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we distinguish in the ova two chief elements - the active formative
yelk (protoplasm) and the passive food-yelk (deutoplasm, wrongly
spoken of as "the yelk"). In the little palingenetic ova, the
segmentation of which we have already considered, the yelk-granules
are so small and so regularly distributed in the protoplasm of the
ovum that the even and repeated cleavage is not affected by them. But
in the great majority of the animal ova the food-yelk is more or less
considerable, and is stored in a certain part of the ovum, so that
even in the unfertilised ovum the "granary" can clearly be
distinguished from the formative plasm. As a rule, the formative-yelk
(with the germinal vesicle) then usually gathers at one pole and the
food-yelk at the other. The first is the ANIMAL, and the second the
VEGETAL, pole of the vertical axis of the ovum.

(FIGURE 1.39. Gastrula of the amphioxus, seen from left side
(diagrammatic median section). (From Hatschek.) g primitive gut, u
primitive mouth, p peristomal pole-cells, i entoderm, e ectoderm, d
dorsal side, v ventral side.)

In these "telolecithal" ova, or ova with the yelk at one end (for
instance, in the cyclostoma and amphibia), the gastrulation then
usually takes place in such a way that in the cleavage of the
impregnated ovum the animal (usually the upper) half splits up more
quickly than the vegetal (lower). The contractions of the active
protoplasm, which effect this continual cleavage of the cells, meet a
greater resistance in the lower vegetal half from the passive
deutoplasm than in the upper animal half. Hence we find in the latter
more but smaller, and in the former fewer but larger, cells. The
animal cells produce the external, and the vegetal cells the internal,
germinal layer.

Although this unequal segmentation of the cyclostoma, ganoids, and
amphibia seems at first sight to differ from the original equal
segmentation (for instance, in the monoxenia, Figure 1.29), they both
have this in common, that the cleavage process throughout affects the
WHOLE cell; hence Remak called it TOTAL segmentation, and the ova in
question holoblastic, or "whole-cleaving." It is otherwise with the
second chief group of ova, which he distinguished from these as
meroblastic, or "partially-cleaving ": to this class belong the
familiar large eggs of birds and reptiles, and of most fishes. The
inert mass of the passive food-yelk is so large in these cases that
the protoplasmic contractions of the active yelk cannot effect any
further cleavage. In consequence, there is only a partial
segmentation. While the protoplasm in the animal section of the ovum
continues briskly to divide, multiplying the nuclei, the deutoplasm in
the vegetal section remains more or less undivided; it is merely
consumed as food by the forming cells. The larger the accumulation of
food, the more restricted is the process of segmentation. It may,
however, continue for some time (even after the gastrulation is more
or less complete) in the sense that the vegetal cell-nuclei
distributed in the deutoplasm slowly increase by cleavage; as each of
them is surrounded by a small quantity of protoplasm, it may
afterwards appropriate a portion of the food-yelk, and thus form a
real "yelk-cell" (merocyte). When this vegetal cell-formation
continues for a long time, after the two primary germinal layers have
been formed, it takes the name of the "after-segmentation."

The meroblastic ova are only found in the larger and more highly
developed animals, and only in those whose embryo needs a longer time
and richer nourishment within the foetal membranes. According as the
yelk-food accumulates at the centre or at the side of the ovum, we
distinguish two groups of dividing ova, periblastic and discoblastic.
In the periblastic the food-yelk is in the centre, enclosed inside the
ovum (hence they are also called "centrolecithal" ova): the formative
yelk surrounds the food-yelk, and so suffers itself a superficial
cleavage. This is found among the articulates (crabs, spiders,
insects, etc.). In the discoblastic ova the food-yelk gathers at one
side, at the vegetal or lower pole of the vertical axis, while the
nucleus of the ovum and the great bulk of the formative yelk lie at
the upper or animal pole (hence these ova are also called
"telolecithal"). In these cases the cleavage of the ovum begins at the
upper pole, and leads to the formation of a dorsal discoid embryo.
This is the case with all meroblastic vertebrates, most fishes, the
reptiles and birds, and the oviparous mammals (the monotremes).

The gastrulation of the discoblastic ova, which chiefly concerns us,
offers serious difficulties to microscopic investigation and
philosophic consideration. These, however, have been mastered by the
comparative embryological research which has been conducted by a
number of distinguished observers during the last few
decades - especially the brothers Hertwig, Rabl, Kupffer, Selenka,
Ruckert, Goette, Rauber, etc. These thorough and careful studies,
aided by the most perfect modern improvements in technical method (in
tinting and dissection), have given a very welcome support to the
views which I put forward in my work, On the Gastrula and the
Segmentation of the Animal Ovum [not translated], in 1875. As it is
very important to understand these views and their phylogenetic
foundation clearly, not only as regards evolution in general, but
particularly in connection with the genesis of man, I will give here a
brief statement of them as far as they concern the vertebrate-stem: -

1. All the vertebrates, including man, are phylogenetically (or
genealogically) related - that is, are members of one single natural
stem.

2. Consequently, the embryonic features in their individual
development must also have a genetic connection.

3. As the gastrulation of the amphioxus shows the original
palingenetic form in its simplest features, that of the other
vertebrates must have been derived from it.

4. The cenogenetic modifications of the latter are more appreciable
the more food-yelk is stored up in the ovum.

5. Although the mass of the food-yelk may be very large in the ova of
the discoblastic vertebrates, nevertheless in every case a blastula is
developed from the morula, as in the holoblastic ova.

6. Also, in every case, the gastrula develops from the blastula by
curving or invagination.

7. The cavity which is produced in the foetus by this curving is, in
each case, the primitive gut (progaster), and its opening the
primitive mouth (prostoma).

8. The food-yelk, whether large or small, is always stored in the
ventral wall of the primitive gut; the cells (called "merocytes")
which may be formed in it subsequently (by "after-segmentation") also
belong to the inner germinal layer, like the cells which immediately
enclose the primitive gut-cavity.

9. The primitive mouth, which at first lies below at the lower pole of
the vertical axis, is forced, by the growth of the yelk, backwards and
then upwards, towards the dorsal side of the embryo; the vertical axis
of the primitive gut is thus gradually converted into horizontal.

10. The primitive mouth is closed sooner or later in all the
vertebrates, and does not evolve into the permanent mouth-aperture; it
rather corresponds to the "properistoma," or region of the anus. From
this important point the formation of the middle germinal layer
proceeds, between the two primary layers.

The wide comparative studies of the scientists I have named have
further shown that in the case of the discoblastic higher vertebrates
(the three classes of amniotes) the primitive mouth of the embryonic
disc, which was long looked for in vain, is found always, and is
nothing else than the familiar "primitive groove." Of this we shall
see more as we proceed. Meantime we realise that gastrulation may be
reduced to one and the same process in all the vertebrates. Moreover,
the various forms it takes in the invertebrates can always be reduced
to one of the four types of segmentation described above. In relation
to the distinction between total and partial segmentation, the
grouping of the various forms is as follows: -

1. Palingenetic (primitive segmentation)

1.1. Equal segmentation (bell-gastrula).

1.1.A. Total segmentation (without independent food-yelk).

2. Cenogenetic segmentation (modified by adaptation).

2.2. Unequal segmentation (hooded gastrula).

2.2.A. Total segmentation (without independent food-yelk).

2.3. Discoid segmentation (discoid gastrula).

2.3.B. Partial segmentation (with independent food-yelk).

2.4. Superficial segmentation (spherical gastrula).

2.4.B. Partial segmentation (with independent food-yelk).

The lowest metazoa we know - namely, the lower zoophyta (sponges,
simple polyps, etc.) - remain throughout life at a stage of development
which differs little from the gastrula; their whole body consists of
two layers of cells. This is a fact of extreme importance. We see that
man, and also other vertebrates, pass quickly through a stage of
development in which they consist of two layers, just as these lower
zoophyta do throughout life. If we apply our biogenetic law to the
matter, we at once reach this important conclusion. "Man and all the
other animals which pass through the two-layer stage, or
gastrula-form, in the course of their embryonic development, must
descend from a primitive simple stem-form, the whole body of which
consisted throughout life (as is the case with the lower zoophyta
to-day) merely of two cell-strata or germinal layers." We will call
this primitive stem-form, with which we shall deal more fully later
on, the gastraea - that is to say, "primitive-gut animal."

According to this gastraea-theory there was originally in all the
multicellular animals ONE ORGAN with the same structure and function.
This was the primitive gut; and the two primary germinal layers which
form its wall must also be regarded as identical in all. This
important homology or identity of the primary germinal layers is
proved, on the one hand, from the fact that the gastrula was
originally formed in the same way in all cases - namely, by the curving
of the blastula; and, on the other hand, by the fact that in every
case the same fundamental organs arise from the germinal layers. The
outer or animal layer, or ectoderm, always forms the chief organs of
animal life - the skin, nervous system, sense-organs, etc.; the inner
or vegetal layer, or entoderm, gives rise to the chief organs of
vegetative life - the organs of nourishment, digestion,
blood-formation, etc.

In the lower zoophyta, whose body remains at the two-layer stage
throughout life, the gastraeads, the simplest sponges (Olynthus), and
polyps (Hydra), these two groups of functions, animal and vegetative,
are strictly divided between the two simple primary layers. Throughout
life the outer or animal layer acts simply as a covering for the body,
and accomplishes its movement and sensation. The inner or vegetative
layer of cells acts throughout life as a gut-lining, or nutritive
layer of enteric cells, and often also yields the reproductive cells.

The best known of these "gastraeads," or "gastrula-like animals," is
the common fresh-water polyp (Hydra). This simplest of all the
cnidaria has, it is true, a crown of tentacles round its mouth. Also
its outer germinal layer has certain special modifications. But these
are secondary additions, and the inner germinal layer is a simple
stratum of cells. On the whole, the hydra has preserved to our day by
heredity the simple structure of our primitive ancestor, the gastraea
(cf. Chapter 2.19.)

In all other animals, particularly the vertebrates, the gastrula is
merely a brief transitional stage. Here the two-layer stage of the
embryonic development is quickly succeeded by a three-layer, and then
a four-layer, stage. With the appearance of the four superimposed
germinal layers we reach again a firm and steady standing-ground, from
which we may follow the further, and much more difficult and
complicated, course of embryonic development.

SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION AND
GASTRULATION OF ANIMALS.

The animal stems are indicated by the letters a-g: a Zoophyta. b
Annelida. c Mollusca.
d Echinoderma. e Articulata. f Tunicata. g Vertebrata.

1. Total Segmentation. Holoblastic ova. Gastrula without separate
food-yelk. Hologastrula.

1.1. Primitive Segmentation. Archiblastic ova. Bell-gastrula
(archigastrula.)
a. Many lower zoophyta (sponges, hydrapolyps, medusae, simpler
corals).
b. Many lower annelids (sagitta, phoronis, many nematoda, etc.,
terebratula, argiope, pisidium).
c. Some lower molluscs.
d. Many echinoderms.
e. A few lower articulata (some brachiopods, copepods: Tardigrades,
pteromalina).
f. Many tunicata.
g. The acrania (amphioxus).

1.2. Unequal Segmentation. Amphiblastic ova. Hooded-gastrula
(amphigastrula).
a. Many zoophyta (sponges, medusae, corals, siphonophorae,
ctenophora).
b. Most worms.
c. Most molluscs.
d. Many echinoderms (viviparous species and some others).
e. Some of the lower articulata (both crustacea and tracheata).
f. Many tunicata.
g. Cyclostoma, the oldest fishes, amphibia, mammals (not including
man).

2. Partial Segmentation. Meroblastic ova. Gastrula with separate
food-yelk. Merogastrula.

2.3. Discoid Segmentation. Discoblastic ova. Discoid gastrula.
c. Cephalopods or cuttlefish.
e. Many articulata, wood-lice, scorpions, etc.
g. Primitive fishes, bony fishes, reptiles, birds, monotremes.

2.4. Superficial Segmentation. Periblastic ova. Spherical-gastrula.
e. The great majority of the articulata (crustaceans, myriapods,
arachnids, insects).


CHAPTER 1.9. THE GASTRULATION OF THE VERTEBRATE.*

(* 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


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