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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


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