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(magnified about ten times). Only the formative yelk (the tread) is
shown in these six figures (A to F), because cleavage only takes place
in this. The much larger food-yelk, which does not share in the
cleavage, is left out and merely indicated by the dark ring without.)

The invagination or the folding inwards of the bird-blastula takes
place in this case also at the hinder pole of the subsequent chief
axis, in the middle of the hind border of the round germinal disk
(Figure 1.59 s). At this spot we have the most brisk cleavage of the
cells; hence the cells are more numerous and smaller here than in the
fore-half of the germinal disk. The border-swelling or thick edge of
the disk is less clear but whiter behind, and is more sharply
separated from contiguous parts. In the middle of its hind border
there is a white, crescent-shaped groove - Koller's sickle-groove (Fig
1.59 s); a small projecting process in the centre of it is called the
sickle-knob (sk). This important cleft is the primitive mouth, which
was described for a long time as the "primitive groove." If we make a
vertical section through this part, we see that a flat and broad cleft
stretches under the germinal disk forwards from the primitive mouth;
this is the primitive gut (Figure 1.60 ud). Its roof or dorsal wall is
formed by the folded upper part of the blastula, and its floor or
ventral wall by the white yelk (wd), in which a number of yelk-nuclei
(dk) are distributed. There is a brisk multiplication of these at the
edge of the germinal disk, especially in the neighbourhood of the
sickle-shaped primitive mouth.

We learn from sections through later stages of this discoid
bird-gastrula that the primitive gut-cavity, extending forward from
the primitive mouth as a flat pouch, undermines the whole region of
the round flat lens-shaped blastula (Figure 1.61 ud). At the same
time, the segmentation-cavity gradually disappears altogether, the
folded inner germinal layer (ik) placing itself from underneath on the
overlying outer germinal layer (ak). The typical process of
invagination, though greatly disguised, can thus be clearly seen in
this case, as Goette and Rauber, and more recently Duval (Figure
1.61), have shown.

(FIGURE 1.58. Vertical section of the blastula of a hen
(discoblastula). fh segmentation-cavity, dw dorsal wall of same, vw
ventral wall, passing directly into the white yelk (wd) (From Duval.)

FIGURE 1.59. The germinal disk of the hen's ovum at the beginning of
gastrulation; A before
incubation, B in the first hour of incubation. (From Koller.) ks
germinal-disk, V its fore and H its hind border; es embryonic shield,
s sickle-groove, sk sickle knob, d yelk.

FIGURE 1.60. Longitudinal section of the germinal disk of a siskin
(discogastrula). (From Duval.) ud primitive gut, vl, hl fore and hind
lips of the primitive mouth (or sickle-edge); ak outer germinal layer,
ik inner germinal layer, dk yelk-nuclei, wd white yelk.

FIGURE 1.61. Longitudinal section of the discoid gastrula of the
nightingale. (From Duval.) ud primitive gut, vl, hl fore and hind lips
of the primitive mouth; ak, ik outer and inner germinal layers; vr
fore-border of the discogastrula.)

The older embryologists (Pander, Baer, Remak), and, in recent times
especially, His, Kolliker, and others, said that the two primary
germinal layers of the hen's ovum - the oldest and most frequent
subject of observation! - arose by horizontal cleavage of a simple
germinal disk. In opposition to this accepted view, I affirmed in my
Gastraea Theory (1873) that the discoid bird-gastrula, like that of
all other vertebrates, is formed by folding (or invagination), and
that this typical process is merely altered in a peculiar way and
disguised by the immense accumulation of food-yelk and the flat
spreading of the discoid blastula at one part of its surface. I
endeavoured to establish this view by the derivation of the
vertebrates from one source, and especially by proving that the birds
descend from the reptiles, and these from the amphibia. If this is
correct, the discoid gastrula of the amniotes must have been formed by
the folding-in of a hollow blastula, as has been shown by Remak and
Rusconi of the discoid gastrula of the amphibia, their direct
ancestors. The accurate and extremely careful observations of the
authors I have mentioned (Goette, Rauber, and Duval) have decisively
proved this recently for the birds; and the same has been done for the
reptiles by the fine studies of Kupffer, Beneke, Wenkebach, and
others. In the shield-shaped germinal disk of the lizard (Figure
1.62), the crocodile, the tortoise, and other reptiles, we find in the
middle of the hind border (at the same spot as the sickle groove in
the bird) a transverse furrow (u), which leads into a flat,
pouch-like, blind sac, the primitive gut. The fore (dorsal) and hind
(ventral) lips of the transverse furrow correspond exactly to the lips
of the primitive mouth (or sickle-groove) in the birds.

(FIGURE 1.62. Germinal disk of the lizard (Lacerta agilis). (From
Kupffer.) u primitive mouth, s sickle, es embryonic shield, hf and df
light and dark germinative area.)

The gastrulation of the mammals must be derived from this special
embryonic development of the reptiles and birds. This latest and most
advanced class of the vertebrates has, as we shall see afterwards,
evolved at a comparatively recent date from an older group of
reptiles; and all these amniotes must have come originally from a
common stem-form. Hence the distinctive embryonic process of the
mammal must have arisen by cenogenetic modifications from the older
form of gastrulation of the reptiles and birds. Until we admit this
thesis we cannot understand the formation of the germinal layers in
the mammal, and therefore in man.

I first advanced this fundamental principle in my essay On the
Gastrulation of Mammals (1877), and sought to show in this way that I
assumed a gradual degeneration of the food-yelk and the yelk-sac on
the way from the proreptiles to the mammals. "The cenogenetic process
of adaptation," I said, "which has occasioned the atrophy of the
rudimentary yelk-sac of the mammal, is perfectly clear. It is due to
the fact that the young of the mammal, whose ancestors were certainly
oviparous, now remain a long time in the womb. As the great store of
food-yelk, which the oviparous ancestors gave to the egg, became
superfluous in their descendants owing to the long carrying in the
womb, and the maternal blood in the wall of the uterus made itself the
chief source of nourishment, the now useless yelk-sac was bound to
atrophy by embryonic adaptation."

My opinion met with little approval at the time; it was vehemently
attacked by Kolliker, Hensen, and His in particular. However, it has
been gradually accepted, and has recently been firmly established by a
large number of excellent studies of mammal gastrulation, especially
by Edward Van Beneden's studies of the rabbit and bat, Selenka's on
the marsupials and rodents, Heape's and Lieberkuhn's on the mole,
Kupffer and Keibel's on the rodents, Bonnet's on the ruminants, etc.
From the general comparative point of view, Carl Rabl in his theory of
the mesoderm, Oscar Hertwig in the latest edition of his Manual
(1902), and Hubrecht in his Studies in Mammalian Embryology (1891),
have supported the opinion, and sought to derive the peculiarly
modified gastrulation of the mammal from that of the reptile.

(FIGURE 1.63. Ovum of the opossum (Didelphys) divided into four. (From
Selenka.) b the four segmentation-cells, r directive body, c
unnucleated coagulated matter, p, albumin-membrane.)

In the meantime (1884) the studies of Wilhelm Haacke and Caldwell
provided a proof of the long-suspected and very interesting fact, that
the lowest mammals, the monotremes, LAY EGGS, like the birds and
reptiles, and are not viviparous like the other mammals. Although the
gastrulation of the monotremes was not really known until studied by
Richard Semon in 1894, there could be little doubt, in view of the
great size of their food-yelk, that their ovum-segmentation was
discoid, and led to the formation of a sickle-mouthed discogastrula,
as in the case of the reptiles and birds. Hence I had, in 1875 (in my
essay on The Gastrula and Ovum-segmentation of Animals), counted the
monotremes among the discoblastic vertebrates. This hypothesis was
established as a fact nineteen years afterwards by the careful
observations of Semon; he gave in the second volume of his great work,
Zoological Journeys in Australia (1894), the first description and
correct explanation of the discoid gastrulation of the monotremes. The
fertilised ova of the two living monotremes (Echidna and
Ornithorhynchus) are balls of one-fifth of an inch in diameter,
enclosed in a stiff shell; but they grow considerably during
development, so that when laid the egg is three times as large. The
structure of the plentiful yelk, and especially the relation of the
yellow and the white yelk, are just the same as in the reptiles and
birds. As with these, partial cleavage takes place at a spot on the
surface at which the small formative yelk and the nucleus it encloses
are found. First is formed a lens-shaped circular germinal disk. This
is made up of several strata of cells, but it spreads over the
yelk-ball, and thus becomes a one-layered blastula. If we then imagine
the yelk it contains to be dissolved and replaced by a clear liquid,
we have the characteristic blastula of the higher mammals. In these
the gastrulation proceeds in two phases, as Semon rightly observes:
firstly, formation of the entoderm by cleavage at the centre and
further growth at the edge; secondly, invagination. In the monotremes
more primitive conditions have been retained better than in the
reptiles and birds. In the latter, before the commencement of the
gastrula-folding, we have, at least at the periphery, a two-layered
embryo forming from the cleavage. But in the monotremes the formation
of the cenogenetic entoderm does not precede the invagination; hence
in this case the construction of the germinal layers is less modified
than in the other amniota.

The marsupials, a second sub-class, come next to the oviparous
monotremes, the oldest of the mammals. But as in their case the
food-yelk is already atrophied, and the little ovum develops within
the mother's body, the partial cleavage has been reconverted into
total. One section of the marsupials still show points of agreement
with the monotremes, while another section of them, according to the
splendid investigations of Selenka, form a connecting-link between
these and the placentals.

(FIGURE 1.64. Blastula of the opossum (Didelphys). (From Selenka.) a
animal pole of the blastula, v vegetal pole, en mother-cell of the
entoderm, ex ectodermic cells, s spermia, ib unnucleated yelk-balls
(remainder of the food-yelk), p albumin membrane.)

The fertilised ovum of the opossum (Didelphys) divides, according to
Selenka, first into two, then four, then eight equal cells; hence the
segmentation is at first equal or homogeneous. But in the course of
the cleavage a larger cell, distinguished by its less clear plasm and
its containing more yelk-granules (the mother cell of the entoderm,
Figure 1.64 en), separates from the others; the latter multiply more
rapidly than the former. As, further, a quantity of fluid gathers in
the morula, we get a round blastula, the wall of which is of varying
thickness, like that of the amphioxus (Figure 1.38 E) and the amphibia
(Figure 1.45). The upper or animal hemisphere is formed of a large
number of small cells; the lower or vegetal hemisphere of a small
number of large cells. One of the latter, distinguished by its size
(Figure 1.64 en), lies at the vegetal pole of the blastula-axis, at
the point where the primitive mouth afterwards appears. This is the
mother-cell of the entoderm; it now begins to multiply by cleavage,
and the daughter-cells (Figure 1.65 i) spread out from this spot over
the inner surface of the blastula, though at first only over the
vegetal hemisphere. The less clear entodermic cells (i) are
distinguished at first by their rounder shape and darker nuclei from
the higher, clearer, and longer entodermic cells (e), afterwards both
are greatly flattened, the inner blastodermic cells more than the
outer.

(FIGURE 1.65. Blastula of the opossum (Didelphys) at the beginning of
gastrulation. (From Selenka.) e ectoderm, i entoderm; a animal pole, u
primitive mouth at the vegetal pole, f segmentation-cavity, d
unnucleated yelk-balls (relics of the reduced food-yelk), c nucleated
curd (without yelk-granules).

FIGURE 1.66. Oval gastrula of the opossum (Didelphys), about eight
hours old. (From Selenka) (external view).)

The unnucleated yelk-balls and curd (Figure 1.65 d) that we find in
the fluid of the blastula in these marsupials are very remarkable;
they are the relics of the atrophied food-yelk, which was developed in
their ancestors, the monotremes, and in the reptiles.

In the further course of the gastrulation of the opossum the oval
shape of the gastrula (Figure 1.66) gradually changes into globular, a
larger quantity of fluid accumulating in the vesicle. At the same
time, the entoderm spreads further and further over the inner surface
of the ectoderm (e). A globular vesicle is formed, the wall of which
consists of two thin simple strata of cells; the cells of the outer
germinal layer are rounder, and those of the inner layer flatter. In
the region of the primitive mouth (p) the cells are less flattened,
and multiply briskly. From this point - from the hind (ventral) lip of
the primitive mouth, which extends in a central cleft, the primitive
groove - the construction of the mesoderm proceeds.

Gastrulation is still more modified and curtailed cenogenetically in
the placentals than in the marsupials. It was first accurately known
to us by the distinguished investigations of Edward Van Beneden in
1875, the first object of study being the ovum of the rabbit. But as
man also belongs to this sub-class, and as his as yet unstudied
gastrulation cannot be materially different from that of the other
placentals, it merits the closest attention. We have, in the first
place, the peculiar feature that the two first segmentation-cells that
proceed from the cleavage of the fertilised ovum (Figure 1.68) are of
different sizes and natures; the difference is sometimes greater,
sometimes less (Figure 1.69). One of these first daughter-cells of the
ovum is a little larger, clearer, and more transparent than the other.
Further, the smaller cell takes a colour in carmine, osmium, etc.,
more strongly than the larger. By repeated cleavage of it a morula is
formed, and from this a blastula, which changes in a very
characteristic way into the greatly modified gastrula. When the number
of the segmentation-cells in the mammal embryo has reached ninety-six
(in the rabbit, about seventy hours after impregnation) the foetus
assumes a form very like the archigastrula (Figure 1.72). The
spherical embryo consists of a central mass of thirty-two soft, round
cells with dark nuclei, which are flattened into polygonal shape by
mutual pressure, and colour dark-brown with osmic acid (Figure 1.72
i). This dark central group of cells is surrounded by a lighter
spherical membrane, consisting of sixty-four cube-shaped, small, and
fine-grained cells which lie close together in a single stratum, and
only colour slightly in osmic acid (Figure 1.72 e). The authors who
regard this embryonic form as the primary gastrula of the placental
conceive the outer layer as the ectoderm and the inner as the
entoderm. The entodermic membrane is only interrupted at one spot,
one, two, or three of the ectodermic cells being loose there. These
form the yelk-stopper, and fill up the mouth of the gastrula (a). The
central primitive gut-cavity (d) is full of entodermic cells. The
uni-axial type of the mammal gastrula is accentuated in this way.
However, opinions still differ considerably as to the real nature of
this "provisional gastrula" of the placental and its relation to the
blastula into which it is converted.

As the gastrulation proceeds a large spherical blastula is formed from
this peculiar solid amphigastrula of the placental, as we saw in the
case of the marsupial. The accumulation of fluid in the solid gastrula
(Figure 1.73 A) leads to the formation of an eccentric cavity, the
group of the darker entodermic cells (hy) remaining directly attached
at one spot with the round enveloping stratum of the lighter
ectodermic cells (ep). This spot corresponds to the original primitive
mouth (prostoma or blastoporus). From this important spot the inner
germinal layer spreads all round on the inner surface of the outer
layer, the cell-stratum of which forms the wall of the hollow sphere;
the extension proceeds from the vegetal towards the animal pole.

(FIGURE 1.67. Longitudinal section through the oval gastrula of the
opossum (Figure 1.69). (From Selenka.) p primitive mouth, e ectoderm,
i entoderm, d yelk remains in the primitive gut-cavity (u).)

The cenogenetic gastrulation of the placental has been greatly
modified by secondary adaptation in the various groups of this most
advanced and youngest sub-class of the mammals. Thus, for instance, we
find in many of the rodents (guinea-pigs, mice, etc.) APPARENTLY a
temporary inversion of the two germinal layers. This is due to a
folding of the blastodermic wall by what is called the "girder," a
plug-shaped growth of Rauber's "roof-layer." It is a thin layer of
flat epithelial cells, that is freed from the surface of the
blastoderm in some of the rodents; it has no more significance in
connection with the general course of placental gastrulation than the
conspicuous departure from the usual globular shape in the blastula of
some of the ungulates. In some pigs and ruminants it grows into a
thread-like, long and thin tube.

(FIGURE 1.68. Stem-cell of the mammal ovum (from the rabbit). k
stem-nucleus, n nuclear corpuscle, p protoplasm of the stem-cell, z
modified zona pellucida, h outer albuminous membrane, s dead
sperm-cells.

FIGURE 1.69. Incipient cleavage of the mammal ovum (from the rabbit).
The stem-cell has divided into two unequal cells, one lighter (e) and
one darker (i). z zona pellucida, h outer albuminous membrane, s dead
sperm-cell.

FIGURE 1.70. The first four segmentation-cells of the mammal ovum
(from the rabbit). e the two larger (and lighter) cells, i the two
smaller (and darker) cells, z zona pellucida, h outer albuminous
membrane.

FIGURE 1.71. Mammal ovum with eight segmentation-cells (from the
rabbit). e four larger and lighter cells, i four smaller and darker
cells, z zona pellucida, h outer albuminous membrane.)

Thus the gastrulation of the placentals, which diverges most from that
of the amphioxus, the primitive form, is reduced to the original type,
the invagination of a modified blastula. Its chief peculiarity is that
the folded part of the blastoderm does not form a completely closed
(only open at the primitive mouth) blind sac, as is usual; but this
blind sac has a wide opening at the ventral curve (opposite to the
dorsal mouth); and through this opening the primitive gut communicates
from the first with the embryonic cavity of the blastula. The folded
crest-shaped entoderm grows with a free circular border on the inner
surface of the entoderm towards the vegetal pole; when it has reached
this, and the inner surface of the blastula is completely grown over,
the primitive gut is closed. This remarkable direct transition of the
primitive gut-cavity into the segmentation-cavity is explained simply
by the assumption that in most of the mammals the yelk-mass, which is
still possessed by the oldest forms of the class (the monotremes) and
their ancestors (the reptiles), is atrophied. This proves the
essential unity of gastrulation in all the vertebrates, in spite of
the striking differences in the various classes.

In order to complete our consideration of the important processes of
segmentation and gastrulation, we will, in conclusion, cast a brief
glance at the fourth chief type - superficial segmentation. In the
vertebrates this form is not found at all. But it plays the chief part
in the large stem of the articulates - the insects, spiders, myriapods,
and crabs. The distinctive form of gastrula that comes of it is the
"vesicular gastrula" (Perigastrula).

In the ova which undergo this superficial cleavage the formative yelk
is sharply divided from the nutritive yelk, as in the preceding cases
of the ova of birds, reptiles, fishes, etc.; the formative yelk alone
undergoes cleavage. But while in the ova with discoid gastrulation the
formative yelk is not in the centre, but at one pole of the uni-axial
ovum, and the food-yelk gathered at the other pole, in the ova with
superficial cleavage we find the formative yelk spread over the whole
surface of the ovum; it encloses spherically the food-yelk, which is
accumulated in the middle of the ova. As the segmentation only affects
the former and not the latter, it is bound to be entirely
"superficial"; the store of food in the middle is quite untouched by
it. As a rule, it proceeds in regular geometrical progression. In the
end the whole of the formative yelk divides into a number of small and
homogeneous cells, which lie close together in a single stratum on the
entire surface of the ovum, and form a superficial blastoderm. This
blastoderm is a simple, completely closed vesicle, the internal cavity
of which is entirely full of food-yelk. This real blastula only
differs from that of the primitive ova in its chemical composition. In
the latter the content is water or a watery jelly; in the former it is
a thick mixture, rich in food-yelk, of albuminous and fatty
substances. As this quantity of food-yelk fills the centre of the ovum
before cleavage begins, there is no difference in this respect between
the morula and the blastula. The two stages rather agree in this.

When the blastula is fully formed, we have again in this case the
important folding or invagination that determines gastrulation. The
space between the skin-layer and the gut-layer (the remainder of the
segmentation-cavity) remains full of food-yelk, which is gradually
used up. This is the only material difference between our vesicular
gastrula (perigastrula) and the original form of the bell-gastrula
(archigastrula). Clearly the one has been developed from the other in
the course of time, owing to the accumulation of food-yelk in the
centre of the ovum.* (* On the reduction of all forms of gastrulation
to the original palingenetic form see especially the lucid treatment
of the subject in Arnold Lang's Manual of Comparative Anatomy (1888),
Part 1.)

We must count it an important advance that we are thus in a position
to reduce all the various embryonic phenomena in the different groups
of animals to these four principal forms of segmentation and
gastrulation. Of these four forms we must regard one only as the
original palingenetic, and the other three as cenogenetic and
derivative. The unequal, the discoid, and the superficial segmentation
have all clearly arisen by secondary adaptation from the primary
segmentation; and the chief cause of their development has been the
gradual formation of the food-yelk, and the increasing antithesis
between animal and vegetal halves of the ovum, or between ectoderm
(skin-layer) and entoderm (gut-layer).

(FIGURE 1.72. Gastrula of the placental mammal (epigastrula from the
rabbit), longitudinal section through the axis. e ectodermic cells
(sixty-four, lighter and smaller), i entodermic cells (thirty-two,
darker and larger), d central entodermic cell, filling the primitive
gut-cavity, o peripheral entodermic cell, stopping up the opening of
the primitive mouth (yelk-stopper in the Rusconian anus).)

(FIGURE 1.73. Gastrula of the rabbit. A as a solid, spherical cluster
of cells, B changing into the embryonic vesicle, bp primitive mouth,
ep ectoderm, hy entoderm.)

The numbers of careful studies of animal gastrulation that have been
made in the last few decades have completely established the views I
have expounded, and which I first advanced in the years 1872 to 1876.
For a time they were greatly disputed by many embryologists. Some said
that the original embryonic form of the metazoa was not the gastrula,
but the "planula" - a double-walled vesicle with closed cavity and
without mouth-aperture; the latter was supposed to pierce through
gradually. It was afterwards shown that this planula (found in several


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