(* Cf. Balfour's Manual of Comparative Embryology volume 2; Theodore
Morgan's The Development of the Frog's Egg.)
The remarkable processes of gastrulation, ovum-segmentation, and
formation of germinal layers present a most conspicuous variety. There
is to-day only the lowest of the vertebrates, the amphioxus, that
exhibits the original form of those processes, or the palingenetic
gastrulation which we have considered in the preceding chapter, and
which culminates in the formation of the archigastrula (Figure 1.38).
In all other extant vertebrates these fundamental processes have been
more or less modified by adaptation to the conditions of embryonic
development (especially by changes in the food-yelk); they exhibit
various cenogenetic types of the formation of germinal layers.
However, the different classes vary considerably from each other. In
order to grasp the unity that underlies the manifold differences in
these phenomena and their historical connection, it is necessary to
bear in mind always the unity of the vertebrate-stem. This
"phylogenetic unity," which I developed in my General Morphology in
1866, is now generally admitted. All impartial zoologists agree to-day
that all the vertebrates, from the amphioxus and the fishes to the ape
and man, descend from a common ancestor, "the primitive vertebrate."
Hence the embryonic processes, by which each individual vertebrate is
developed, must also be capable of being reduced to one common type of
embryonic development; and this primitive type is most certainly
exhibited to-day by the amphioxus.
It must, therefore, be our next task to make a comparative study of
the various forms of vertebrate gastrulation, and trace them backwards
to that of the lancelet. Broadly speaking, they fall first into two
groups: the older cyclostoma, the earliest fishes, most of the
amphibia, and the viviparous mammals, have holoblastic ova--that is to
say, ova with total, unequal segmentation; while the younger
cyclostoma, most of the fishes, the cephalopods, reptiles, birds, and
monotremes, have meroblastic ova, or ova with partial discoid
segmentation. A closer study of them shows, however, that these two
groups do not present a natural unity, and that the historical
relations between their several divisions are very complicated. In
order to understand them properly, we must first consider the various
modifications of gastrulation in these classes. We may begin with that
of the amphibia.
The most suitable and most available objects of study in this class
are the eggs of our indigenous amphibia, the tailless frogs and toads,
and the tailed salamander. In spring they are to be found in clusters
in every pond, and careful examination of the ova with a lens is
sufficient to show at least the external features of the segmentation.
In order to understand the whole process rightly and follow the
formation of the germinal layers and the gastrula, the ova of the frog
and salamander must be carefully hardened; then the thinnest possible
sections must be made of the hardened ova with the microtome, and the
tinted sections must be very closely compared under a powerful
microscope.
The ova of the frog or toad are globular in shape, about the twelfth
of an inch in diameter, and are clustered in jelly-like masses, which
are lumped together in the case of the frog, but form long strings in
the case of the toad. When we examine the opaque, grey, brown, or
blackish ova closely, we find that the upper half is darker than the
lower. The middle of the upper half is in many species black, while
the middle of the lower half is white.* (* The colouring of the eggs
of the amphibia is caused by the accumulation of dark-colouring matter
at the animal pole of the ovum. In consequence of this, the animal
cells of the ectoderm are darker than the vegetal cells of the
entoderm. We find the reverse of this in the case of most animals, the
protoplasm of the entoderm cells being usually darker and
coarser-grained.) In this way we get a definite axis of the ovum with
two poles. To give a clear idea of the segmentation of this ovum, it
is best to compare it with a globe, on the surface of which are marked
the various parallels of longitude and latitude. The superficial
dividing lines between the different cells, which come from the
repeated segmentation of the ovum, look like deep furrows on the
surface, and hence the whole process has been given the name of
furcation. In reality, however, this "furcation," which was formerly
regarded as a very mysterious process, is nothing but the familiar,
repeated cell-segmentation. Hence also the segmentation-cells which
result from it are real cells.
(FIGURE 1.40. The cleavage of the frog's ovum (magnified ten times). A
stem-cell. B the first two segmentation-cells. C four cells. D eight
cells (4 animal and 4 vegetative). E twelve cells (8 animal and 4
vegetative). F sixteen cells (8 animal and 8 vegetative). G
twenty-four cells (16 animal and 8 vegetative). H thirty-two cells. I
forty-eight cells. K sixty-four cells. L ninety-six cells. M 160 cells
(128 animal and 32 vegetative).
(FIGURES 1.41 TO 1.44. Four vertical sections of the fertilised ovum
of the toad, in four successive stages of development. The letters
have the same meaning throughout: F segmentation-cavity. D covering of
same (D dorsal half of the embryo, P ventral half). P yelk-stopper
(white round field at the lower pole). Z yelk-cells of the entoderm
(Remak's "glandular embryo"). N primitive gut cavity (progaster or
Rusconian alimentary cavity). The primitive mouth (prostoma) is closed
by the yelk-stopper, P. s partition between the primitive gut cavity
(N) and the segmentation cavity (F). k k apostrophe, section of the
large circular lip-border of the primitive mouth (the Rusconian anus).
The line of dots between k and k apostrophe indicates the earlier
connection of the yelk-stopper (P) with the central mass of the
yelk-cells (Z). In Figure 1.44 the ovum has turned 90 degrees, so that
the back of the embryo is uppermost and the ventral side down. (From
Stricker.)).
The unequal segmentation which we observe in the ovum of the amphibia
has the special feature of beginning at the upper and darker pole (the
north pole of the terrestrial globe in our illustration), and slowly
advancing towards the lower and brighter pole (the south pole). Also
the upper and darker hemisphere remains in this position throughout
the course of the segmentation, and its cells multiply much more
briskly. Hence the cells of the lower hemisphere are found to be
larger and less numerous. The cleavage of the stem-cell (Figure 1.40
A) begins with the formation of a complete furrow, which starts from
the north pole and reaches to the south (B). An hour later a second
furrow arises in the same way, and this cuts the first at a right
angle (Figure 1.40 C). The ovum is thus divided into four equal parts.
Each of these four "segmentation cells" has an upper and darker and a
lower, brighter half. A few hours later a third furrow appears,
vertically to the first two (Figure 1.40 D). The globular germ now
consists of eight cells, four smaller ones above (northern) and four
larger ones below (southern). Next, each of the four upper ones
divides into two halves by a cleavage beginning from the north pole,
so that we now have eight above and four below (Figure 1.40 E). Later,
the four new longitudinal divisions extend gradually to the lower
cells, and the number rises from twelve to sixteen (F). Then a second
circular furrow appears, parallel to the first, and nearer to the
north pole, so that we may compare it to the north polar circle. In
this way we get twenty-four segmentation-cells--sixteen upper,
smaller, and darker ones, and eight smaller and brighter ones below
(G). Soon, however, the latter also sub-divide into sixteen, a third
or "meridian of latitude" appearing, this time in the southern
hemisphere: this makes thirty-two cells altogether (H). Then eight new
longitudinal lines are formed at the north pole, and these proceed to
divide, first the darker cells above and afterwards the lighter
southern cells, and finally reach the south pole. In this way we get
in succession forty, forty-eight, fifty-six, and at last sixty-four
cells (I, K). In the meantime, the two hemispheres differ more and
more from each other. Whereas the sluggish lower hemisphere long
remains at thirty-two cells, the lively northern hemisphere briskly
sub-divides twice, producing first sixty-four and then 128 cells (L,
M). Thus we reach a stage in which we count on the surface of the ovum
128 small cells in the upper half and thirty-two large ones in the
lower half, or 160 altogether. The dissimilarity of the two halves
increases: while the northern breaks up into a great number of small
cells, the southern consists of a much smaller number of larger cells.
Finally, the dark cells of the upper half grow almost over the surface
of the ovum, leaving only a small circular spot at the south pole,
where the large and clear cells of the lower half are visible. This
white region at the south pole corresponds, as we shall see
afterwards, to the primitive mouth of the gastrula. The whole mass of
the inner and larger and clearer cells (including the white polar
region) belongs to the entoderm or ventral layer. The outer envelope
of dark smaller cells forms the ectoderm or skin-layer.
In the meantime, a large cavity, full of fluid, has been formed within
the globular body--the segmentation-cavity or embryonic cavity
(blastocoel, Figures 1.41 to 1.44 F). It extends considerably as the
cleavage proceeds, and afterwards assumes an almost semi-circular form
(Figure 1.41 F). The frog-embryo now represents a modified embryonic
vesicle or blastula, with hollow animal half and solid vegetal half.
Now a second, narrower but longer, cavity arises by a process of
folding at the lower pole, and by the falling away from each other of
the white entoderm-cells (Figures 1.41 to 1.44 N). This is the
primitive gut-cavity or the gastric cavity of the gastrula, progaster
or archenteron. It was first observed in the ovum of the amphibia by
Rusconi, and so called the Rusconian cavity. The reason of its
peculiar narrowness here is that it is, for the most part, full of
yelk-cells of the entoderm. These also stop up the whole of the wide
opening of the primitive mouth, and form what is known as the
"yelk-stopper," which is seen freely at the white round spot at the
south pole (P). Around it the ectoderm is much thicker, and forms the
border of the primitive mouth, the most important part of the embryo
(Figure 1.44 k, k apostrophe). Soon the primitive gut-cavity stretches
further and further at the expense of the segmentation-cavity (F),
until at last the latter disappears altogether. The two cavities are
only separated by a thin partition (Figure 1.43 s). With the formation
of the primitive gut our frog-embryo has reached the gastrula stage,
though it is clear that this cenogenetic amphibian gastrula is very
different from the real palingenetic gastrula we have considered
(Figures 1.30 to 1.36).
In the growth of this hooded gastrula we cannot sharply mark off the
various stages which we distinguish successively in the bell-gastrula
as morula and gastrula. Nevertheless, it is not difficult to reduce
the whole cenogenetic or disturbed development of this amphigastrula
to the true palingenetic formation of the archigastrula of the
amphioxus.
(FIGURE 1.45. Blastula of the water-salamander (Triton). fh
segmentation-cavity, dz yelk-cells, rz border-zone. (From Hertwig.)
FIGURE 1.46. Embryonic vesicle of triton (blastula), outer view, with
the transverse fold of the primitive mouth (u). (From Hertwig.)
FIGURE 1.47. Sagittal section of a hooded-embryo (depula) of triton
(blastula at the commencement of gastrulation). ak outer germinal
layer, ik inner germinal layer, fh segmentation-cavity, ud primitive
gut, u primitive mouth, dl and vl dorsal and ventral lips of the
mouth, dz yelk-cells. (From Hertwig.))
This reduction becomes easier if, after considering the gastrulation
of the tailless amphibia (frogs and toads), we glance for a moment at
that of the tailed amphibia, the salamanders. In some of the latter,
that have only recently been carefully studied, and that are
phylogenetically older, the process is much simpler and clearer than
is the case with the former and longer known. Our common
water-salamander (Triton taeniatus) is a particularly good subject for
observation. Its nutritive yelk is much smaller and its formative yelk
less obscured with black pigment-cells than in the case of the frog;
and its gastrulation has better retained the original palingenetic
character. It was first described by Scott and Osborn (1879), and
Oscar Hertwig especially made a careful study of it (1881), and
rightly pointed out its great importance in helping us to understand
the vertebrate development. Its globular blastula (Figure 1.45)
consists of loosely-aggregated, yelk-filled entodermic cells or
yelk-cells (dz) in the lower vegetal half; the upper, animal half
encloses the hemispherical segmentation-cavity (fh), the curved roof
of which is formed of two or three strata of small ectodermic cells.
At the point where the latter pass into the former (at the equator of
the globular vesicle) we have the border zone (rz). The folding which
leads to the formation of the gastrula takes place at a spot in this
border zone, the primitive mouth (Figure 1.46 u).
Unequal segmentation takes place in some of the cyclostoma and in the
oldest fishes in just the same way as in most of the amphibia. Among
the cyclostoma ("round-mouthed") the familiar lampreys are
particularly interesting. In respect of organisation and development
they are half-way between the acrania (lancelet) and the lowest real
fishes (Selachii); hence I divided the group of the cyclostoma in 1886
from the real fishes with which they were formerly associated, and
formed of them a special class of vertebrates. The ovum-segmentation
in our common river-lamprey (Petromyzon fluviatilis) was described by
Max Schultze in 1856, and afterwards by Scott (1882) and Goette
(1890).
Unequal total segmentation follows the same lines in the oldest
fishes, the selachii and ganoids, which are directly descended from
the cyclostoma. The primitive fishes (Selachii), which we must regard
as the ancestral group of the true fishes, were generally considered,
until a short time ago, to be discoblastic. It was not until the
beginning of the twentieth century that Bashford Dean made the
important discovery in Japan that one of the oldest living fishes of
the shark type (Cestracion japonicus) has the same total unequal
segmentation as the amphiblastic plated fishes (ganoides).* (*
Bashford Dean, Holoblastic Cleavage in the Egg of a Shark, Cestracion
japonicus Macleay. Annotationes zoologicae japonenses, volume 4 Tokio
1901.) This is particularly interesting in connection with our
subject, because the few remaining survivors of this division, which
was so numerous in paleozoic times, exhibit three different types of
gastrulation. The oldest and most conservative forms of the modern
ganoids are the scaly sturgeons (Sturiones), plated fishes of great
evolutionary importance, the eggs of which are eaten as caviar; their
cleavage is not essentially different from that of the lampreys and
the amphibia. On the other hand, the most modern of the plated fishes,
the beautifully scaled bony pike of the North American rivers
(Lepidosteus), approaches the osseous fishes, and is discoblastic like
them. A third genus (Amia) is midway between the sturgeons and the
latter.
(FIGURE 1.48. Sagittal section of the gastrula of the water-salamander
(Triton). (From Hertwig.) Letters as in Figure 1.47; except--p
yelk-stopper, mk beginning of the middle germinal layer.)
The group of the lung-fishes (Dipneusta or Dipnoi) is closely
connected with the older ganoids. In respect of their whole
organisation they are midway between the gill-breathing fishes and the
lung-breathing amphibia; they share with the former the shape of the
body and limbs, and with the latter the form of the heart and lungs.
Of the older dipnoi (Paladipneusta) we have now only one specimen, the
remarkable Ceratodus of East Australia; its amphiblastic gastrulation
has been recently explained by Richard Semon (cf. Chapter 2.21). That
of the two modern dipneusta, of which Protopterus is found in Africa
and Lepidosiren in America, is not materially different. (Cf. Figure
1.51.)
(FIGURE 1.49. Ovum-segmentation of the lamprey (Petromyzon
fluviatalis), in four successive stages. The small cells of the upper
(animal) hemisphere divide much more quickly than the cells of the
lower (vegetal) hemisphere.
FIGURE 1.50. Gastrulation of the lamprey (Petromyzon fluviatilis). A
blastula, with wide embryonic cavity (blastocoel, bl), g incipient
invagination. B depula, with advanced invagination, from the primitive
mouth (g). C gastrula, with complete primitive gut: the embryonic
cavity has almost disappeared in consequence of invagination.)
All these amphiblastic vertebrates, Petromyzon and Cestracion,
Accipenser and Ceratodus, and also the salamanders and batrachia,
belong to the old, conservative groups of our stem. Their unequal
ovum-segmentation and gastrulation have many peculiarities in detail,
but can always be reduced with comparative ease to the original
cleavage and gastrulation of the lowest vertebrate, the amphioxus; and
this is little removed, as we have seen, from the very simple
archigastrula of the Sagitta and Monoxenia (see Figures 1.29 to 1.36).
All these and many other classes of animals generally agree in the
circumstance that in segmentation their ovum divides into a large
number of cells by repeated cleavage. All such ova have been called,
after Remak, "whole-cleaving" (holoblasta), because their division
into cells is complete or total.
(FIGURE 1.51. Gastrulation of ceratodus (from Semon). A and C stage
with four cells, B and D with sixteen cells. A and B are seen from
above, C and D sideways. E stage with thirty-two
cells; F blastula; G gastrula in longitudinal section. fh
segmentation-cavity. gh primitive gut or gastric cavity.)
In a great many other classes of animals this is not the case, as we
find (in the vertebrate stem) among the birds, reptiles, and most of
the fishes; among the insects and most of the spiders and crabs (of
the articulates); and the cephalopods (of the molluscs). In all these
animals the mature ovum, and the stem-cell that arises from it in
fertilisation, consist of two different and separate parts, which we
have called formative yelk and nutritive yelk. The formative yelk
alone consists of living protoplasm, and is the active, evolutionary,
and nucleated part of the ovum; this alone divides in segmentation,
and produces the numerous cells which make up the embryo. On the other
hand, the nutritive yelk is merely a passive part of the contents of
the ovum, a subordinate element which contains nutritive material
(albumin, fat, etc.), and so represents in a sense the provision-store
of the developing embryo. The latter takes a quantity of food out of
this store, and finally consumes it all. Hence the nutritive yelk is
of great indirect importance in embryonic development, though it has
no direct share in it. It either does not divide at all, or only later
on, and does not generally consist of cells. It is sometimes large and
sometimes small, but generally many times larger than the formative
yelk; and hence it is that it was formerly thought the more important
of the two. As the respective significance of these two parts of the
ovum is often wrongly described, it must be borne in mind that the
nutritive yelk is only a secondary addition to the primary cell, it is
an inner enclosure, not an external appendage. All ova that have this
independent nutritive yelk are called, after Remak,
"partially-cleaving" (meroblasta). Their segmentation is incomplete or
partial.
(FIGURE 1.52. Ovum of a deep-sea bony fish. b protoplasm of the
stem-cell, k nucleus of same, d clear globule of albumin, the
nutritive yelk, f fat-globule of same, c outer membrane of the ovum,
or ovolemma.)
There are many difficulties in the way of understanding this partial
segmentation and the gastrula that arises from it. We have only
recently succeeded, by means of comparative research, in overcoming
these difficulties, and reducing this cenogenetic form of gastrulation
to the original palingenetic type. This is comparatively easy in the
small meroblastic ova which contain little nutritive yelk--for
instance, in the marine ova of a bony fish, the development of which I
observed in 1875 at Ajaccio in Corsica. I found them joined together
in lumps of jelly, floating on the surface of the sea; and, as the
little ovula were completely transparent, I could easily follow the
development of the germ step by step. These ovula are glossy and
colourless globules of little more than the 50th of an inch. Inside a
structureless, thin, but firm membrane (ovolemma, Figure 1.52 c) we
find a large, quite clear, and transparent globule of albumin (d). At
both poles of its axis this globule has a pit-like depression. In the
pit at the upper, animal pole (which is turned downwards in the
floating ovum) there is a bi-convex lens composed of protoplasm, and
this encloses the nucleus (k); this is the formative yelk of the
stem-cell, or the germinal disk (b). The small fat-globule (f) and the
large albumin-globule (d) together form the nutritive yelk. Only the
formative yelk undergoes cleavage, the nutritive yelk not dividing at
all at first.
The segmentation of the lens-shaped formative yelk (b) proceeds quite
independently of the nutritive yelk, and in perfect geometrical order.
When the mulberry-like cluster of cells has been formed, the
border-cells of the lens separate from the rest and travel into the
yelk and the border-layer. From this the blastula is developed; the
regular bi-convex lens being converted into a disk, like a
watch-glass, with thick borders. This lies on the upper and less
curved polar surface of the nutritive yelk like the watch glass on the
yelk. Fluid gathers between the outer layer and the border, and the
segmentation-cavity is formed. The gastrula is then formed by
invagination, or a kind of turning-up of the edge of the blastoderm.
In this process the segmentation-cavity disappears.
The space underneath the entoderm corresponds to the primitive
gut-cavity, and is filled with the decreasing food-yelk (n). Thus the
formation of the gastrula of our fish is complete. In contrast to the
two chief forms of gastrula we considered previously, we give the name
of discoid gastrula (discogastrula, Figure 1.54) to this third
principal type.
Very similar to the discoid gastrulation of the bony fishes is that of
the hags or myxinoida, the remarkable cyclostomes that live
parasitically in the body-cavity of fishes, and are distinguished by
several notable peculiarities from their nearest relatives, the
lampreys. While the amphiblastic ova of the latter are small and
develop like those of the amphibia, the cucumber-shaped ova of the hag
are about an inch long, and form a discoid gastrula. Up to the present
it has only been observed in one species (Bdellostoma Stouti), by Dean
and Doflein (1898).
It is clear that the important features which distinguish the discoid
gastrula from the other chief forms we have considered are determined
by the large food-yelk. This takes no direct part in the building of
the germinal layers, and completely fills the primitive gut-cavity of
the gastrula, even protruding at the mouth-opening. If we imagine the
original bell-gastrula (Figures 1.30 to 1.36) trying to swallow a ball
of food which is much bigger than itself, it would spread out round it
in discoid shape in the attempt, just as we find to be the case here
(Figure 1.54). Hence we may derive the discoid gastrula from the
original bell-gastrula, through the intermediate stage of the hooded
gastrula. It has arisen through the accumulation of a store of
food-stuff at the vegetal pole, a "nutritive yelk" being thus formed
in contrast to the "formative yelk." Nevertheless, the gastrula is
formed here, as in the previous cases, by the folding or invagination
of the blastula. We can, therefore, reduce this cenogenetic form of
the discoid segmentation to the palingenetic form of the primitive
cleavage.
(FIGURE 1.53. Ovum-segmentation of a bony fish. A first cleavage of
the stem-cell (cytula), B division of same into four
segmentation-cells (only two visible), C the germinal disk divides
into the blastoderm (b) and the periblast (p). d nutritive yelk, f
fat-globule, c ovolemma, z space between the ovolemma and the ovum,
filled with a clear fluid.)
This reduction is tolerably easy and confident in the case of the
small ovum of our deep-sea bony fish, but it becomes difficult and
uncertain in the case of the large ova that we find in the majority of
the other fishes and in all the reptiles and birds. In these cases the
food-yelk is, in the first place, comparatively colossal, the
formative yelk being almost invisible beside it; and, in the second
place, the food-yelk contains a quantity of different elements, which
are known as "yelk-granules, yelk-globules, yelk-plates, yelk-flakes,
yelk-vesicles," and so on. Frequently these definite elements in the
yelk have been described as real cells, and it has been wrongly stated
that a portion of the embryonic body is built up from these cells.
This is by no means the case. In every case, however large it is--and
even when cell-nuclei travel into it during the cleavage of the
border--the nutritive yelk remains a dead accumulation of food, which
is taken into the gut during embryonic development and consumed by the
embryo. The latter develops solely from the living formative yelk of
the stem-cell. This is equally true of the ova of our small bony
fishes and of the colossal ova of the primitive fishes, reptiles, and
birds.
(FIGURE 1.54. Discoid gastrula (discogastrula) of a bony fish. e
ectoderm, i entoderm, w border-swelling or primitive mouth, n
albuminous globule of the nutritive yelk, f fat-globule of same, c
external membrane (ovolemma), d partition between entoderm and
ectoderm (earlier the segmentation-cavity).)
The gastrulation of the primitive fishes or selachii (sharks and rays)
has been carefully studied of late years by Ruckert, Rabl, and H.E.
Ziegler in particular, and is very important in the sense that this
group is the oldest among living fishes, and their gastrulation can be
derived directly from that of the cyclostoma by the accumulation of a
large quantity of food-yelk. The oldest sharks (Cestracion) still have
the unequal segmentation inherited from the cyclostoma. But while in
this case, as in the case of the amphibia, the small ovum completely
divides into cells in segmentation, this is no longer so in the great
majority of the selachii (or Elasmobranchii). In these the
contractility of the active protoplasm no longer suffices to break up
the huge mass of the passive deutoplasm completely into cells; this is
only possible in the upper or dorsal part, but not in the lower or
ventral section. Hence we find in the primitive fishes a blastula with
a small eccentric segmentation-cavity (Figure 1.55 b), the wall of
which varies greatly in composition. The circular border of the
germinal disk which connects the roof and floor of the
segmentation-cavity corresponds to the border-zone at the equator of
the amphibian ovum. In the middle of its hinder border we have the
beginning of the invagination of the primitive gut (Figure 1.56 ud);
it extends gradually from this spot (which corresponds to the
Rusconian anus of the amphibia) forward and around, so that the
primitive mouth becomes first crescent-shaped and then circular, and,
as it opens wider, surrounds the ball of the larger food-yelk.
Essentially different from the wide-mouthed discoid gastrula of most
of the selachii is the narrow-mouthed discoid gastrula (or
epigastrula) of the amniotes, the reptiles, birds, and monotremes;
between the two--as an intermediate stage--we have the amphigastrula
of the amphibia. The latter has developed from the amphigastrula of
the ganoids and dipneusts, whereas the discoid amniote gastrula has
been evolved from the amphibian gastrula by the addition of food-yelk.
This change of gastrulation is still found in the remarkable ophidia
(Gymnophiona, Coecilia, or Peromela), serpent-like amphibia that live
in moist soil in the tropics, and in many respects represent the
transition from the gill-breathing amphibia to the lung-breathing
reptiles. Their embryonic development has been explained by the fine
studies of the brothers Sarasin of Ichthyophis glutinosa at Ceylon
(1887), and those of August Brauer of the Hypogeophis rostrata in the
Seychelles (1897). It is only by the historical and comparative study
of these that we can understand the difficult and obscure gastrulation
of the amniotes.
The bird's egg is particularly important for our purpose, because most
of the chief studies of the development of the vertebrates are based
on observations of the hen's egg during hatching. The mammal ovum is
much more difficult to obtain and study, and for this practical and
obvious reason very rarely thoroughly investigated. But we can get
hens' eggs in any quantity at any time, and, by means of artificial
incubation, follow the development of the embryo step by step. The
bird's egg differs considerably from the tiny mammal ovum in size, a
large quantity of food-yelk accumulating within the original yelk or
the protoplasm of the ovum. This is the yellow ball which we commonly
call the yolk of the egg. In order to understand the bird's egg
aright--for it is very often quite wrongly explained--we must examine
it in its original condition, and follow it from the very beginning of
its development in the bird's ovary. We then see that the original
ovum is a quite small, naked, and simple cell with a nucleus, not
differing in either size or shape from the original ovum of the
mammals and other animals (cf. Figure 1.13 E). As in the case of all
the craniota (animals with a skull), the original or primitive ovum
(protovum) is covered with a continuous layer of small cells. This
membrane is the follicle, from which the ovum afterwards issues.
Immediately underneath it the structureless yelk-membrane is secreted
from the yelk.
(FIGURE 1.55. Longitudinal section through the blastula of a shark
(Pristiuris). (From Ruckert.) (Looked at from the left; to the right
is the hinder end, H, to the left the fore end, V.) B
segmentation-cavity, kz cells of the germinal membrane, dk
yelk-nuclei.
FIGURE 1.56. Longitudinal section of the blastula of a shark
(Pristiurus) at the beginning of gastrulation. (From Ruckert.) (Seen
from the left.) V fore end, H hind end, B segmentation-cavity, ud
first trace of the primitive gut, dk yelk-nuclei, fd fine-grained
yelk, gd coarse-grained yelk.)
The small primitive ovum of the bird begins very early to take up into
itself a quantity of food-stuff through the yelk-membrane, and work it
up into the "yellow yelk." In this way the ovum enters on its second
stage (the metovum), which is many times larger than the first, but
still only a single enlarged cell. Through the accumulation of the
store of yellow yelk within the ball of protoplasm the nucleus it
contains (the germinal vesicle) is forced to the surface of the ball.
Here it is surrounded by a small quantity of protoplasm, and with this
forms the lens-shaped formative yelk (Figure 1.15 b). This is seen on
the yellow yelk-ball, at a certain point of the surface, as a small
round white spot--the "tread" (cicatricula). From this point a
thread-like column of white nutritive yelk (d), which contains no
yellow yelk-granules, and is softer than the yellow food-yelk,
proceeds to the middle of the yellow yelk-ball, and forms there a
small central globule of white yelk (Figure 1.15 d). The whole of this
white yelk is not sharply separated from the yellow yelk, which shows
a slight trace of concentric layers in the hard-boiled egg (Figure
1.15 c). We also find in the hen's egg, when we break the shell and
take out the yelk, a round small white disk at its surface which
corresponds to the tread. But this small white "germinal disk" is now
further developed, and is really the gastrula of the chick. The body
of the chick is formed from it alone. The whole white and yellow
yelk-mass is without any significance for the formation of the embryo,
it being merely used as food by the developing chick. The clear,
glarous mass of albumin that surrounds the yellow yelk of the bird's
egg, and also the hard chalky shell, are only formed within the
oviduct round the impregnated ovum.
When the fertilisation of the bird's ovum has taken place within the
mother's body, we find in the lens-shaped stem-cell the progress of
flat, discoid segmentation (Figure 1.57). First two equal
segmentation-cells (A) are formed from the ovum. These divide into
four (B), then into eight, sixteen (C), thirty-two, sixty-four, and so
on. The cleavage of the cells is always preceded by a division of
their nuclei. The cleavage surfaces between the segmentation-cells
appear at the free surface of the tread as clefts. The first two
divisions are vertical to each other, in the form of a cross (B). Then
there are two more divisions, which cut the former at an angle of
forty-five degrees. The tread, which thus becomes the germinal disk,
now has the appearance of an eight-rayed star. A circular cleavage
next taking place round the middle, the eight triangular cells divide
into sixteen, of which eight are in the middle and eight distributed
around (C). Afterwards circular clefts and radial clefts, directed
towards the centre, alternate more or less irregularly (D, E). In most
of the amniotes the formation of concentric and radial clefts is
irregular from the very first; and so also in the hen's egg. But the
final outcome of the cleavage-process is once more the formation of a
large number of small cells of a similar nature. As in the case of the
fish-ovum, these segmentation-cells form a round, lens-shaped disk,
which corresponds to the morula, and is embedded in a small depression
of the white yelk. Between the lens-shaped disk of the morula-cells
and the underlying white yelk a small cavity is now formed by the
accumulation of fluid, as in the fishes. Thus we get the peculiar and
not easily recognisable blastula of the bird (Figure 1.58). The small
segmentation-cavity (fh) is very flat and much compressed. The upper
or dorsal wall (dw) is formed of a single layer of clear, distinctly
separated cells; this corresponds to the upper or animal hemisphere of
the triton-blastula (Figure 1.45). The lower or ventral wall of the
flat dividing space (vw) is made up of larger and darker
segmentation-cells; it corresponds to the lower or vegetal hemisphere
of the blastula of the water-salamander (Figure 1.45 dz). The nuclei
of the yelk-cells, which are in this case especially numerous at the
edge of the lens-shaped blastula, travel into the white yelk, increase
by cleavage, and contribute even to the further growth of the germinal
disk by furnishing it with food-stuff.
(FIGURE 1.57. Diagram of discoid segmentation in the bird's ovum
(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),