheton) that
the headprocess of the Amniotes is the homologue of the metenteron
of the Anamnia, the part of the gut, formed by overgrowth and
mgrowth (invagination) of cells at the „deuteroporic" lips. The distinct
metenteron of the Anamnia is reduced in size in the Amniotes, the
wall of the metenteron being a prolongation of the primitive streak
(= the changed „deuteropore" of the Anamnia) in Birds and Mam-
mals. In this organ, the headprocess, is to be found in \'many cases
a longitudmal canal, the metenteron or
LiEBERKUHN-canal. In both,
Anamnia and Amniote^, the wall of the metenteron fuses with that
part of the gut-lining, which has originated from cells, formed in
most cases by delamination of the embryonic knob, and called by
me hypoblast for short. A communication between the archenteron,
the cavity enclosed by the hypoblast, and the metenteron is esta-

-ocr page 61-

blished. In Anamnia the posterior part of the gut is formed by a
large part of the metenteron; consequently we can expect that this
IS also the case in Amniotes.

Rabl tries to prove the transition of cells of the headprocess to
the definite gut-lining as indicated above. Against this the following
facts must be objected:

, I . Strahl (1916) is of opinion that these cells in the live embryo,
in which all cells are movable with respect to each other, can pass
as Well into the mesoblast, generally lying in close connection with the
notochord.
Strahl does not prove this, mentions it only against
Rabl\'s view.

. 2 • It is not certain that these cells of the chordal plate take part
in the formation of the gut-lining. Keibel (1916) argues that it is
Very well possible that, whilst the notochord separates from the hypo-
last, these cells move in such a way that they become situated behind
each other, contributing in this manner to the growth in length of
the notochord
(Keibel 1889, 1916, „Scheinwachstum" of the notochord).

As we have observed, Manis has a very broad chordal plate in many
eases, sometimes even with as many nuclei as Rabl describes for the
rabbit. In later stages in
Manis we also find a cylindrical notochord
jn the same region of the embryo, where the broad chordal plate was
® be found. In most cases every section of the notochord consists of
^nch less cells than the sections of the chordal plate (cf figs. 36, 37,
42, plate IV and 59, 60, plate VI). Here a table is given, in
^^nich is indicated the number of nuclei from chordal plate and
notochord in sections through different somites (table 4).

Table 4.

of embryo.

Som. i

Som. iv

Som. viii

64 (8 a 9 som.)..

13

«5

24

(10 a II som.).

\'5

«7

26

53 (\'8 som.).....

7

12

«7

-ocr page 62-

From this table it follov^s that Rabl\'s observations in the rabbit
are corroborated in
Mam\'s. [In connection with this, it is to be noted
that
Keibel described (1889) cross-sections of the notochord with only
two ^ nuclei
{Cavm)]. These observations are not a proof of Rabl\'s
opinion, according to me, for though I do not believe that Strahl\'s
supposition that in the live embryo cells of the chordal plate pass
into the mesoblast, is right, I think that
Keibel\'s opinion is not im-
probable. The data mentioned above may even support it. It is also
in favour of this opinion that in stages in which the notochord is no
longer incorporated in the hypoblast, the cranial and older part always
has a small diameter with few nuclei, while the caudal and younger
part has a large diameter with numerous compactly arranged cells.
It seems as if in this younger part of the notochord, the cells are
not yet situated behind each other.
Keibel\'s „Scheinwachstum" will
still take place here. I believe that consequently the observations in
Manis^ are in favour of Keibel\'s opinion and that Rabl\'s view must
be rejected. It is probable that the formation of the metenteron of
the Anamnia is reduced in the Amniotes to such an extent that the
headprocess no longer participates in the formation of the future ali-
mentary tract. It would even be very difficult to trace elements pas-
sing from the headprocess into the lining of the gut, for it is almost
impossible to ascertain the transition of elements from one organ to
another, when these organs are intimately fused. So I beheve that
the second question (p. 42) must be answered in the negative.

I must still point to some facts, which become clearer, when we
compare the different stages. In doing so it is evident that the primi-
tive streak is a centre of growth, from which the headprocess and also
the chorda-anlage (derived from the headprocess), are formed. We
can distinguish a part of the headprocess, with which the mesoblast
is connected, and a part, where this is the case no longer, the latter
laying more cranially. Both parts move caudalward, just as the chor-
dal canal, formed in it. More cranialward a short transitional part follows
and then the chordal plate extends a great distance, being incorporated
in the hypoblast and thus forming the roof of the future alinientary
tract In stages, in which about 13 pairs of somites are formed, the
chordal plate begms to separate from the hypoblast. Before the chor-
dal plate a fifth part of the chorda-anlage follows, separating from the

-ocr page 63-

hypoblast later on. It is not certain, whether this part is formed by
the so-called chordal plate. ^

The notochord can be divided into a part, formed out of the head-
process (the metenteron of the Anamnia) and a part, lying caudally
of this and formed out of the primitive streak, the homologue of
the changed deuteroporic lips. Perhaps an anterior, accessory part
can be distinguished, but it is not certain, whether this is formed out of
the chordal plate itself, or out of the prochordal plate. When the latter
js the case, and this seems probable, it would be formed from the
lining of the archenteron.

Summary.

In Manis a tongue-shaped organ, the so-called headprocess, develops
out of the cranial part of the primitive streak and grows cranialward
Ween epiblast and hypoblast. The place, where it is formed out
the primitive streak, is generally swollen and is then called

HENSEN-knob.

bla"^^^ headprocess reaches as far as a thickened part of the hypo-
ast, the prochordal plate. The hypoblast, extending beneath the
^ process and primitive streak, is originally equally thick every-
^re, but gradually, before the headprocess fuses with it, the hypo-
ast becomes thinner in the mid-axis. From the sides of the head-
m the gastrai mesoblast is formed, out of which the somites, lying
he cephalic region of the embryo, derive.

he anterior border of the headprocess fuses with the prochordal
and^^ ^^ ^^^ th^ headprocess incorporates in the hypoblast,

the K " ^^^ chordal plate is formed out of it. The incorporation of
^ ? chorda-anlage occurs in
Manis as follows. A lumen, the chor-
panal, appears in the centre. (A
LiEBERKtJHN-canal, observed in the
emf headprocess in other Mammals, I did not find in the
Manis-
s j.^^yos at my disposal). Then the wall, lying beneath the canal,
Pu h ^^^ hypoblast is cleft here, and the chorda-anlage spreads, laterally
cleft the hypoblast, which process is visible, as the edges of the

hypoblast are distinctly compressed,
b ^ parts of the chordal canal can extend along a great distance,
usually they are only traceable in a few sections. They can open

-ocr page 64-

cranially into the cavity enclosed by the hypoblast. The largest and
most distinct canals occur in the mid-axis of the headprocess, but they can
also be observed on the left or on the right, and in many cases, even
on both sides. Generally the lumina, which appear double, are also
traceable along some distance. They are all considered the homologue
of the canalis neurentericus of the Anamnia. The cell-membranes and
nuclei are radially arranged around these canals.

The incorporation of the chorda-anlage in the hypoblast takes place
from before backward. When the chordal plate is entirely incorporated
in the cranial region of the embryo (i.e. when the borders of the
chordal plate pass into the hypoblast, which then does not extend beneath
it) the incorporation still continues more caudalward. Then all trans-
itional stages of this process are traceable in the same embryo.

There are indications of the anterior part of the notochord being
formed by the prochordal plate, a thickened part of the hypoblast.
If this anterior part grew independently out of the chordal plate, already
incorporated in the hypoblast, these observations could be made as well.

Originally the length of the headprocess is not considerable. Later
on it derives from the primitive streak, which shortens from before
backwards. This posterior part also incorporates in the hypoblast. This
is still the case in an embryo with 13 pairs of somites and a 14"^
pair forming. In an embryo with 16 pairs of somites the incorporation
comes to an end, and in an embryo with 18 pairs of somites the
notochord is for the greater part separated from the hypoblast. Then
in the region of only 5 pairs of somites and- in the most anterior part
of the chorda-anlage this is not the case. Still further caudalward a
part of the chorda-anlage, which does not incorporate in the hypoblast,
is formed out of the primitive streak. Here too parts of the chordal
canal are to be found. On comparing younger and older embryos,
we see that the chordal canal moves caudalward, just as the neurenteric
passage of the Anamnia.

In an embryo, possessing 8 pairs of somites and a 9\'" forming,
the gastral mesoblast is still connected with the anterior part of the
chordal plate. In the posterior part of the headprocess the mesoblast is
always connected with it.

In younger stages, in which the chordal plate not yet separates
from the hypoblast, 4 zones of the chorda-anlage are to be found.

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They are from back to front: i°. The part just formed out of the
primitive streak, connected with the mesoblast. 2°. The part, no longer
connected with the mesoblast. These parts are both covered by the
hypoblast; parts of the chordal canal can be observed in them.
3 • A transitional zone. 4\'. The chordal plate incorporated in the hypo-
blast. Later on the chordal plate separates from the hypoblast, the
most anterior part of the chorda-anlage separating still later.

Originally the hypoblast extends as an uninterrupted layer under the
primitive streak. Secondarily in the mid-axis, in the anterior part of
the primitive streak, the hypoblast fuses intimately with the tissue of
the primitive streak. This place moves caudalward, when the primitive
streak shortens from before backward. Laterally epiblast, mesoblast
and hypoblast are distinctly separated in this region of the embryo.

It could not be proved that cells of the headprocess pass into the
hning of the archenteron. It is probable that the cells of the chorda-
anlage place themselves behind each other, contributing in this way
to the growth in length of the notochord. This is probably the expla-
nation of the fact that in young embryos a very broad chordal plate,
consisting of numerous cells, occurs, while later on in the same region

the embryo the diameter of the notochord is small, each section con-
taming few cells. In embryos, in which the chorda has separated from
the hypoblast a long time ago, and lies free between medullary tube and
gnt, the anterior part is always thinner than the posterior part, having
oeen formed out of the primitive streak last. This observation might
also be in favour of this view.

C 4

Kon. Aktid. v. Wetensch. (20 Sectie). Di. XXI.

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THE SO-CALLED PROCESS OF GASTRULATION

IN MAMMALS.

s

Before discussing more fully the so-called process of gastrulation
in Mammals,. I must make some general remarks on gastrulation.
Haeckel, who formulated the idea „gastrula", defined it („Gastraea-
theorie", 1874,
p. 15) as „einen einaxigen ungegliederten hohlen Körper
ohne Anhänge, dessen einfache Höhle (Urdarm) sich an einem Pole
der Axe durch eine Mündung (Urmund) öflnet und dessen Körper-
wand aus zwei Zellenschichten oder Blättern besteht: Entoderm oder
Gastralblatt und Exoderm oder Dermalblatt." The gastrula is formed
in many different ways. The formation of it can take place by means
of invagination, delamination, epiboly etc. A two-layered embryo is
always formed with epiblast on the outside {= exoderm) and hypoblast
on the inside. (= entoderm). Both layers enclose a cavity, which com-
municates with the exterior by an aperture, the blastopore or primitive
mouth Urmund). At present
Haeckel\'s view that the process of
mvagination is the more primitive is still opposed to
Ray Lankester\'s
view that the process of delamination is the more primitivje, which
is nothing but a matter of taste, according to me. The blastopore,
formed by invagination, is the natural consequence of the process of
invagination itself, whilst in the two-layered stage, formed by dela-
mination, the blastopore secondarily opens into the exterior.

In the descriptive part we have seen how in Mam\'s the hypoblast is
formed out of the embryonic knob by delamination and how afterwards
It grows round the inside of the trophoblast and finally forms a closed
sac. The formation of the hypoblast by delamination has been observed
m a great number of Mammals. Lately, however,
(Hill 1910,
Hartman 1919) a somewhat different process is described in Marsu-
pials, viz. „immigration" (p. 17).

In the embryonic knob (epiblast or hypoblast) an opening is not

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early developmental stages of manis javanica desm. 51

to be found in Manis. Yet in the literature on the first developmental
stages in Mammals some cases are communicated, from which it would
evident that in the two-layered germdisc a true blastopore is to

In the first place Selenka (1886) mentions that in blastocysts of

^de/phys vifginiana (in three cases out of eight), he has observed an

opening in both germ-layers. At the inside of the opening a coagu-

a ion is found. Later on, neither opening nor coagulation are visible.

TTK ^^^^ too (1889, p. 53) has found in the rabbit a place, where „ein

bertreten von Zellen aus der oberen in die untere Schicht (scheint)

statt zu finden." Later on, he could never find similar formations.

-Weape (1883) figures somewhat older stages of the mole, in which

j^mesoblast-cells" are developing. A spot is visible, where the hypo-

ast passes into the epiblast. Here there is an indication of a „blastopore",

communication of the blastocyst-cavity with\'the exterior is

o be found. Heape himself considers it to be an early LiEBERKtJHN-

canal. Here the very first anlage of the headprocess is to be observed,

^^ to the right of the „blastopore" (fig. 31) a row of cells is visible,

^Xtending cranialward between epiblast and hypoblast. These cells

re called „mesoblast-cells" by Heape, but evidently they are the

of the headp rocess.

young shields of the dog (1897), Bonnet has found openings,

l^^\'ietrating the epiblast, but .not the hypoblast. According to him, they

j^e not artificial. Bonnet utters his opinion with great reservation,

^ owever, for according to him, it is not at all certain that they can be

n^Pared to the openings, described by Selenka or by Heape.

Ta Hubrecht describes different „blastopores" (1890 Sorex, 1902

and Erinaccas, cf also Hubrecht 1908 and 1912). In Sorex,

ch A ^^"^terior part of the embryo (sliowing amongst others a pro-

^^^oidal plate, a thick layer of epiblast and a distinct primitive streak,

por^\' ^^^^ mesoblast develops), he finds a rudimentary „blasto-

to k\' ^"^te penetrating the germdisc. Usually no „blastopore" is

tion ^^^^^^^^ in Tarsins (1902), but in one case Hubrecht men-

emh ^^^^ pit in front of the primitive streak (consequently, the

^fyo is not a two-layered germdisc). According to Hubrecht this

iQo^^ ^ rudimentary blastopore. Young Erinacei/s-stagcs (Hubrecht

^ 1908, 1912), however, often show a distinct canal in the pos-

c 4*

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terior part of the germdisc. In the figures, sketched by him (1902:
figs. 8 and 9, plate XII, 1908: fig. 53, 1912: figs. 46^ and 46^) only
epiblast and hypoblast are to be seen, the „blastopore" penetrating
both. In the sections of the
Erimiceus-cmhryos (which I have exami-
ned), between epiblast and hypoblast cells are distinctly visible, howe-
ver, which have disappeared as a distinct layer in the reproduction.

Must these cases be considered examples of blastopores, where
hypoblast and epiblast pass into each other.? I do not think so. In
many cases, I take this „blastopore" to be the
lieberkuhn-canal, of
which I hope to explain the theoretical signification in subsequent
pages (p. 53, sqq.). In
Tarsius and Sorex the „blastopore" must
certainly not be regarded as such, the reasons being, 1°. that the stage
of the embryo, in which a great part of the mesoblast, a primitive
streak and a prochordal plate have originated, is too far developed;
2°. that the place of these openings or pits lies just in front of the
primitive streak (in the
HENSEN-knob); 3^ the presence of a head-
process in
Eri/iaceus, which is not reproduced well in the figure of
Hubrecht.

This became evident after a renewed examination of the sections,
present in the
HuBRECHT-Laboratory at Utrecht. Also for Ennaceiis
I think that this opinion holds good. The germdiscs (though Hubrecht
reproduced but two sections with a „blastopore", yet openings occur
in many blastocysts) are much younger, however, than those of
Tarsius
and So}-ex, also described by him. Between epiblast and hypoblast a
row of cells is to be found — not reproduced as a distinct layer —
lying in the embryo, figured in 1902, 1908, and 1912, to the right
of the opening. These cells are the first anlage of the headprocess,
according to me. The primitive streak lies (in the figure) above the
opening, representing the
lieberkuhn-canal, which has developed early
in
Erinaceus. (Perhaps this is due to the very early attachment of the
embryo to the uterine wall.)

Heape\'s figure is to be interpreted in the same way. This inves-
tigator thinks, he has to do with an early „neurenteric canal"
(= LiebeR-
kuhn
-canal). To the right of the opening Heape\'s figure (1883, fig-
31) shows a headprocess, to the left a distinct primitive streak.

Hartman examined (1919)-a great number of young blastocysts
of the opossum
[Dide/phys virginiam), but he never found the ope-

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njng, described bySELENKA in 1886, which according to Hartman
must be considered the place, where a mother-cell of the hypoblast
migrates from the epiblast („immigration"). Consequently, in this case
v^e must reject the view that the opening, described by
Selenka, is
a blastopore.

Finally Keibel and Bonnet are so reserved in their opinions on the
theoretical signification of the openings, described by them that we
need not consider them at greater length.

So it has not at all been proved that a blastopore is found in the
didermic germdisc of Mammals, and it is certain that in many cases
the opening described is a short
lieberkuhn-canal, which has opened
into the blastocyst-cavity. If this is true, Haeckel\'s definition of the
gastrulation is not applicable to Mammals. Hence we cannot speak
of a Mammalian gastrula.

Now we will proceed to a discussion on the formation of the
alimentary tract in Vertebrates. As a starting point the holoblastic ova,
found in Amphibia (Dipnoi, Ganoids and
Petromyzon) are chosen,
these representing phenomena easy to explain, as they are microlecithal.

In Amphibia we observe the following facts in the mam. After the
ormation of the blastula a process begins, called „clivage gastruleen
by Brächet (1902). After this process (the separation of the micro-
"^eres and macromeres by a slit) the macromeres spread over the inside
the blastula-cavity and consequently the wall in this part of the
^ii^hryo then consists of two layers, epiblast and hypoblast
, I\'rom the place, which was always designated by the name blastopore,
f^t must be called deuteropore, an overgrowth and ingrowth of cells
place simultaneously, the result being a cavity, the roof of which
by micromeres. This cavity, formed by invagination, fuses
the lumen, formed out of the blastocoel during „clivage gastru-
Like
Assheton, I will call the latter archenteron, the former
\'l\\\'\'<-\'nteron. Moreover,
Assheton accepts two developmental phase

Protogenesis, during which the archenteron is formed in a radially
^symmetrical larva, and the
deuterogenesis, during which the growth in

^ngth of the embryo is established. 1 v ^n

The archenteron originates directly out of the blastocoel; it then
ses with the metenteron and both form the primitive gut.
^i-om the wall of the metenteron the notochord and a great pare

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of the mesoblast derive. After separation of the notochord from the
metenteron-wall, the latter closes beneath it

In Amphioxm ho^h developmental phases are also present; during
protogenesis a radially symmetrical gastrula is formed by invagination

The hl^^r -d hypoblast enclosing an\\rchLteron

The blastocoel, however, disappears. During deuterogenesis complicated
processes take place (
Cereontaine ,906) with the re^ilt that the arch „-
«ron IS enlarged caudalward by a metenteron. In the bilaterally sym-
metrical larva notochord and mesoblast develop

is L" be\'found\'h?\'"\' \'"5™Wastic ova of the Selachii a blastocoel
is to be found between germdisc and yolk-mass. Here the hypoblast is

formed from the germdisc by delamination. As in most of the Amph bia
n wtch\'t.\'^f^ \'u archenteron. T^wa^

Ind Se? b- ^ " different, however, in Amphibfa

gLtrdin^\'a\'" \'\'y delamination, in theform^ by „clivage

fk t very distinct in SelachH,

viz. the formation of the metenteron by invagination. Here archen-
teron and metenteron also fuse and form the primitive gut The

o itTrrWvC however, foLs the grtat r part

o t. Probably this is ,n connection with the extension ohhe Iwpo-

blast over the bulky yolk-mass. A consequence of this process s ^fthe

formation of the trunk anticipates the formation of the head F om the roof

In Amnio es \'^r If i" the groups discussed

of tt r^iote. ^ ™ disappeared as such. The primitive streak

toric fe of tt; with the deute-

Zph Tah^ n »"d Urmund",

prir^Wve streak nf I the opinion that tlie

iLr- ^ \'Elongated deuteropore of the Anamnia. Later on Balfour
on -dependent of h m expressed the same view. Bonnet ( ^

parrof 1 meslw I" Anamnia a great

fn kit rot: i ^

primitive streak in bntr elongated deuteropore, the

Lute opor flit bv ■ i" of \'he

aeuteroponc lips by invagination. From this organ the notochord and
a considerable part of the mesoblast originate "°tochord ana

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In Reptiles we also see that the hypoblast separates from the
germdisc by delamination during protogenesis. The cavity, situated
between hypoblast and yolk-mass, is called archenteron here too. The
metenteron is formed by invagination. The cell-mass, in which the
metenteron occurs, grows cranialward between epiblast and hypoblast
and is called headprocess. Hence the headprocess is the homologue
of the metenteron-wall of the Anamnia. The metenteron communi-
cates by an aperture with the exterior; it is the rest of the deute-
ropore. In Reptiles archenteron and metenteron fuse: the wall of
the metenteron connects itself with the archenteron-wall (= hypoblast)
and the metenteron opens into the archenteron. In Reptiles too the
notochord is formed out of the roof of the metenteron and a great
part of the mesoblast out of its sides. The definite gut-lining consists
principally of hypoblast. The metenteron-wall forms but a small part
of it; the production of notochord and mesoblast is the chief function
of the metenteron-wall now. Moreover, it is difficult to trace, whether
cells of the metenteron-wall pass
into the lining of the alimentary tract.
. In Birds the metenteron is still more reduced. Though a lumen
sometimes to be observed (generally called canalis neurentericus —
Gasser 1878—), yet in most cases a so-called headprocess without
Inmen is found, extending from the primitive streak in the direction

of the future head-region. The headprocess fuses with the protogenetic

hypoblast, formed from the germdisc by delamination. In Birds it is
^till more difficult to trace, whether cells of the metenteron-wall
participate in the formation of the future gut-hning.

I^eptiles, Birds and Monotremata are all megalecithal forms. Lht
processes above described are about equal in these groups.

The other Mammals, which are microlecithal, (in Marsupials the
yolkbody is extruded in early cleavage-stages, cf
Hill 1910, 191«)
show the same organs and developmental phases.
. Starting from the two-layered developmental stage, we soon see tha ,
^\\the posterior part of
tL germdisc, the primitive streak develops,
is a thickened part of the epiblast, and from which myoblast pro-
^^^^rates. In front we see the hensen-knob and at the posterior bord
the shield the caudal knob. These two parts are the homology
the dorsal and ventral deuteroporic lips, the primitive streak the
^^omologue of the closed elongated deuteropore of the Anamnia.

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true deuteropore the epiblast and hypoblast pass into each other; this
^es not take place, however, in the primitive streak. From the
HENSEN-knob the headprocess grows out in a cranial direction. In
most cases a canal appears in the headprocess. The headprocess must
be considered the homologue of the metenteron-wall of the Anamnia
the lumen m it the metenteron.

The canal in the headprocess was first found by Lieberkuhn in
Mammals 1882) and is still often called
lieberkuhn-canal.

Ljeberkuhn himself designated it by the name chordal canal. Very
often the name neurenteric passage or canalis neurentericus is used.

I^or some time the headprocess extends freely between epiblast and
hypoblast, which has been formed from the embryonic knob by delami-
nation (, immigration" in Marsupials) and encloses the umbilici vesicle,
trom which a part of the gut will derive.

Consequently in Mammals a part of the blastocyst-cavity becomes
the archenteron. \' \'

Then the headprocess fuses with the wall of the archenteron, after
hanng formed mesoblast out of its sides. The chorda-anlage is then
said to have incorporated in the hypoblast: the chordal plate occupies

f P" After that Ihe notochord

separates from , the hypoblast, which closes again beneath it. Probably

he headprocess ,s not participating in the formation of the lining of

^e alimentary tract i„ Mammals (cf the descriptive part pp. 44-46).

hJn the\'metenttJon has

Reotilfra, [ r\'- "" ova of Selachii,

Mono relt \' / "«"\'\'ely -«all; in Mammals, the

wan trrt, r\'\'V \' " " P""\' metente on-

wall takes ,n the formation of the definite gut-lining, becomes less

ventral deuteroporic

Te Z,If TT\'TVi^r\'"\'"\' """ also fLis

one by th activity of which the embryo grows in length.

na L Ttl - generally formed by delami-

In Mimmat rri\'^f^e diffère« groups,
(cf. ÎÎ,
5T-53) ^ does not occur%ither

eentticTndfeJTb t from the proto-

6 m tne deuterogenetic hypoblast (i.e. headprocess) and

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primitive streak in different Mammals. The names given (in the
descriptive part) to the four parts of the mesoblast are partly
topographical terms {cephalic mesoblast, peripheral mesoblast, both
protogenetic) and partly terms in connection with the theoretical
signification of the organs, from which they develop.\'These two deu-
terogenetic mesoblast parts are called
gastral mesoblast, because it origi-
nates from the metenteron-wall and
peristomal mesoblast, because it
IS formed out of the wall around the stoma i.e. deuteropore.

The above survey gives a comparative morphological description
of the same ontogenetical processes in Vertebrates. It most resembles
the opinion of
Assheton, who has given a clear account of it in
1907 and 1909. For some pecuharities I made use of Bonnet\'s view,
which he states in the last edition of his „Lehrbuch".

In proceeding to a concise description of the different opinions of
the authors on the so-called gastrulation-process and on the formation
^ the primitive gut in Mammals, we must mention in the first place
I^auber, who, as early as 1876, homologized the primitive streak of
the Amniotes and the „blastopore" (i.e. deuteropore) of the Anamnia.
Independent of him,
Balfour compares the primitive groove of the
Amniotes to the primitive mouth of the Anamnia.
Van Beneden comes
to agree with this opinion in
1886. He then thinks that the Lieberkuhn-
canal (very marked in Fespertilio murinus) is homologous with the
"§astrula"in-growth of the Amphibia. Consequently he calls the
Lie-
i^EHKuHN-canal archenteron. , ., ,

At that time it was already known that the hypoblast, which
participates in the formation of the gut-lining, originates from the
jmbryonic knob by delamination before the invagination of the archen-
It was thought that the principal part of the gut-epithelium
formed by „precocious segregation", though a pait of it was

\'"Paginated (this being indicated in the name archenteron). The early

of the hypoblast was connected with the increase in yolk in
Amniotes and with the secondary loss of it in Mammals.

Gradually manv investigators began to say that the gastrulation ot
Vertebrates (especially of
the Amniotes) occurs in two phases. In

first phase the hypoblast is formed by delamination, in the second

the material for the notochord and mesoblast finds its way into

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the interior of the embryo. So later on, two sharply opposed opinions
are found side by side. The one holds that the invagination of the
metenteron is the most important process of gastrulation
(Rabl,
SoBOTTA, Greil), the other that the gastrula of the Mammals is the
two-layered germdisc and that the invagination process has nothing to
do with gastrulation
(Hubrecht, Keibel).

Formerly [Hubrecht and Keibel accepted a gastrulation in two
phases in Mammals, called caenogenetic and palingenetic phase by the
former and first and second gastrulation-phase by the latter.

In 1902, however Hubrecht changed his point of view, which
he explained more fully in 1905. From that time onwards he thinks
that only the first phase is gastrulation and that the second phase has
nothing to do with it. The second phase is a process, to which he
gives the name of notogenesis in connection with
Lwoff\'s opinion
(1894). Only in
A?nphioxiis the gastrula is formed by invagination,
in all other Vertebrates by delamination. When the delamination-
gastrula is formed i.e. when „an intestinal entoderm is differentiated
as against an integumentary ectoderm"
(Hubrecht 1905, p. 408) the
second phase of development begins. The gastrulation (= cephalo-
genesis) is followed by a process, which starts with the formation of
notochord and mesoblast (notogenesis). The radially symmetrical larva
has become bilaterally symmetrical.

Properly speaking, the terms cephalogenesis and notogenesis do not
express what happens in reality. As
Hubrecht himself remarks, only
the most anterior portion of the head is formed during cephalogenesis
and moreover trunk-segments enter into the composition of the head.
For that reason, I have used in the above
Assheton\'s terms protoge-
nesis and deuterogenesis, which are almost identical with
Hubrecht\'s
cephalo- and notogenesis.

Like Hubrecht, Keibel too accepts (since 1905) a definition ofgas-
^iilation applicable to the Invertebrates as well as to the Vertebrates.
This definition however, does not agree with the definition of
Haeckel
^74, who defines the gastrula as a didermic stage with a blastopore.
Hubrecht s definition includes all ways in which the inner layer,
the hypoblast, is formed (1905, p. 408).

Hubrecht does not indicate the lieberkuhn-canal and the head-
process by the names formerly usually used. The primitive streak does

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not represent a changed blastopore (in the old sense) of the Anamnia.
The stomodaeum and the oral slit of the Actinia correspond, accor-
dmg to
Hubrecht, to the notochord and the primitive groove of the
Amniotes
(Hubrecht 1905, p. 412).

We have seen (pp. 51, 52) that Hubrecht thought" to have found a
blastopore in several Mammals, investigated by him, and that it is my
opinion that this opening is not a real blastopore, but that it is the
inetenteron-lumen (lieberktihn-canal), which has opened into the
archenteron and is in communication with the exterior. Consequently,
bis „blastopore" is not a „cephalogenetic" but a „notogenetic" formation.

The view, elaborated by Assheton (1894/^, 1905, 1909) I have
treated for the greater part in preceding pages (pp.
53—57)- Here
I will mention only the principal facts, especially in connection with
Hubrecht\'s theory. Since 1894 Assheton accepts two growth-centra

the embryo (primary and secondary growth-centra, 1894^). Later
on he distinguished (1905), as well as
Hubrecht, two developmental
phases, protogenesis and deuterogenesis, which, however, are not iden-
tical with cephalo- and notogenesis: during protogenesis the anterior
part of the embryo is formed. The embryo now is in the „gastrula"-
stage, according to
Assheton and has a radially symmetrical shape.
In his phylogenetical speculations
Hubrecht supposes that tlieaboral
side of this gastrula-stage (botli
Hubrecht and Asshetom think, in this
connecton, of the gastrula of the Actinia with a blastopore) forms the
J^ntral side of the older embryo, and that the oral side forms die
dorsal side of it (hence
Hubrecht\'s term notogenesis). According
to
Assheton, however, — who came to his conclusions by actual

experimental observations — the aboral side of the gastrula becomes
the a - - -

- xiiiu-iiAib ui inc gastrula (to compare ......... ^ y -r- ■

. Inuring protogenesis the archenteron-wall is formed, either by inva-
gination
{Amphioxus) or by delamination (most other Vertebrates).
^Juring deuterogenesis the metenteron-wall is formed, in what way is
no great importance (by epiboly, invagination and so-called „m-

«exion" in Amphioxus, according to Cerfontaine (1906), overgrowth

.^»id ingrowth of cells in Selachii, Amphibia and Amniotes). It is ot
\'^Portance that by the primary growth-centrum a radially symme-

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trical larva is formed (protogenesis), becoming bilaterally symmetrical
in a later stage (deuterogenesis). Archenteron and metenteron always
fuse. From the roof of the metenteron the notochord originates, from
the sides and also from the „blastopore"-lips the mesoblast is formed
for the greater • part. The metenteron gradually reduces in size in
„higher" Vertebrates, the wall of the metenteron and the headprocess
are homologous organs. According to
Assheton, a considerable part
of the gut, including the pharynx, and certainly the heart, are pro-
togenetic (1909, p. 244). The notochord is deuterogenetic.
Assheton
is of opinion that no accessory protogenetic part of the notochord is
formed, which is the case according to
Hubrecht, Bonnet and Heape.

Bonnet (here I communicate the view out of his „Lehrbuch der
Entwicklungsgeschichte", 1920) to explain the gastrulation and the
formation of the germinal layers in Amniotes, starts from the same
processes in Amphibia.

Gastrulation is never an independent process. It is always connected
with other processes, especially with the formation of the notochord
and a considerable part of the mesoblast.

In the microlecithal ova of the Amphibia (e.g. Ra/m, Bufo, and
Salamandra) a part of the blastocoel fuses with that part of the future
gut, which is formed by invagination from the „blastopore". The part
of the gut, derived from the blastocoel, he calls „Ergiinzungshohle",
the invaginated part of the gut „Urdarm" (= primitive gut or
archenteron).

The wall of the gut is composed of two parts: primitive or „Prot-
entoblast", and „Dotterentoblast" or lining of the „Ergiinzungshohle".
The thickened part between both components of the gut is the „Er-
ganzungsplatte". From the wall of the „Urdarm" the notochord and
a part of the mesoblast (gastral mesoblast) originate. Consequently,
the formation of the gut-Hning, often called gastrulation, is not a
simple process.
Bonnet distinguishes Assheton\'s archenteron and
metenteron, but he calls the former „Erganzungshohle" and the
latter „Urdarm".
Bonnet is of opinion that his „Urdarm" is the oldest
part of the gut; according to
Assheton this part is formed later,
however. I believe that
Assheton is right: the part of the gut
formed by „clivage gastrulden"
(Bonnet does not mention this name,
though the process, described by him, is identical with it) develops

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first and must be called archenteron for that reason. Later on the

metenteron opens into the archenteron.

The gastrulation-process, as we have seen it in Amphibia, so
Bonnet continues, is greatly reduced in Amniotes („bis zur Unkennt-
lichkeit verwischt".
Bonnet 1920, p. 93)- This is in connection widi
the increase of volk in Sauropsida and the secondary loss of yolk in
Mammals. The „Dotterentoblast" is formed in an early stage by „pre--
cocious segregation". This is a phenomenon, which must be called
heterochrony, often to be observed in the ontogeny. Hence
Bonnet
considers the formation of both „Dotterentoblast" and „1 rotentoblast
to belong to the process of gastrulation. In Amphibia both forma-
tions occur almost simultaneously. Because
Bonnet calls the archen-
teron of
Assheton „Erganzungshohle", it is evident that he considers
the formation of the gut-part by invagination the most important
phenomenon.

Bonnet then describes in Amniotes the same processes we have
already mentioned in preceding pages. A few remarks must stdl be
made. In Amniotes the anterior part of the „archenteron -wall -
metenteron-wall of
Assheton) fuses with a thickened part ot the
^dotterentoblast", the „Erganzungsplatte". (It is the «ii^^y^^f ^
we have called in the descriptive part prochordal plate.) The „1 otte
hlatt" of the Reptiles and ^\'rds is
homologrzed by hnn with all ^
makromeres of the Amphibian blastula. The primitive ^trf^ is con-
sidered to be the
strongly elongated An^^hibian „blastopore , »

the sides are tused intimLely. In Amphibia already a lineai bto^^^
IS to be found. The primitive streak, however, is

pore", for in a real ,:blastopore" epiblast and hypoblast pass into each

«ther and in a primitive streak this is not -the case.
, In Mammals, having secondarily become yolk-less, an „Erganzungs
^lohle" and an „Urdarm" are also to be found and fuse
^ I^Rachet (1902 and later on) distinguishes m

lation-process\\ divided into two separate phases. His ,,chvag ga^t u^
^s the first phase, the wall of the cavity then formed is cm^
of makromi-es. This part of the future gut is the «fme A
sshet^
called archenteron. A „second temps de gastrulation
-livage gastruLen". The blastopore, vimial ^^P in

The „second temps de la gastrulation" is a complicated p

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which an overgrowth and ingrowth oi cells take place. It is to be
noted that the part of the gut formed in the second phase is called
Archcnteron by Brächet. For that reason Assheton\'s name meftn-
teron is to be preferred. Brächet and Assheton are of about the
same opinion, but they name the organs differently.

In 1915 C. Rabl published an elaborate paper on: „Edouard van
•Beneden
und der gegenwärtige Stand der wichtigsten von ihm be-
handelten Probleme". Besides giving a critical discussion of the different
investigations of
van Beneden, he also criticizes the theories of Hu-
brecht, Keibel, Brächet
and others, and gives own investigations
of the development of
Hatferia and the rabbit, after the process of
gastrulation in Vertebrates has been treated.

After\'the year 1886 van Beneden is of opinion that the LiEber-
kühn
-canal in Mammals is homologous with the gut of the Amphibia,
formed by invagination. The protogenetic hypoblast of
Assheton is
called lecithophore by him and compared to the paraderm in Reptiles
(v.
Kupffer, or the „Dotterentoblast" of Bonnet). Both layers of the
didermic stage of the Amniotes cannot be homologized with the
„ectoderm" and „entoderm" of
Amphioxus, for the formation of these
two layers takes place before the process of gastrulation. For that reason
the names blastophore and lecithophore are used. The headprocess
(= archenteron in
van Beneden\'s sense) fusing with the lecithophore,
the latter does not exist as such any longer and then both parts of
the pt-hning are called hypoblast by him. The early formation of the
lecithophore is considered by
van Beneden and others to be connected
with the early attachment of the embryo to the uterine wall. The
lecithophore becomes a nutritive organ and because of this a differentia-
tion of the hypoblast in lecithophore and archenteron-wall takes place.

After the disctission of van Beneden\'s investigations, Rabl, agreeing
with
van Beneden on the whole, proceeds to the description of the
different theories of gastrulation in Vertebrates, principally in Mammals.
Especially with
Hubrecht\'s and Keibel\'s opinion he cannot at all
agree. In the first place he criticizes
Hubrecht\'s definition of gastru-
lation
(Hubrecht 1905, p. 408 and antea p. 58). According to Rabl
gastrulation is not a process of differentiation, but a process of growth,
(Kabl 1915, p. 230: „dass also die Gastrulation in erster Linie nicht
em Differenzierungs-, sondern ein Wachstumsprozess ist."), because

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it follows from the „cell-lineage" that the lining of the gut is to be

found in much earlier stages.

Apart from this, Rabl\'s greatest objection is that according to
Hubrecht and Keibel, the gastrulation comes to an end, when the
hypoblast has been formed by delamination.
Rabl thinks that then
the formation of the gut-lining has not quite stopped in Mammals.
During the process of invagination not only material for notochord
and mesoblast finds its way into the interior of the embryo, but also
material for the gut-lining is invaginated. During the incorporation
of the headprocess in the hypoblast (= lecithophore of
van BenedEn),
cells of the headprocess pass into the hypoblast. We have seen (pp.
44—46) in what manner
Rabl, who even thinks that the greater
part of the gut-lining derives from the headprocess, tries to prove
this, and moreover, how
Strahl (1916) and Keibel (1916, p. 9)
refute his arguments. . ,

When Rabl asks Hubrecht (1915^ P- ^55) on what grounds he proves
that the gut-lining is derived from the hypoblast, formed hy delami-
nation of the embryonic knob, this question can be answered by the
following question: What conclusive proof does
Rabl give ot h^is
opinion that such an important part of the gut-lining origmates from t^he
headprocess.? It is true that
Hubrecht never asked himself, whether
^ells of the gut-epithelium are formed out of the headprocess, but
that such an important part of the lining of the alimentary trac in
Mammals arises from the headprocess, as
Rabl will have it, is alto-
gether improbable. , . . r^.

Keibel (1016 and earlier) admits that, besides the formaUon of the
notochord and mesoblast, also a small part of the gnt-hning migh
^^enve from the headprocess, but he thinks it more probable a
this is no longer- the case in Mammals. We have seen (PP- 45> 4^)
J^Eibel explains, by his so-called „Scheinwachstum , the fact t ia
younger stages the chordal plate
is conn^osed of many cells, a d ^^^
«Ider stages tht notochord confains, in
the same region of the embryo
^^^y few .cells.
Hubrecht makes a sharp distinction betw^i^
processes of gastrulation and of mesoblast- and "«to^^^^^f^, ~
The incorpomtion of the chorda-anlage in the hypoblast and the s pa^
••^tion of the notochord from it later on, is
never interpreted theoreti

\'\'^"y by Hubrecht.

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To summarize, we can say that Rabl considers the process of
mvagmation of the headprocess the process of gastrulation in Mam-
mals. The early development of the hypoblast in Mammals is
explamed by him as follows. In Sauropsida, where the yolk-mass of

the ovum has increased enormously, a part of the gut-lining is necessary

tor the nutritive function. For that reason it is formed earher than the
rest ot the hypoblast. In Mammals, which have become yolk-less
apm a remnant of the former state is still to be found, thfe hypo-
blast developing prematurely here.. Against the view of
Rabl it can
be objected that he has not traced the formation of the gut in all
groups of Vertebrates. In doing so it is evident that in all Vertebrates,
in megalecithal as well as in microlecithal or yolk-less groups the
alimentary tract is formed in two parts, the archenteron and metenteron.
1 will be concise in discussing the opinions of other investigators

rruu"^ uT \'\' ^ supporter of the „invagination-theory".

The hypoblast called subgerminal layer by him, is not homologous
with the hypoblast of
Amphioxus, formed by invagination. The cells
of the germdisc of the meroblastic Amniotes, divided by him into a
germinal and a subgerminal layer and a subgerminal syncytium, arise
m connection with the yolk-mass. „Was wir bis jetzt erörtert haben,
vollzieht sich mi Blastulazustände und umfasst Episoden des Ringens
des Keimes mit seinem Dotterballaste", (p. 228).

wil^\'.K^\'f ^\'""uf \'\' r\'\' g"t-wall („Urdarm"), homologous

with the hypoblast of Amphioxus, its formation is the gastrulation.

Later on, when the , Urdarm" is reduced in size, the subgerminal layer
akes the place of the „Urdann"-wall as gut-lining. The Mammals
a e derived f^m the Sauropsida by loss of yolk. We see that
Greil
differs from Rabl only in details. According to him the gastrulation
IS also the invagination of the headprocess.

SOBOTTA, who Ijas investigated the mouse especially, expresses hiin-

IZ rz (^i- ^9-). So also Jen-

knson (1906, 1913): the paraderm .(= hypoblast) is formed by

^^ ofthe arcl J

fr?rL Who has investigated MegMa-

Hrli ! conclusion, differing little from that of

Hubrecht, Brächet and Assheton. \'

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ScHLATER (1907, 1909) supports Hubrecht\'s vicw on the ground
of theoretical considerations and agrees also with his phylogenetical
speculations (to compare the comparative part, p.
76).

Other investigators I will pass over in silence, except Triepel (1916,
1917\'^),
who holds an opinion, keeping the mean between
Hubrecht\'s and Rabl\'s views. All Metazoa pass through a morula-,
blastula- and gastrula-stage. In Vertebrates also a chordula-stage must
be distinguished, (according to Schlater). The gastrula is the „die
erste Darmanlage enthaltende(n) Keim der Metazoa"
(1917^?; p. 288).
It is of less importance, whether the
gastrula develops by invagination,
epiboly or delamination. „Die Chordula is durch das Auftreten des
die Chorda-anlage enthaltenden Mesoblasts charakteresiert". Because
Primates (and in
Tatusia, Patterson 1912) a part of the mesoblast
arises very early in front of the axial organs, the chordula-stage is
defined by him as „den eine Chorda-anlage zeigenden dreischichtigen
Keim der Chordaten". In
Amphioxus he distinguishes an „entodermal
ehordulation, in other Vertebrates an „ectodermal" chordulation. Whe-
ther this distinction is correct or not, I will not decide (cf
Cerfon-
Taine 1906). The hypoblast, perhaps participating in the formation
of the anterior part of the notochord (cf pp. 42—44) an „ectodermal-
cntodermal" chordulation must be distinguished in Craniota to be
correct. During the „ectodermal" chordulation, cells, which will form
\\Part of the intestinal epithelium, find their way into the interior
of the embryo. These cells are called „Urentoderm" by him. ^

^any facts are in flivour of this opinion. It agrees s^ obviously
^ith Assheton\'s opinion, however, that the greatest diff-erence is a
difference of terminology. For gastrulation and protogenesis, chor-

"^tdation and deuteiwenesis are identical conceptions. As Strahl (19löj

[^"^arks, the gastrulation-problem is partly a problem of denomination.

due to this that so many investigators, misunderstanding each
^f^cr\'s terms, come to totally different conclusions, though the facts
Observed by them differ slightly. . ,

, Summarizing the above tlieories and opinions, we see in the tiist
P^^^e that Hubrecht and Keibel, Assheton and Brächet in the mam
agree. These investigators accept two developmental phases

Hübrecht makes a too sharp distinction between „cephalogenes s
»notogenesis", according to me. He does not point
to the fact that

Kon. Akad. v. Wetensch. (2c Sectie). D). XXI. ^

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the headprocess (notogenetic) fuses with the cephalogenetic hypoblast
and that this must have a theoretical signification. Moreover, the terms
cephalogenesis and notogenesis are not quite correct (cf
p. 58). As
we must accept
Haeckel\'s definition of gastrulation, Hubrecht\'s
definition cannot be right, because his definition does not mention \'a
blastopore. The opening, described by
Hubrecht as a real blastopore
in different Mammals, is not a cephalogenetic, but a notogenetic
formation, according to me.

Keibel, who agrees with Hubrecht in the main, also excludes
the formation of the notochord and mesoblast from the gastrulation.
It IS possible, however, that cells of the headprocess pass into the gut-
linmg, though It is very difficult to trace, whether this is the case
m Mammals. The „Scheinwachstum" of the notochord is, in connec-
tion with the formation of the notochord out of the chorda-anlage,
more probable.
Keibel does not make such a sharp distinction between
cephalogenesis and notogenesis.

I think that Assheton\'s theory, founded on a comparative morpho-
logical investigation of the formation of the gut, notochord and
mesoblast in all Vertebrates, is the most logical. To both developmental
phases he gives names, indicating only that the one comes about
earher than the other
[proto- and deutero-gtxxt^x^). During protogenesis
the archenteron, during deuterogenesis the metenteron develops. Both
fuse in
all Vertebrates, which explains the incorporation of the head-
process in the hypoblast in Mammals. The metenteron, easy to dis-
tinguish in Anamnia, decreases in size in Amniotes, the archenteron
becoming more distinct in the latter. The theoretical signification of
the primitive streak, headprocess and
lieberkühn-canal is clear, accor-
ding to his opinion. For that reason I agree with Assheton.

Brächet, who investigated especially Anamnia and whose opinions
cannot be treated in detail, because of that, accepts the same deve-
lopmental phases. By „clivage gastrul6en" the acrogenetic part of the
embryo ,s formed. After that, during cephalogenesis, the head is
rormed as far as the nervus vagus, and in the third place
Brächet
^stinguishes a cormogenesis, during which trunk and tail are formed,
me part of the gut, opening into the exterior by the\'„blastopore",
is the archenteron, according to
Brächet. De Selys Longchamps calls
It deutenteron in
Petromyzon, however (1910).

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De Lange too distinguishes a cephalo- or protogenesis, a somato-
or deuterogenesis, and moreover an uro- or tritogenesis.
In the first
phase, the head is formed as far as the auditory vesicle, the para-
chordal mesoblast excepted. In the second phase the anterior part
of the trunk, and the parachordal mesoblast of the head is develo-
ped, and in the third phase the other parts of trunk and tail are

formed.

A distinct survey of the different opinions of Hubrecht, Brächet,
AsshetoxN
and de Lange is to be found in de Lange (1912) and
I^Rachet (1914).

Though the data, given by Bonnet, agree with the above, he
does not mention two different developmental phases in the embryo.
The formation of both „Protentoblast" and „Dotterentoblast" belongs
to the gastrulation, though the „Dotterentoblast" is formed in an
earlier stage by precocious
segregation. This occurs in connection
^jth the volk-increase in Sauropsida and the secondary yolk-loss in
^lammals.\' The invagination, the formation of the headprocess, is
the most important process
of gastrulation. According to me. Bonnet
fttaches too little importance to the flict that tlie „Dotterentoblast
hypoblast) is formed so early in a// \\^ertehrates.
Bonnet, in his ex-
planation of the facts, agrees witli the so-called invagination gastrula-
or »Urdarm"-theory, of which Rabl and van Beneden m;e the chief
supporters. The invagination of the headprocess is, according to this-

"»eory, the gastrulation-process. ^ , 1

, According to Rabl, the liypoblast derived from tl>e embryon c
\'\'"Ob by delamination, even takes an inconsiderable part in the toi-
"Wtion of tlie gut-lining of the Mammals. (Jastrulation ■•\'"\'1 >»«\'8\'-
"Mion are identical. Conseu uently the opinions of Hubrecht an

sliarply opposed. Rarl attaches little importance to the proto-
g\'>etic liypobhtst in connection with the gastrulation; "««he «.
"\'"\'
HKct cull attention to the headprocess and the \'"»rpo u o„ ot
•he cliorda-anlage in, and the separation of the notochord fr^i I t
•^yPoMast.
Ra„l giv^s an explamition of the fact that the choid,.-
""\'^ge incorporates in the hypoblast (p. 44)- , ,,

, S\'arting ffom the definition of Hakck,a, «e l"ve ^een hat
applicable to none of the processes, which take place dunng the
"\'«ogeny of the \\\'ertebrates. ,

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EARLY DEVELOPMENTAL STAGES OF MANIS JAVANICA DESM.

The invagination of the metenteron is no gastrulation, because it
takes place in a two-layered stage, already existing.

The process of delamination, by which the future wall of the
archenteron is formed,-is no gastrulation either, because I believe
that no primitive mouth is formed. Only in
Amphioxus amongst
Vertebrates an invagination-gastrula is found. Consequently a process
of gastrulation does not take place in Craniotes.

Protogenesis, cephalogenesis and acrogenesis are not identical with
gastrulation.

We have also seen that the gut of the Vertebrates consists of a
protogenetic (archenteron) and a deuterogenetic part (metenteron).

In „lower" Vertebrates the formation of the metenteron is most
important, with regard to the formation of the alimentary tract.
In Amniotes the most important part is formed by delamination
(archenteron). In Mammals the last process is so momentous that during
the invagination of the headprocess probably no cells ofthe gut-lining
hnd their way into the interior of the embryo.

68

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COMPARATIVE PART.

The number of investigations on the first developmental stages of
Mammals having greatly increased, it is impossible to give a com-
plete survey of the literature existing. This is not necessary either,
as here I will only treat facts in connection with the investig tions,
above described.

I intend to give a survey arranged as much as possible aftei* the
organs. Only the more recent investigators will be treated in detail,
in order to be concise. In this part the hypoblast is not mentioned
because it has been discussed fully in the chapters on the origin ot
tbe hypoblast and the gastrulation of the Mammals. What must be
said about the primitive streak, is to be found in the descriptive and
Comparative parts on the development of the notochord.

I. Earfy segmentation-stages.

I" 1875 Ed. van Beneden investigated the segmentation of
^be Mammalian ovum in detail. It was Iready known that this
ovum is holoblastic. Van Beneden now found that the two tirs
blastomeres have a different size in the rabbit and supposed that
^ le epiblast and hypoblast become separated from each otiier, when
tbe two first cleavaLre-cells arise. Besides differing in size the blasto-
are differently stained. The „epiblast\'-cell, the larger one
before the „hypoblast"-cell and consequently a three-cell stage
often to be found (to compare also
Van Beneden et Julin 1880
the bat). In the four-cell stage the line connecting the c n ^
^^ the „epiblast"-cells is at right angles to the hne, connecting t e
of the „hypoblast"-cells. Then a change takes plac , ^
\'\'bypoblasf\'-cell pZsses into the centre, surrounded on on.

»epiblast"- and on the other by three „hypoblast -cells. Hie

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segmentation of the „epiblast"-cells anticipating the segmentation of
the „hypoblast"-cells, later on a so-called
metagastriila is formed,
differing from the real gastrula in this only that a lumen is not to
be found. The place, where the „hypoblast"-cells lie superficially was
supposed to be the blastopore.

Later on van Beneden (v. Beneden et Julin 1884), in consequence
of
Koellikers investigations (e. g. 1882), came to the conclusion that
in such early stages the hypoblast could not be present as a germ-layer
and that a blastopore could not be found either. Then the theoretical
signification of the metagastrula was rejected by him.

Heape, however, found that the segmentation of the ovum of the
mole is irregular.
A „metagastrula" is present, but the layers, com-
posing it, are not derived from two different cleavage-cells. Out of
the interior layer epi- and hypoblast are stipposed to develop.

Differences in size and staining in the blastomeres could not be
found in the rabbit, sheep, and pig by
Assheton (1894^, 1898^,
1898^), in the hedge-hog by Keibel (1888) and mouse by Tapani
(1889)
and SoBOTTA (1895).

Sobotta explained the phenomenon often observed that one of
the two cleavage-cells is larger than the other. He found that a
blastomere about to divide, increases in size, that its plasma is then
less deeply stained and that the larger cell cleaves before the other.
For that reason three-cell stages are often to be found [to compare
also
KuNSEMtiLLER (1906, E/vz/^m/j-) and Hartman {\\c)\\c)j:>ide/p/iys).

In Mafus .a distinct difference in size of the cleavage-cells is not
present (figs. 2, 3 and 4, plate 1), but I have found a three-cell
stage in the oviduct.

The young Mams-ovTi are all covered by follicle-cells. This is
also described for the mole (
Heape 1883, mature eggs); Sobotta
found (1895) that the ovum of the mouse passing into the oviduct,
is still surrounded by the corona radiata. In two-cell stages these
cells have disappeared.
Huber observed (1915) in the rat unsegmented
ova in the oviduct, surrounded by a few follicle-cells, and about
the same is found in the ferret (
Robinson 1903) and hedgehog
(Keibel 1888 and Kunsemuller 1906). In Mams the cells of the
corona radiata surround the ovum for a considerable time; the cells
of the discus proligerus are to be observed in an unsegmented ovum-

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In Galeopithecus too, these cells can be distinguished. In the descriptive
part
I have already mentioned that Hubrecht (i910, 1919) supposed
that the nuclei of them were future trophoblastic nuclei (cf the
descriptive part pp. 12—14).

„ R. Hertwig mentions (1903) that in many cases the zona pellu-
cida still surrounds the ovum, after the cells of the corona radiata
have disappeared. All kinds of differences can be observed. Now the
zona surrounds the unsegmented egg only
{Sorex, Tupaja, Hubrecht
1890 and 1895), now this is the case only in very early stages
(Tarsius, Hubrecht 1902) or not at all. In Manis I could not find
a zona pellucida anywhere. Some authors think (for example van
Beneden) that acids dissolve the zona pellucida, but this was not
confirmed by others
(Sobotta). The M.z;;/>-embryos were preserved-

in picro-sulphuric acid.

The stage, in which the young embryo passes into the uterus, is

also very different. Hubrecht (1902) found with

cells still in the oviduct, whilst Keibel (1888) observed two-cell stages
of
Erinaceus close to the uterus. The three- and four-cell stages o
^anis lie in the oviduct. The oviducts, being sectioned in
1907 and
earlier, are only kept partly; so I cannot trace in what part ot the

oviduct these ova are to be found.

The succeeding Manis-stagc, in whicli trophoblast and embryonic

l^nob are developed, is to be found in the uterus.

II. Trophoblast and embryonic knob.

Bischoff already observed the difference between the hlast^
^all and embryonic knob (1842-1854) and traced the tieveio).
tl>e compact mnlberry-stage to the hollow blastocyst.
I" the years between ,875 -a 1884 n dispute

BenkdL and Koe.uker, principally concernnig the nit

f the embryonic knob. \\\'an Beneden j\' \' ^ f h^

•he outer layL- ofthe embryonic knob Ivi the

the inner layer is the hypoblast (visible already the meta
g^«rula) and that the tis,sue between these layers must be consideied

IndepSently, RAtmER (.875) \'^nt the outer layer (called

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rauber-layer after him) disappears later on and consequently does
not participate in the formation of the embryonic epiblast. It is a
part of the trophoblast. The investigations of
Koelliker (1879, 1882)
proved that
van Beneden was wrong. The layer below the Rauber-
layer is the embryonic epiblast, and not the mesoblast, this arising in
a later stage.
Van Beneden came to agree with this opinion in 1884
(van Beneden et Julin 1884).

The blastocyst with its inner mass of cells is found in all kinds
of Mammals.
Hubrecht called the part of the blastocyst, not parti-
cipating in the formation of the embryo, trophoblast (1888) and the
inner cell-mass embryonic knob (1890).

Hubrecht (1902, 1908) distinguishes the trophoblast and the
embryonic knob in the first segmentation-stages, principally by dif-
ferent reactions against staining reagents
{Tupaja, 1895) or different
position of the cleavage-cells. He interprets a figure given by
Selenka in the following way (Hubrecht 1908, plate A, fig. 4): he
regards the central cell as the mother-cell of the embryonic knob,
and the peripheral cells as the trophoblastic cells.
Hill, however, has
shown (1918) that the larger cells (the central cell too) are tropho-
blastic cells and that the smaller ones are the mother-cells of the
embryonic knob. Consequently in Marsupials the trophoblast never
covers the embryonic anlage.

According to Rauber and Koelliker the cells of the rauber-layer
disappear, whereas
Heape {Talpa, 1883) and Balfour (rabbit, 1881
p. 220) hold that these cells are absorbed by the definite epiblast.

O. Hertwig (1903) feels inclined to compare the embryonic knob
to the germdisc of the macrolecithal eggs of the Monotremata,
Birds and Reptiles, the trophoblast then being homologous with the
unsegmented part of these ova.

The usual opinion that the trophoblast is epiblastic, is denied by
Assheton (1898^7). He thinks that the trophoblast is of hypoblastic
origin. Like
van Beneden (1899/^), he accepts two different layens
m the morula, but the part
van Beneden calls epiblast is called
hypoblast by
Assheton and the reverse. A clear survey of the
opinions of
Assheton and of van Beneden (and also of Hubrecht\'s
opinion before 1910) is to be found in Assheton 1908, fig. n, i2
and
13. Hubrecht\'s last opinion I have treated on pp. 12—14.

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III. Mesoblast.

•-X

We have seen that in Manis the mesoblast originates from several
places of the embryonic shield viz. partly out of the primitive streak
and headprocess, partly out of the hypoblast.

All investigators agree about the origin of the mesoblast from the
primitive streak and headprocess in Mammals. Different names are
used for these two mesoblast-parts, which can be united under the
name axial mesoblast. The fact that so many names are used, is due
to the different theoretical significations attached to the primiUve streak
and headprocess by the authors. To give an example:
Hubrecht
(1908) calls the mesoblast, derived from the primitive streak, ventra
mesoblast,
Rabl (1889, 1915) and Bonnet (1920) call it peristomal
mesoblast to express that this mesoblast-part is formed around the
stoma (= the elongated blastopore, with which the primitive streak

can be homologized). , . 1 .1

Opinions, however, differ very mod, about the question, whe he.
tWs mesoblast is partly formed out of tlie hypoblast in Matiimals.

Tsukaguchi (.912) gives a survey of the investigators, who deny
\'hat it is formal out of the hypoblast and are of opinion that the
mesoblast (tlie headprocess is also considered by

\'igators to belong [o the mesoblast) is formed exclusively from the

does not agree with different other authors, who tl,ink that
a part of the mesoblast can develop from the protogenetic hypoblast^
The protogenetic mesoblast is-formed in different Mammals Th
two parts of the hypoblast participating in its formation are
prochordal plate and tlie annular zone of proliferation
^ The procUrdal plate [protocliordal (H™!\'\'\'-™^)]
by
Hubrkcht in L™ (1890). According to him

tlie notochord is derived from it. Later on «""kecht 1 o fig«
the plate in
Tarsias (1902) and mentioned it in 0

^AliMEisTKR {1913) did not find it), »V/, W
(>908). In ses the origin of the \'-\'ephal-c mesoblas from tte

plate is evident. Usually it arises earlier than -

the plate in primitive streak-stages, hut where » ^mct cepha
"»esoblast-formation takes place in an embryo, in which .15 1\'

-ocr page 90-

of somites have developed. In Gaieopithecus too, Hubrecht has
described (1919) a protochordal plate. Probably ^the part, described
as such by
Hubrecht, belongs to the annular zone of proliferation,
however, the protochordal plate being present in a more cranial part
of the hypoblast (cf the remark of
de Lange in Hubrecht 1919,
p. 18). By
Bonnet the prochordal plate is described under the name
„Ergänzungsplatte".

The mesoblast described by Heape in the mole (1883), by Selenka
(1882) in the mouse and by Robinson (1892) in the mouse and
rat, as originating from the hypoblast, probably is cephalic meso-
blast too. The correctness of these observations, however, is often
called in question. In the mouse and the rat recent investigators
as
Sobotta (191 i) and Huber (1915) could not confirm it.

Assheton (1894^) observed that the cephalic mesoblast develops
from a thickened part of the hypoblast in the rabbit.
Newman and
Patterson ( i 9 i o) describe a protochordal plate, which gives rise to
the cephalic mesoblast in
Tatusia. Not only in Mammals the cephalic
mesoblast is observed. To give some examples:
Balfour and Deighton
(1882) observed it in the chick; Brächet (1914) in Chrysemys and
de Lange (1913) in Megalobatrachus. Moreover Brächet and de
Lange
distinguish a prochordal plate (plaque prochordal).

Many authors, however, deny the existence of a prochordal plate,
participating in the formation of the cephalic mesoblast. According
to
Rabl (1915, p. 227) this plate is „Teil der unteren Keimschicht
mit dem sich -später das Vorderende des Kopffortsatzes verbindet"; 011
p. 239, however, „die Protochordalplatte ist das vorderste Ende des
im Lecithophor vorgeschobenen Kopffortsatzes oder Urdarmsäckchens;
das daraus hervorgehende Mesoderm ist also „gastrales" in dem er-
wähnten Sinne. Dieser Teil des gastralen Mesoderms bleibt weitaus am
längsten mit dem Entoderm in Verbindung und löst sich erst sehr
spät von ihm ab." Yet a real cephalic mesoblast exists, as is evident
from the observations made by
Hubrecht and Bonnet.

In Manis the existence of it cannot be called in question. When
we examine only the
Manis-tiwhx^o N°. 83, where the chordal plate
passes into the prochordal plate, from which the cephalic mesoblast
arises, this observation seems to be in favour of
Rabl\'s opinion. In the
younger stages (151 and 180), however, we see that
Rabl\'s opinion

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cannot be correct, because here a prochordal plate is present too, with
which the chorda-anlage has not yet fused.

In sagittal sections of young rabbit-areas (Rabl 1915, fig. 8 plate
and figs.
i and 2, plate VI), we see in front of the anterior border
of the headprocess some mesoblast-cells, distinctly connected with the
hypoblast.
Rabl supposed these cells to originate from the headprocess.
The figures, however, may be interpreted also in such a way that
the cells must be regarded as developing directly from the hypoblast.

Thickened parts of the hypoblast just in front of the headprocess
are often figured e.g. by
Carius (1888), who was the first to draw
attention to this pait in the rabbit and considered it to be the primary
pharyngeal membrane. In the sections figured by him, just in front
of this thickened part, witli which the chorda-anlage is connected,
He some mesoblast-cells, out of which the pericard is formed in latei
stages. The great difference between his
figures^and mine is that in
mine (figs. 33 and 34), the cephalic mesoblast is distinctly connec ed
with the prochordal plate, and that this is not the case with the pnmaiy

pharyngeal membrane of Cakihs. . • / «q„\\

Keibei. describes a primary pharyngeal membrane .n Cavm (i ««9 ,
the mesoblast being separated from it here sagittal se^ .

figured by van Be^kden (F.\'s^c-rMh .888, and

(G™«, ,9,8) show the same, Tsukagucm, describes how in the goat
(\' 9. Z) the notochord ends in an indifferent »Mesoderin-en ode mm^
Newman and Patterson (19.0) figure a protochordal
the same being described by W
ilIon and Hai. (.907);"
(here, liowever, formed ov,t of the headprocess), by bTRAHL (.9.4,

"916) and Grosser (Ndmo, 1913)-

\'v\'oj and (jrosser Uio/m, i\')\'J ■ , into

These observations all differ very much. They h iivuW ^
observations, from which it is evident that the ceph c nicsd ^

8\'ving rise to the formation of the pericard amon^t othei , 01 ly^

tom\'the prochordal plate [dog, T.,^, ^

(Assheton)]; 2°. obse vations pointing to the fact hat " / ^ J
Ptimary p,laryngeal membrane niesoblast-cells are to be foi ul ^oK b y
="«0 giving to the pericard), not connected witl th tl

-mterior cells.of the gastrai mesoblast, formed o
[rabbit (
Carius), Qivhi, VcspcrtiUo nwrmus, etc.

-ocr page 92-

Opinions also differ about the existence of the annular zone of
proliferation and the peripheral mesoblast originating from it.

In 1890 this zone is described by Hubrecht in Sorex and later on
in
Tarsius (1902) and Galeopithecus (1919). In 1884 Bonnet had already
figured (in the sheep) the so-called „Mesoblasthof," also a peripheral,
ringshaped hypoblast-thickening, from which extra-embryonic meso-
blast develops principally.
Keibel, however, tracing the mesoblast-
formation in the sheep too (1894), could not confirm
Bonnet\'s
observations.

Neither in Cavia and the rabbit, nor in the pig (1891) Keibel
could find the peripheral mesoblast. It is described, however, by
Robinson (1892) in the mouse and rat. Kollmann (1884) gave an
example of the existence of the peripheral mesoblast in Birds, descri-
bing the so-called „Randwulst" in the chick.

We must not forget that according to Branca (1912) the con-
nection between mesoblast and hypoblast can be secondary.

The peripheral mesoblast originates partly out of the region of the
embryonic shield, lying in front of the primitive streak.

Tsukaguchi (1912) shows that in the goat and (1913) in the rabbit,
in front of the primitive streak, mesoblast-cells develop out of the
epiblast. He denies the origin of the mesoblast from the hypoblast.

Baumeister (1913) finds mesoblast in Erinaceus at the anterior
border of the shield, but shows that these cells are in connection
with the mesoblast, derived from the primitive streak.

The statements above described, concerning the existence of a peri-
pheral mesoblast, are confirmed in
Manis. Here the peripheral meso-
blast is in connection with the anterior part of the prochordal plate.
This is also stated by
Hubrecht (1908, p. 33) in other Mammals.
The ring-shaped zone, producing the peripheral mesoblast, extends
along the borders of the germdisc.

When it was discovered that both in Vertebrates and Invertebrates
the mesoblast is developed in many different ways, some investigators
rejected the dogma of the specificity of the germ-layers. They came
to consider the mesoblast a topographical layer.

Schlater (1907) does not agree with this conception ofthe meso-
blast. The fact that the mesoblast in Vertebrates is formed out of different
sources, is interpreted phylogenetically by him. In his explanation, lie

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starts from the very early development of the mesoblast in Primates,
before primitive streak and headprocess are visible. This primitive three-
layered stage of the Vertebrates corresponds to a so-called mesenchy-
miila-stage, which all Metazoa, the Coelenterata excepted, have passed
through, according to him. In Vertebrates a new organ, the axial skele-
ton, makes its appearance; the Chordata-type has originated and in con-
nection with this, new mesoblast, the secondary ijnesoblast or axial
mesoblast, is formed. The phylogenetic stage, which all Vertebrates
have passed through, is called
chorda/a by Schlater, in connection
with the appearance of the notochord. The mesoblast formed in the
mesenchymula (protogenesis corresponds phylogenetically to the for-
mation of the mesenchymula) is to be found in a few cases only
and among them
Schlater reckons all cases, in which the mesoblast
is formed in an earlier stage than the primitive streak-stage and out
of other sources than primitive streak and headprocess. Examples of
this are the early mesoblast-formation in Primates
(Homo, Semnopithcci/s,
TarsiusJ
and also in Tatusia (Patterson 1912) and Gakopithccus
(Hubrecht 1919).! In Manis the peripheral mesoblast, originating in
primitive streak-stages, and the cephalic mesoblast, developing still later,
must be considered the primary mesoblast, the gastral and peristomal
mesoblast the axial or secondary mesoblast, according to
Schlater.

Phylogenetically speaking, cephalic and pheripheral mesoblast belong
to the mesenchymula, gastral and peristomal mesoblast to the chordula.

Rabl denies the existence of cephalic and of peripheral mesoblast.
Only the axial mesoblast, formed in close relation with the process
of invagination is to be found, according to
Rabl. In the chapter
„Lehre von den Keimblättern", in his „Handbuch" 1903, O.
Hertwig
does not mention either the peripheral or the cephalic mesoblast

(cf also his „Lehrbuch" 1915)-

The authors, who accept the existence of the peripheral mesoblast
agree about its\'derivatives. According to both
Hubrecht (1908) and
van Beneden (1912) the bloodvessels and the blood of the umbilical
vesicle are derived from the hypoblast or „l^citophore".
Kollmann
regards (1884) the „Randkeim" or „Akroblast" as a „Bindegewebs-
blutkeim".
Ruckert agrees with this view (1906) and Triepel

(1916) accepts it.

To sum up: the protogenetic as well as the deuterogenetic part

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of the future gut-lining can produce mesoblast. As a result a sheet
of cells is formed between epiblast and hypoblast, in the embryonic
as well as in the extra-embryonic part of the blastocyst, This layer
I have called mesoblast for short.

IV. Notochord.

It is clear that the question, to which „germinal layer" the noto-
chord belongs, is answered in many different ways, so many opinions
on the gastrulation-process existing.

Hubrecht, for example, holds that the notochord originates from
the epiblast and hypoblast, others consider the headprocess and noto-
chord to belong to the mesoblast, derived from the epiblast, whereas
Rabl; Sobotta and Bonnet think that the notochord is hypoblastic.

O. Hertwig (1903) accepts, besides the germ-layers, a separate
„chordal layer". In
Manis there are indications, but no proof that
the anterior part of the notochord is formed from the wall of the
archenteron. It is clear that the posterior part of the notochord is
not formed, like the principal part out of the metenteron-wall, but
out of the primitive streak, which we considered to be homologous
with the changed deuteropore of the Anamnia.

We have mentioned Bonnet\'s, Hubrecht\'s and also Heape\'s
opinion that the so-called protochordal plate or „Erganzungsplatte"
forms not only the cephalic mesoblast, but also the anterior part ot
the notochord;
Jenkinson (1906, 1913) agrees with this opinion.

It is not clear to me, what were the grounds of Hubrecht\'s opinion
that the protochordal plate forms a part of the notochord. In his
paper, in which the name protochordal plate is first used
1890), this name is given to the thickening „developed in situ ns
part of the hypoblast" (p. 501) and later, p. 508 he states: „part ot
this patch will develop into the anterior portion of the notochord.
For this reason I will call it the protochordal plate". As far as I can
trace, he never proved this. He says only (p. 509) „that the forma-
tion of a protochordal plate has preceded the appearance of the very
first indications of a gastrula-ridge (primitive streak)" and on p. 5 1
after he has described the protochordal plate and the development
of the cephalic mesoblast, he mentions that the size of the proto-

-ocr page 95-

chordal plate decreases and that then „the front portion of the noto-
chord folds off, this being the last phase in the developmental phe-
nomenon of what we have called the protochordal plate. This has
been figured by
Heape for the mole and is not further entered upon
in this paper" (to compare also
Assheton 1909).

Consequently he quite agrees with Heape (1883) and Bonnet (1889),
who stated that the cephalic mesoblast forms out of the thickened
part of the hypoblast and who found that later on the notochord
develops here.
Heape did not prove either that this part of the
notochord did not arise from the anterior part of the headprocess.

In 1889 Bonnet had already shown that in the sheep the anterior
part of the notochord originates from the hypoblast. In the dog,
however, he was able to prove it (cf antea p. 42). He recognized
the „Erganzungsplatte", not only because it produces the cephalic meso-
hlast, but also because it possesses peculiar chromatophile granules.
In older stages he saw that the notochord proceeds more and more
cranialward and observed also that this anterior part of the notochord
IS formed out of a part of the hypoblast, in which these granules are
\'^Iso to be found. This hypoblast-part must be considered a remnant

r\\C - .. , 1 1 ........\'iU T^ «-v-in-L- nf cnnntr-\'v:

^ or somites tne notocuuiu, ------ 1 • 1

fonnd as flir as the primary pharyngeal membrane whidi, as it a so
possesses these granules, must also be considered a derivative ot the

"Ei\'ganzungsplatte". ,, • , 1 i 1

In the descriptive part we have seen that in Manfs the chordal
J^^te forms out of the headprocess after incorporating in the hypoblast,
^he headprocess, extending cranialward between epiblast and iiypo-
does not receive cells from either of these layers, as they aie
clearly separated from it.

, About the length of the primitive streak opinions difter. Kkibll
example (pig, .893, 1895) thinks that it ongj^\'^^y/^^^^^^ \'

^^^ as tl/e anteHor border of the embryonic shield and becomes

S\'-^duaily shorter, the headprocess simultaneously developmgcaudalwa 1
\'915 Rahl figured a great number embryonic areas of he

\'•^^b^^it, fron, which it is clear that in the rabbit at least l e pn
^\'^^tive
streak gradually becomes longer up to a certain stage,

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8o early developmental stages

never reaches the anterior border of the shield. (Ruber\'s observation
(1918) of the tirst anlage ot the. headprocess in
Cavia, does not
agree with the opinion of
Keibel either, the primitive streak being
short here.). When the primitive streak has reached its greatest length
a headprocess is already formed and the primitive streak then gra-
dually shortens from before backwards. During this process, the
posterior part of the headprocess (the so-called „Primitivstreifenteil
des Chordas") forms out of the primitive streak.

In a recent investigation on the first developmental stages in
Mammals
(Tsukaguchi on the goat, 1912) the author describes four
primitive streak-stages, in which the primitive streak reaches a length
of 75 of the germdisc and the headprocess extends as far as the
anterior border of the shield. A lumen is not always to be found in the
headprocess; discontinuous canals and slits are described. The anterior
part of the headprocess passes into an „indifferente Mesoderm-
entodermmassa," compared by the author to
Bonnet\'s „Erganzungs-
platte". Stages with free headprocess (found in the pig, rabbit,
Cavia,
V•jspertilio
and also in Manis) Tsukaguchi has not found in the goat.

The notochord is described in detail in an older embryo with 5 pairs
of somites. He distinguishes five different zones from back to front.

1°. The part in front of the primitive streak, lately formed out
of it (the so-called „Primitivstreifenteil des Chordas"). It is distinctly
separated from the epiblast and is no longer connected with the
mesoblast on both sides. Canals are partly visible.

2°. An „Umbildungszone", differentiated distinctly in a dorsal and
a ventral part with a canal lying between. Like zone I, it is covered
by the hypoblast. It passes gradually into

3°. the chordal plate, incorporated in the hypoblast. During the
process of incorporation the ventral mass of the chorda-anlage,
described
in 2°, has spread, the hypoblast being cleft simultaneously. The chor-
dal plate is principally formed out of the dorsal cells, mentioned under
2°; the ventral cells are used for the growth in length of the noto-
chord by „Zellumlagerung" (= „Scheinwachstum",
Keibel 1889). Thin
zone is very long. The growth in length of the notochord is possible,
because the „Primitivstreifenteil des Chordas" (zone I) is very thick-
4°. A short zone, in which the notochord is separated from the
hypoblast; a secondary chordal canal is found here (cf
Keibel 1889)-

-ocr page 97-

5°. a still shorter zone, which is not yet separated from the hypo-
blast, and passes into the same „Mesoderm-entodermmassa", in which
the headprocess of the primitive
streak-stage ended. Tsukaguchi
cannot decide, whether this part of the notochord develops from the
hypoblast here, or whether \'this zone is formed out of cells derived

from the headprocess.

In young M^///>-embryos, in which the separation of the notochord

has not progressed very far, we can distinguish about the same zones.

Yet there are differences. The „Primitivstreifenteil des Chordas , also

present in Manis, is connected witli the mesoblast, this not being the

case in the goat. In front of this zone a second region is to be found,

"o longer connected with the mesoblast. In Manis these two zones,

both covered by the hypoblast, contain many discontinuous lumina,

some times forming long canals. Like Tsukaguchi I thnik that these

lumina are not homologous with tlie metenteron-Iumen, but with the

canalis neurentericus of tlie Anamnia. In some few embryos described

this chordal canal opens into the future gut and just f™"\' ° tte

place the chorda-anlage passes into tlie third zone: the chorda i^ate

incorporated in the hypoblast. Consequently the transitional zone

(Tsukaguciii\'s second zone) is very sliort.

In Manis 1 could not find a secondary chordal canal. Tsukaguchi s

4"\' and c\'l\' zone are present in Mams. . , •„

The L,R«EKKOHN-canal or metenteron-lumen is

process in many Mammals. In Manis, the pg (Ke.„ri. \'he %

(Bonnet ,90,) it probably does not occur. The blastc^» e d u 1 d.

by HurkecVi in EwL (.9°^. \'goS, 19\'

L.KBKKK(,HN-canal. [By Baumeustek it was not found " ^
>9.3), hut P
etekmaU observed it liere (.907)]- - p- hlc lat

■-\'r \'"v\'Trlit;;;;

ihe primary chordal canal (Kkibel looy; looi^

;:[ Man!mals fe. g. dog with 8 . 9 P;^- ^
^^hhit with 11 pairs ot somites (Carihs i HH«), ana 7 a o ^

often mistaken for the L\'-ekkOhn^
the hi.dproce,s,s proper in priniifve streak-stages. ^^J\'^hor.
\'°8y of the e canals is very confusing cf. p. 7)-«
•»ke the canalis neurentericus to be the caudal rest of the
L.ebekki ^^

^«^t-han,,. Kon. Akad. v. Wctcnsch. (ic Scctic). Dl. XXI.

-ocr page 98-

§2 early developmental stages

canal (e.g. Grosser 1913, Strahl 1916), which moves caudalward.
Moreover
Triepel (1914, 1917^) thinks that the presence of the canalis
neurentericus is a convergent phenomenon in Vertebrates.

These few examples may suffice to show that the problem of the
relation of metenteron and canalis neurentericus has not yet been solved.

Strahl (1916) distinguishes a posterior „Ergänzungsstück" of the
notochord, formed out of the primitive streak. The same is stated
by
Bonnet (1901), who also mentions (1920) that this part of the
notochord is always solid, and sometimes shows some small traces
of canals. In a
Mams-&mhvyo with 18 pairs of somites, however, we
can still observe a canalized part of the notochord, which will never
incorporate in the hypoblast.

Rabl (1915) does not lay stress on .the part of the notochord,
derived from the primitive streak. Yet the development of the noto-
chord in the rabbit is treated elaborately by him.
He thinks that
the headprocess participates in the formation of the gutlining (cf.
antea, p.
44). According to me this is not proved. From his sections
it further follows that in rabbit-embryos with 12 pairs of somites
the notochord has for the greater part separated from the hypoblast.
This is the case in
Manis, when 18 pairs of somites are formed. In
rabbit-embryos with 12 pairs of somites the anterior part of the
chorda-anlage is still connected with the hypoblast. From his des-
cription of the chorda-anlage of an embryo with 6^7 pairs of
somites (
Rabl 1915, pp. 433—434), I will still mention the following
facts. A chördal canal is not described. The chordal plate
becomes
flatter and flatter and gradually contains less cells. At the level of
the pair of somites the chordal plate-section consists of
7 nuclei.
According to
Rabl all cells of the chordal plate have then passed into
the gut-epithelium. Still more in front the chordal plate is no longer
distinguishable from the hypoblast and it then passes into an „inter-
epitheliale Zellplatte (über), aus der nach rechts und links auch
jetzt
noch, wie übrigens auch später, das Mesoderm mächtig hervorsprosst.
Hier bleibt also das Mesoblast am längsten mit der dorsalen Darm-
wand in genetischem Zusammenhang". [Here we have a clear des-
cription of the prochordal plate, of which he denied the existence in
preceding pages (pp.
227 and 239)]. „Leider lässt sich von allen
diesen Dingen, von denen sehr wenig bekannt ist, ohne zahlreiche

-ocr page 99-

Abbildungen keine gute, klare Beschreibung geben; von einer grös-
seren Zahl von Abbildungen wollte ich aber absehen, da der Gegen-
stand nicht unmittelbar zum Thema der Abhandlung gehört". And
also on p. 419 he mentions this plate, in describing the hypoblast
in primitive streak-stages: „Sodann folgt unter dem Vorderende der
Hirnplatte eine dicke Strecke der unteren Zelllage. Von dieser Strecke
sieht man auf Querschnitten rechts und links Mesoderm her vorsprossen.\'
Später wird dieses Epithel noch dicker, und die Zellwucherung
mächtiger. Die Strecke entspricht der von
Rex bei der Ente und
Möwe beschriebenen interepithelialen Zellmasse des Entoderms , and
this one
must also be compared to the prochordal plate.

Recently Huber (19.8) has elaborately traced the anlage and
morphogenesis of the notochord in
Cavia. He describes i . pnmKive
streak-stages with the finst anlage
of the headprocess, originating from
a comparatively short primitive streak; 2°. a primary pharyngeal
membrLie, in L„t of which me,soblast-cells occur (the anterior cells
> of the mesoblast-wings developing from the primitive .streak, ct. antea

p. 76); 3°. a LiEBEKimtN-canal, opening ""o the cavity, enclosed by

the hypoblast, by different apertures. When tl>e headproce^ has becmne
a
chordal plate, the hypoblast still covers this plate in different places.

Tliis was not observed in Manis. . .

The olde,st stage, in which the chordal P\'^\'e is still enti ely in»-
porated in the hypoblast in
Cavia, is a stage, m ^^h.ch no sonii e^
»■■e to be found. In
Manis this is still the case in ^biyo w. h
■3 pains of somites. The separation ofthe notochord takes place
t^o ways in
Cavia (also according to Keidel \'»»9)- , , ;

, t". By undergrowth of the hypoblast-edges. A > ° ^^^

•hen formed, which becomes cylindrical later on. 2 . 1 X
•he borders ;f the cliordal plate, by which a
^rectly formed. In botli cases a secondary chordal ^

T^hese two methods of separation Huber z l

\'^■«bryo. In Manis only the .second method (infolding ^^ f ™^

Hmkh does no, mention that in the part of the notocted, oiig matmg

of the primitive streak last (zone 1 and II
\'«^ntions only the presence of a L.EBERKÜHN-canal >" \'
. In Manis we hale observed that the chordal canal app rs do We
many cases. Sometimes (e.g.
Cav.a, v. Spee .888 and Huber

-ocr page 100-

84 early developmental stages of manis javanica desm.

1918) such a formation is also observed in the headprocess, before
the formation of somites. Evidently a double metenteron is found
here, in these young stages.

The appearance of a double metenteron as well as of a double
chordal canal requires an explanation. According to me, the appear-
ance of the canals (single or double) is connected with the mechanical
■function of the incorporating organ, in which they occur. The
Lie-
berkühn-canal and the chordal canal are two phylogenetically different
formations (metenteron and canalis neurentericus), but the ontogenetical
function of them is the same: i. e. the incorporation in the hypoblast,
and this occurs by spreading of the wall of both canals. In both
cases this can come about from a single or a double canal.

Before concluding this survey of the literature, treating the deve-
lopmental stages of the notochord, I will state:

Bonnet\'s „Ergänzungsplatte" probably occurs also in Majiis. Be-
cause I cannot prove that the anterior part of the notochord is
formed out of this protogenetic part of the hypoblast, I have called
this plate prochordal plate.

A following part of the notochord develops from the headprocess
proper, in which a canal appears in many Mammals. In
Manis I
could not find this canal. This part of the notochord is derived from
the metenteron-wall.

The posterior part of tlie notochord is the „Primitivstreifenteil des
Chordas", the „Zuwachsstück" of
Bonnet. It is formed out of tlie
primitive streak. In this part a distinct canal appears in many cases
in
Manis. The lumina in it made many authors think: „Der Chorda-
kanal rückt in den hinteren Theil der Keimscheibe"
(Lieberkühn
1882, p. 421) or „dass der Chorda-kanal sich in den Primitivstreifen
hinein fortsetzt"
(Bonnet 1901, p. 267). This is supposed to happen
after the formation of the first pair of somites.

Though I dare not state that this opinion is wrong, yet I believe
that these lumina can be more successfully compared to the canalis

neurenticus of the Anamnia. The LiEBERKiinN-canal and these canals

«

cannot be considered the same lumina. However, there is a close
relation between these formations, as well as between metenteron
and canalis neurentericus in Anamnia.

-ocr page 101-

LITERATURE CITED.

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*) Not seen by me.

-ocr page 102-

86 early developmental stages

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-ocr page 103-

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-ocr page 104-

88 early developmental stages , "

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Hertwig, O. 1903. Die Lehre von den Keimblättern in „Hand-
buch der vergleichende und exp. Entwickelungslehre der Wir-
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„ 1918. Some observations on the early development of Didel-
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Amer. anat. mem. n°. 5.

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„ 1890. Studies in mammalian embryology. II. The develop-
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„ 1894. Spolia nemoris. Quart. Journ. Micr. Sei. Vol. 36.

„ 1895. Die Phylogenese des Amnions und die Bedeutung des
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„ 1902. Furchung und Keimblattbildung bei Tarsius spectrum.
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„ 1905. The gastrulation of the Vertebrates. Quart. Journ.
Micr. Sei. Vol. 49; also Anat. Anz. Bnd. 26.

„ 1908. Early ontogenetic phenomena in Mammals and their
bearing on our-interpretation of the phylogeny of the Verte-
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1910. Is the trophoblast of hypoblastic origin as AsshktoN
will have it.? Quart. Journ. Micr. Sei. Vol. 55.

„ 1912. Frühe Entwickelungsstadien des Igels und ihre Bedeu-
tung für die Vorgeschichte (Phylogenese) des Amnions. Zool.
Jahrb. suppl. XV
(SpENGEL-Festschrift).

-ocr page 105-

Hubrecht, A. A. W. 1919. Früheste Entwickhingsstadien und Pla-
centation von
Galeopithecus. Bearbeitet von Dr. Dan. de
Lange Jr
. Verb. Kon. Akademie v. Wetensch. Amsterdam.

(II). Deel 16. N°. 6.
Huxley, T. H. 1864. Elements of comparative anatomy.
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Vertebrates and on the significance of germinal layers in
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„ 1913. Vertebrate Embryology. Oxford. ^ 1 ^r u

Keibel, Fr. 1888. Zur Entwickelungsgeschichte des Igels. Verb.

fsso.\' ^Zur Entwickelungsgeschichte der Chorda bei Säugern
(Meerschweinchen und Kaninchen). Arch, flir Anat. und I hys.

Anat. Abt. \' , . , 1 o 1 • a .

1891. Uber die Entwickelungsgeschichte des Schweines. Anat.
„ Xl S5. Suidien zur
Entwickelungsgeschichte des Schweines.

Schwalbe\'s Morph. Arbeiten. Bnd. 3 und 5.
„ 1894. Die
EntWickelung des Mesoblasts beim Schaf Vei-

handl. Anat. Ges. 1894. t^ j a

„ 1905. Zur Gastrulationsfrage. Anat. Anz. Bnd. 26.

„ 1916. Zu Carl Rabl\'s „Edouard van ^^

gegenwärtige Stand der wichtigsten von ihm behandelten 110-

bleme". Arch. Mikr. Anat. Bnd. 89.
Koelliker, A. 1879. Entwickelungsgeschichte des Menschen und

der höheren Thiere. 2"= Auflage. Leipzig.
\'\' . 1882. Die
EntWickelung der Keimblätter beim Kaninchen.

Festschr. Würzburg. Bnd. I. , , tt ^^r ^tnty-

Kollmann, J. 1884. Der Randwulst und der Ursprung der Stutz

der\'wH.dtit und die Bedeutung des Primitivstreifs. Arch.

- la^ j:; "die

/«W,« Schlegel. Anat. Hefte. Bnd. 32-

-ocr page 106-

c)0 early developmental stages

de Lange Jr., Dan. 1912, 1913. Mitteilungen zur Entwicklungsge-
schichte des japanischen Riesensalamanders.

L Anat. Anz. Bnd. 42. IL Anat. Anz. Bnd. 43.
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Anat. und Phys. An. Abt.
Lwoff, B. 1894. Die Bildung der primären Keimblätter und die
Entstehung der Chorda und des Mesoderins bei den Wirbel-
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banded armadillo from the primitive streak-stage to birth.
Journ. of Morph. Vol. 21.
Patterson, J. T. 1912"^ A preliminary report on the demonstration
of polyembryonic development of the armadillo
{Tatusia).
Anat.\'Anz. Bnd. 41.
„ 1913- Polyembryonic development in
Tatusia novemcincta.
Journ. of Morph. Vol. 24.
Petermann, W. 1907. Zur Kenntnis der frühen Entwicklungsvor-
gänge am Ei des Igels
{Erinaceus curopaeus L.) vor Ausbildung
der Medullarrinne. Zeitschr. Wiss. Zool. Bnd. 85.
Rabl, C. 1889, 1892. Theorie des Mesoderms. Morph. Jahrb. Bnd.
15 und 19.

„ 1915- Edouard van Beneden und der gegenwärtige Stand
der wichtigsten von ihm behandelten Probleme. Arch. Mikr.
Anat. Bnd. 88.

Rauber, A. 1875. Die erste Entwickelung des Kaninchens. Sitz.
Ber. Naturf Ges. II. Leipzig.
„ 1876. Primitivrinne und Urmund. Beitrag zur Entwicke-
lungsgeschichte des Plühnchens. Morph. Jahrb. Bnd. 2.
Resink, A. J. 1903. Die Stammentwickelung der embryonalen

Organe. Tijdschr. Ned. Dierk. Vereeniging. (2). Deel 8.
Robinson, A. 1892. Observations upon the development of the
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and the amnion in Mammals. Quart. Journ. Micr. Sei.
Vol. 33.

„ 1903. Lectures on the early stages of the development of
Mammalian ova and on the formation of the placenta in
different groups of Mammals. Journ. Anat. and Phys. Vol. 38.

-ocr page 107-

Rückert, j. 1906. Entstehung der Gefässe und des Blutes in
O.
Hertwig\'s „Handbuch der Entwickelungslehre". Bnd. I.
Teil I. Jena.

Schlater, G. 1907. Uber die phylogenetische Bedeutung des soge-
nannten mittleren Keimblattes. Anat. Anz. Bnd. 31.
1909. Zur Frage vom Ursprung der Chordaten nebst einigen
Bemerkungen zu den frühesten Stadien der Primaten-Embryo-

genese. Anat. Anz. Bnd. 34.
Selenka, E. 1882. Keimblätter und Gastrulaform bei der Maus. Biolo-
gisches Centraiblatt. Bnd. II. . , ,
E. 1886, 1887. Das Opossum. Studien über Entwickelungs-
geschichte der Tiere. Bnd. 4. Wiesbaden.
de Selys-Longchamps, M. 1910. Gastrulation et formation des
feuillets chez
Petromyzon Pianeri. Arch, de Biologie. T. 25.
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Maus. Arch. Mikr. Anat. Bnd. 45.
„ 19,1. Die
EntWickelung des Eies der Maus, vom ersten

Auftreten des Mesoderms an bis zur Ausbildtmg fer Embryo- "
anlage und dem Auftreten der Allantois. Arch. Mikr. Anat.

„ fgiVzur Frage der Wanderung des Säugetiereies durch

den Eileiter. Anat. Anz. Bnd. 47.
V.
Spee, F. 1888. Uber die Entwickelung^svorgange vom Knoten

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Arch. An. und l^hys. An. Abt. Menschen

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und solche von Mycetes. ^erhandl. An. Gesellscl .
„ 1916. Uber einen jimgen menschlichen Emi^o ■ eb^^^^^^^
kungen zu C.
Rabl\'s Gastrulationstheorie. ^^^

^afani, a 1889. La fécondation et la segmentaüon étudiees
les oeufs des rats. Arch. Ital. de Biologie. T

(ZeiWhr. f. Anat. und angewannte Konstitutionslehre).

-ocr page 108-

92 early developmental stages of manis javanica desm.

Triepel, H. 1917/^. Lehrbuch der Entwickehingsgeschichte. Leipzig.
Tsukaguchi, R. 1912. Zur Entwickehingsgeschichte der Ziege. An.
Hefte. Bnd. 46.

„ 1913. Ein Beitrag zur Theorie des Mesoderms. Anat. Anz.
Bnd. 44.

^^Turner, W. On the placentation of the sloths. Trans. Royal. Soc.

of Edinburgh. Vol. 27.
Weber, M. 1891. Zoologische Ergebnisse einer Reise in Nieder-
ländisch Ost-Indien. Bnd. II. Beiträge zur Anatomie und Ent-
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Mams. Leiden.
„ 1904. Die Säugetiere. Jena.
Wenckebach, K. F. 1891. Der Gastrulationsprozess bei Lacerta

agilis. Anat. Anz. Bnd. 6.
Wilson and Hill, 1907. Observations on the development of
Ornithorhynchus. Phil. Trans. Roy. Soc. London. Series B.
Vol. 199.

-ocr page 109-

description of plates.

List of abbreviations.

^ pr. = aorta primitiva.

a\' z. = annular zone of proliferation.

Br. = brain.

ch. = notochord.

ch. can. = chordal canal.

ch, p. = chordal plate.

cor. rad. = corona radiata.

disc. prol. = discus proligerus.

e = embryonic knob.

ej-jtl\' = heart-endothelium.

ep^ = epiblast.

H. k. = HENSEN-knob.

li. p. = headprocess.

hyp = hypoblast.

ex. hyp. = extra-embryonic hypoblast.

med. = medulla.

med. gr. ■ medullary groove.

mes. = mesoblast.

ceph. mes. = cephalic mesoblast.

gast. mes. = gastral mesoblast

per. mes. = peripheral mesoblast.

ps mes. = peristomal mesoblast.

ov. ep. = oviduct-epithelium.

p c = pericardium.

ph. m. = pharyngeal membrane.

p jr == primitive groove.

p p = prochordal plate.

p s = primitive streak.

som\'. I etc. - first etc. pair of somites.

tj.^ = trophoblast.

„t. = uterine wall.

-ocr page 110-

94 early developmental stages

Explanation of figures.

Plates I and II.

Fig. I. A section through an ovum not yet divided. The discus
proHgerus and the corona radiata, surrounding the ovum, are
visible. The nucleus is not found
{n\\ a nucleus of the corona
radiata). The ovum lies in the oviduct. X 410.

HUBRECHT-Lab. Cat. N°. Manh, 51 A III, 5.

Fig. 2. A section of a three-cell stage, surrounded by corona radiata
and lying against the ciliated oviduct-epithelium. One nucleus
of the cleavage-cells is visible (;/). x 525.

HUBRKCHT-Lab. Cat. N°. Mam\'s, \\-jie III, n.

Flg. 3. A section of a four-cell stage, surrounded by corona radiata
and lying close against the oviduct-epithelium. Three cleavage
cell-nuclei are visible. X 525.

HuBRECHT-Lab. Cat. N°. Manis, 44f, II, 8.

Fig. 4. A section of the same stage. Two nuclei are visible. X 525.

HuBRECHT-Lab. Cat. N°. Matiis, ^^c, II, 9.

5- A section through a strongly compressed blastocyst, lying in
utero. The wall of the blastocyst consists of the trophoblast only,
with the embryonic knob at one place. In this section no
hypoblast is visible. X 220.

HuBRECHT-Lab. Cat. N°. Manis, 87 ^ II, 5.

Fig. 6. A section through the same blastocyst. The first anlage ot
the hypoblast is visible. X 325.

HuBRECHT-Lab. Cat. N°. Manis, g II, 4.

Fig. 7. Schema of folded blastocyst with embryonic knob, consisting
of
epiblast and hypoblast and still covered by the trophoblast.
The hypoblast has spread over the whole inside of the tropho-
blast. The uterine wall is indicated. X

HuBRECHT-Lab. Cat. N°. Manis, 113,/. II, 6.

Fig. 8. Embryonic knob and part of the same blastocyst. Epiblast
with few nuclei, hypoblast beneath it with many tall or
cubical nuclei; extra-embryonic hypoblast consisting
of flat-
tened cells. X 225.

-ocr page 111-

Fi^s. 9 and i o (plate II), 11 and 12 (plate 13—16 (plate II),
ij and 18 (plate I) and 19 (plate II) show surface-vtews of

the embryonic shields (cf. pp. 17 and ^V\'
Fig. g. A primitive streak-stage. Mams, N°. 180. X lO-
Fig. 10. Embryo with 4 a 5 pairs of somites.
Mams, 83. X 10.
Figs. II and 12. „ „ 8a9 " " . " N.64.X10.

Fig. II dorsal, fig. 12 ventral view.

Fig. ,3. Embrvowithioa 11 pairs of somites. S^-Xio.

Fit 4. ^^ " " fXio.

fe; andi8.: : :;: : N°.68.XI3.

Fig. 17 dorsal, fig. 18 ventral view.
Fie 10 Embryo with 18 pairs of somites.
Mams, N . 53 X 10.
. \' Fi^t Tplate I) ah^gs. 21-28 (plate III) represent the

primitive streak-stage N"". 151. 1 r 1 j

Fie ^o General view of a section through the strongly folded
\' " Mas ;c; ; lymg against the uterine wall.
Extra-emhryon>c hy-
poblast consisting of flattened cells.
Eml,., embryo. X ^o.

HuBRKCHT-Lab. Cat. N°. Mams, .5"^,. I- \'\'>•

plate 111.

prolifcration. The situation of the sections figs. 22

tke .,éryo,nc shield

huUcakd in ,fig. 2 > • posterior border) through

F.g. 22 The 8 .section (coun ^ ^^ ,1 e peristomal mesoblast
the primitive streak, hoin wh cn t j groove is

originates. The probab e ,,\' ; \' \' columnar

shown. The hypob ast .f"""" ,es laterally

cells under the primitive streak. 1 lit epioi i

into the trophoblast. X \'9\'\'

IIU1,RKC11T-L»b. Cat. N\'. Mam,. 151«. J\'

-ocr page 112-

Fig. 23. A section (9 sections in front of that, shown in fig. 22) through
the primitive streak, covered by the hypoblast. The embryonic
shield rises above the level of the trophoblast. X 191-

HUBRECHT-Lab. Cat. N°. Manis, 151 <2,„ I, 6.

Fig. 24. The section in front of that, shown in fig. 23, passing
through the
HENSEN-knob. Here the hypoblast forms an un-
interrupted layer too. X i94-

HuBRECHT-Lab. Cat. N°. Manis, II, 5.

Fig- 25. The section in front of that, shown in fig. 24, passing
through the headprocess. To the left the annular zone of pro-
liferation. X 198.

HuBRECHT-Lab. Cat. N°. Manis, 151 III, 7.

Fig. 26. The section in front of that, shown in fig. 25, passing
through the anterior border of the headprocess. The annular
zone is visible on the left, on the right not yet. X 200.,

HuBRECHT-Lab. Cat. N". Manis, 151 I, 4.

Fig. 27. The section in front of that, shown in fig. 26, passing
through the prochordal plate. The annular zone is visible on
the left as well as on the right. The very first formation of
the peripheral mesoblast is to be seen (at both sides a mitotic
figure). X 200.

HuBRECHT-Lab. Cat. N°. Manis, 151«,, II, 2.

Fig. 28. A section through the prochordal plate and annular zone,
with distinct peripheral mesoblast-formation, 7 sections in front
of that, shown in fig. 27. X 200.

IIUBRECHT-Lab. Cat. N°. Manis, 151 I, i."

Figs. 29—32. Sagittal sections throuv-h the primitive streak-
stage N\\
180.

Fig. 29. Schema of the median section. The primitive streak from the
caudal border (on the left) to Here the headprocess extends
cephalad between epiblast and hypoblast, as far as the
prochordal
plate. The peristomal mesoblast originates from the primitive
streak. The hypoblast in the posterior part of the
embryonic
shield is a layer composed of cubical cells, gradually becoming
thinner in the anterior part. Just in front of the prochordal
plate the wall of the blastocyst is torn off. X 96-

HuBRECHT-Lab. Cat. N°. Manis, iSo^i, II, 14.

-ocr page 113-

Fig. 30. Enlarged middle part of fig. 29. The much flattened hypo-
blast is to be noted.
 X 265. \' . ,
Fig. 31. Enlarged cephalic part of the same germdisc, showing the

headprocess and the prochordal plate. X 345-

HUBRECHT-Lab. Cat N°. Ma/iis, 180«, II, i.

Fig. .2. Formation of the peripheral mesoblast out of the extra-
embryonic hypoblast (annular zone of proliferation) behind the
embryonic shield. The mitotic figure
fmj is to be noted. The
trophoblast lies against the uterine wall, fused widi it m some
plaœs (this is not visible in the figure). On the right between
primitive streak and hypoblast the peristomal mesoblast is to

be seen. X 280.

HuBRECHT-Lab. Cat. N". Manh, lîoa, I, «o.

Plate IV.

F/W n Sagittal sections through the embryo N\\ 83,

msessin^ L pairs of somites ivith a s" pair formwg-
Fig. 33\'lSa\'o^f the^nid-sagittal section, ^he peristo^
^ blast is formed out of the primitive
of the chorda-anlage is torn from the P^"^ ^^

pericardial space has not been formed as yet. X 5 •

hunreclli-.i-ab. cm. Mm«, 8j". 9- , , „f

Pi, 34. Tl. cephalic regio,^^^^^^^^^^^^^^

— arevlible. X-,

ilunrhxiit-LMi. Cal. N». Mm SJ". \'X\' , , , , „.n In it

Pig. 35 The HKNSKN-knob with the lumina = chordal cana^ >n ^
"greatly enlarged. Cranialward the chordal plate.
 X

Il™i.KCliT-Lab. Cai. N». Mm:, 8;». 1-\'\',

-ocr page 114-

Figs. 36.—41. Cross-sections, passing through the chorda-
anlage of embryo N°.
64 with Sag pairs of somites (cf. fig. bzj.

Fig. 36. A section through the anterior part of the broad chordal
plate, quite incorporated in the hypoblast. The undifferentiated
mesoblast still connected with the chordal plate-borders.

X 340-

HuBRECHT-Lab. Cat. N°. Manis, 64«, IV, 8.

Fig. 37. A section through the 7\'^ pair of somites. The broad
chordal plate passes into the hypoblast, which is torn from it
on the right. X 34°-

HuBRECHT-Lab. Cat. N°. Manis, 64« IV, 28.

Fig. 38. A section through the 8\'^ pair of somites. The chordal plate
is slightly higher and incorporated in the hypoblast. X 340-

HUBRECHT-Lab. Cat. N°. Manis, 64«^ I, 4.

Fig. 39. A section through the undifferentiated mesoblast. The
chordal plate, higher than the hypoblast, is not quite incorpo-
rated in it. X 340-

HuBRECHT-Lab. Cat. N°. Manis, 64«, III, 11.

Fig. 40. A section still further caudalward, showing the very first
beginning of the incorporation of the chordal plate in the
hypoblast. X 342.

HuBRECHT-Lab. Cat. N°. Manis, 64^;, III, iS.

Fig. 41. The headprocess, not yet incorporated in the hypoblast, is
connected with the undifferentiated mesoblast: the connection
in the \'section being broken, however. X 34°.

IIUBRECHT-Lab. Cat. N°. Manis, 64«, IV, 3.

Figs. 42—44. Cross-sections through the chorda-anlage of
embryo N\\
32 with \\o a 11 pairs of somites, fcf Jig. 63/

Flg. 42. A section through the 5\'\'\' pair of somites, showing\'a very
broad chordal plate, not quite incorporated in the hypoblast on
the right side. The hypoblast is slightly compressed
against
the borders of the chordal plate. On the left and on the riglit
the primitive aortae. X 340.

HuBRECHT-Lab. Cat. N°. Manis, 32«, VI, ,5.

Flg. 43. A section through the undifferentiated mesoblast. Distinct
broad and tall chordal plate; the hypoblast extending under
the borders of it. X 340.

HuBRECHT-Lab. Cat. Mauis, 32^7, II, 7.

-ocr page 115-

OF MANIS JAVANICA DESM. 99

t

Plate V.

Fig. 44. A section through the headprocess, with chordal canal and
connected with the mesoblast. Very first beginning of the in-
corporation of the chorda-anlage in the hypoblast. X 340-

HuBRECHT-Lab. Cat. N\'. Manis, 32 III, n - \'

p/gs. 45—50. Cross-sections through the embryo N\\ 89 %vith

n pairs of somites (cf. fig. 64^\'.
Fig. 4c. A section through the undifferentiated mesoblast of the
head and anterior part of the chordal plate, incorporated in
the hypoblast, into which it passes directly.
 X 34°-

IIUBRKCHT-Lab. Cat. N°. Manis, 89«, VII, n.

Fig. 46. A section, througl, the 4"\' P»"\' »mites The chordal
plate is not very broad, and
incorporated n> the hypoblast,
{he latter being strongly compressed by the borders of the

chordal plate. X 34°-

iiubreciit-ul). Cat. N°. Mmh. 89«, VI, 10.

Fig. 7. A section through the P^ir of somites, "".ch resem-
bling fig. 46.
The hypoblast ..u,ch co.npressed -he.e too.

X 340-

HuiiKKCIlT-Ul.. Cal. N". Mam, M", 7- , , ■ •

Fig. 48. A section th,-ough the ^

^"rcavity, encsed by
• Ti^ (at^ientero,/). Begi.,..ing
of .ncoiporafon ol

the chordal plate in the hypoblast. X 34°-

llimRKcin-Ub. Cal. N°. 89". >• ">■

X 340-

HunRKcm-Lab. Cat. AU»,is, 89^\'. 1, .,nn(-ir-

50. A section through the "«dprocess w.«^^
ing double. The hypoblast extends beneath .t. A 34

IIU»R,:c„r-Lab. Cal. Jlta", »9«.

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loo ■ early developmental stages

Figs. 51—55. Cross-sections through the chorda-anlage of em-
bryo N°.
31 with 13 ^ 14 pairs of somites (cf. fig.

Fig. 51. Anterior part of the chorda-anlage, not yet separated from
the hypoblast. X 34°-

HUBRECHT-Lab. Cat. N°. Manis, V, ii.

Fig. 52. A little further caudalward the notochord is almost com-
pletely separated from the hypoblast; only a very small part
is not yet covered by the hypoblast. X 340-

HuBRECHT-Lab. Cat. N°. Manis, ^la^ VHI, 15.

Fig. 53. A section through the pair of somites. Chordal plate with
hypoblast, slightly thickened under its borders. X 340-

IIUBRECHT-Lab. Cat. N°. Manis, 31«, VHI, 14.

Fig. 54. A section through the 14»\'\' pair of somites, which is
forming. The chordal plate is incorporating in the hypoblast,
which extends under the greater part of it. X 34°-

HuBRECHT-Lab. Cat. N°. Manis, 31 a^ VI, 6.

Fig. 55. A section through the headprocess with chordal canal and
connected with the undifferentiated mesoblast. X 340-

HuBRECHT-Lab. Cat. N°. Manis, 11, 9.

Plate VI.

Fig. 56. Sagittal section through the fore-gut of embryo N°. io8,
with 13 pairs of somites. The chordal plate forms the roof ot
the primitive gut, except in the most cephalic part, where it ends
in the mesoblast. The pharyngeal membrane is cut tangentially.
The heart-endothehum and the pericardial space are visible.
The hypoblast surrounding the anterior part of the fore-gut
is distinctly thickened. X 2oo.

HuBRECHT-Lab. Cat. N°. Manis, 108 II, 6.

Fig- 57- ^ section through the well-developed chordal-canal in the
headprocess of the embryo N°. 68 with i6 pairs of somites.
The undifferentiated mesoblast is not connected with the head-
process (cf fig. 66.). X 340-

HuBRECHT-Lab. Cat. N°. Manis, 68 II, 6.

-ocr page 117-

Pjgs, —6i. Cross-sections through the chorda-anlage of em-
bryo N°.
53 ivith 18 pairs of somites (cf. fig. byj.

Fig. 58. A section through the anterior part of the chorda-anlage,
not yet separated from the hypoblast. X 34°-

HuBRECHT-Lab. Cat. N°-. Manis, 53 V, 2.

Fig. 59. The section caudalward of that, shown in fig. 58. The
notochord is quite separated from the hypoblast, which extends
beneath it and is somewhat thinner here. X 340-

HuBRECHT-Lab. Cat. N°. Manis, 53^7, V, 16.

Fig. 60. A section through the pair of somites, showing a large
cylindrical notochord. The hypoblast is somewhat thinner
beneath it.
 X 340-

HuBRECHT-Lab. Cat. N°. Manis, ^I, 15.

Fig. 61. A section through the 1pair of somites. The notochord
is not yet quite separated from the hypoblast. X 340-

HuBRECHT-Lab. Cat. N°. Manis, 53 II, 13-

F/gs, 62_.67. Reconstructed sagittal sections through the mid-

axis of the hinder part of the chorda-anlage ofthe embryos N\\

64, 32, 89, 31, 68, and 53. , t

The circumference. of the somites, projected on the mid-

sagittal plane is indicated by a---line, the upper face

of the medullary groove, projected in the same way, marked

l^y ^___line. Primitive streak and medulla are dotted, the

chorda-anlage hatched and the hypoblast is black. Tlie sections
described are indicated. All figs.
X 67, except fig. 68 (X 5^)-

Fig 62 A section through embryo N°. 64 with 8^9 pairs of somites.
Chordal plate and lieadprocess, covered by hypoblast, are visible.
In tlie anterior part of the primitive streak a regi6n is visible, in
which the hypoblast is fused intimately with the primitive streak.

Fig 63 A section through embryo N°. 32 with 10 -A 11 pairs of
somites A short narrow cliordal canal is visible in the head-
process. The hypoblast in the anterior part of the primitive

streak as in embryo N°. 64. ,

Fig 64. A section through embryo N°. 89 with 13 pairs of somites.
Two parts of the chordal canal are visible in the headprocess.
The anterior part opens into the archenteron. Hypoblast fused
with primitive streak somewhat more caudalwa^l.

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i02 early developmental stages of manis javanica desm.

Fig. 65. A section through embryo N°. 31 with 13 a 14 pairs of

somites. Two small parts of the chordal canal are visible
Flg. 66 A section through embryo 68 with 16 pairs of somites.
A very distinct and long chordal canal is visible, which opens
into the archenteron. \'

Fig. 67_ A section through embryo 53 with 18 pairs of somites.

The notochord has separated along a great distance from the

hypoblast, except in the region of the somites XI—XV Three

small parts of the chordal canal are to be seen in the head-
process.

-ocr page 119-

STELLINGEN.

L Een gastrula s.str. komt bij de Vertebraten niet voor.

II. De darm van alle Vertebraten wordt gevormd door vereeniging
van het archenteron en het metenteron.

III. De „entodermalen Wanderzellen", welke V. Danchakoff waar-
nam in de bloedvaten van jeugdige kip-embryonen (Anat.
Hefte, Bd. 37, 1908), zijn primaire geslachtscellen.

IV. De secundaire geslachtskenmerken staan onder den invloed

van hormonen, die afgescheiden worden door de interstitieele

cellen van de geslachtsklier (z.g. puberteitsklier).

t

y. In de elementen van den plastischen spiertonus bestaat ook de
mogelijkheid van een tonische contractie, die o.a. opgewekt
wordt door het regelende centrum.

VI. De kleine kernen, door Huhrecht in zich klievende Zoog-
diereicren
{Galeopithecus, Manis) gevonden, zi)n geen toekomstige
trophoblastkernen, zooals
Huhrf.cht meende, doch lollikel-
epitheelkernen.

VII. Oxyuris equi (Schrank) en Oxyuris mastigodes Nitsch zijn
zelfstandige soorten.

VIII. Het melksap beschermt de planten tegen diervraat.

-ocr page 120-

IX. Het verdient aanbeveling de Aristolochiaceae als Polycarpicae
te beschouwen.

X. De Rotatorien en de Nematoden kunnen van een zelfden stam-
vorm afgeleid worden.

XI. De onderzoekingen van Heribert Nilsson (Hereditas, Bd. I,
1920) maken het zeer waarschijnlijk, dat de wetten van Mendel
ook voor Oenothera Lamarckiana geldig zijn.

XII. De Monograptidae zijn verwant met Rhabdopleura (A. Sche-
potieff.
Neues Jahrbuch für Mineralogie, Bd. II, 1905).

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