There is a substantial agreement throughout the animal world in the
first changes which follow the impregnation of the ovum and the
formation of the stem-cell; they begin in all cases with the
segmentation of the ovum and the formation of the germinal layers. The
only exception is found in the protozoa, the very lowest and simplest
forms of animal life; these remain unicellular throughout life. To
this group belong the amoebae, gregarinae, rhizopods, infusoria, etc.
As their whole organism consists of a single cell, they can never form
germinal layers, or definite strata of cells. But all the other
animals--all the tissue-forming animals, or metazoa, as we call them,
in contradistinction to the protozoa--construct real germinal layers
by the repeated cleavage of the impregnated ovum. This we find in the
lower cnidaria and worms, as well as in the more highly-developed
molluscs, echinoderms, articulates, and vertebrates.
In all these metazoa, or multicellular animals, the chief embryonic
processes are substantially alike, although they often seem to a
superficial observer to differ considerably. The stem-cell that
proceeds from the impregnated ovum always passes by repeated cleavage
into a number of simple cells. These cells are all direct descendants
of the stem-cell, and are, for reasons we shall see presently, called
segmentation-cells. The repeated cleavage of the stem-cell, which
gives rise to these segmentation-spheres, has long been known as
"segmentation." Sooner or later the segmentation-cells join together
to form a round (at first, globular) embryonic sphere (blastula); they
then form into two very different groups, and arrange themselves in
two separate strata--the two primary germinal layers. These enclose a
digestive cavity, the primitive gut, with an opening, the primitive
mouth. We give the name of the gastrula to the important embryonic
form that has these primitive organs, and the name of gastrulation to
the formation of it. This ontogenetic process has a very great
significance, and is the real starting-point of the construction of
the multicellular animal body.
The fundamental embryonic processes of the cleavage of the ovum and
the formation of the germinal layers have been very thoroughly studied
in the last thirty years, and their real significance has been
appreciated. They present a striking variety in the different groups,
and it was no light task to prove their essential identity in the
whole animal world. But since I formulated the gastraea theory in
1872, and afterwards (1875) reduced all the various forms of
segmentation and gastrulation to one fundamental type, their identity
may be said to have been established. We have thus mastered the law of
unity which governs the first embryonic processes in all the animals.
Man is like all the other higher animals, especially the apes, in
regard to these earliest and most important processes. As the human
embryo does not essentially differ, even at a much later stage of
development--when we already perceive the cerebral vesicles, the eyes,
ears, gill-arches, etc.--from the similar forms of the other higher
mammals, we may confidently assume that they agree in the earliest
embryonic processes, segmentation and the formation of germinal
layers. This has not yet, it is true, been established by observation.
We have never yet had occasion to dissect a woman immediately after
impregnation and examine the stem-cell or the segmentation-cells in
her oviduct. However, as the earliest human embryos we have examined,
and the later and more developed forms, agree with those of the
rabbit, dog, and other higher mammals, no reasonable man will doubt
but that the segmentation and formation of layers are the same in both
cases.
But the special form of segmentation and layer formation which we find
in the mammal is by no means the original, simple, palingenetic form.
It has been much modified and cenogenetically altered by a very
complex adaptation to embryonic conditions. We cannot, therefore,
understand it altogether in itself. In order to do this, we have to
make a COMPARATIVE study of segmentation and layer-formation in the
animal world; and we have especially to seek the original,
PALINGENETIC form from which the modified CENOGENETIC (see Chapter
1.1) form has gradually been developed.
This original unaltered form of segmentation and layer-formation is
found to-day in only one case in the vertebrate-stem to which man
belongs--the lowest and oldest member of the stem, the wonderful
lancelet or amphioxus (cf. Chapters 2.16 and 2.17). But we find a
precisely similar palingenetic form of embryonic development in the
case of many of the invertebrate animals, as, for instance, the
remarkable ascidia, the pond-snail (Limnaeus), and arrow-worm
(Sagitta), and many of the echinoderms and cnidaria, such as the
common star-fish and sea-urchin, many of the medusae and corals, and
the simpler sponges (Olynthus). We may take as an illustration the
palingenetic segmentation and germinal layer-formation in an
eight-fold insular coral, which I discovered in the Red Sea, and
described as Monoxenia Darwinii.
(FIGURE 1.29. Gastrulation of a coral (Monoxenia Darwinii). A, B,
stem-cell (cytula) or impregnated ovum. In Figure A (immediately after
impregnation) the nucleus is invisible. In Figure B (a little later)
it is quite clear. C two segmentation-cells. D four
segmentation-cells. E mulberry-formation (morula). F blastosphere
(blastula). G blastula (transverse section). H depula, or hollowed
blastula (transverse section). I gastrula (longitudinal section). K
gastrula, or cup-sphere, external appearance.)
The impregnated ovum of this coral (Figure 1.29 A, B) first splits
into two equal cells (C). First, the nucleus of the stem-cell and its
central body divide into two halves. These recede from and repel each
other, and act as centres of attraction on the surrounding protoplasm;
in consequence of this, the protoplasm is constricted by a circular
furrow, and, in turn, divides into two halves. Each of the two
segmentation-cells thus produced splits in the same way into two equal
cells. The four segmentation-cells (grand-daughters of the stem-cell)
lie in one plane. Now, however, each of them subdivides into two equal
halves, the cleavage of the nucleus again preceding that of the
surrounding protoplasm. The eight cells which thus arise break into
sixteen, these into thirty-two, and then (each being constantly
halved) into sixty-four, 128, and so on.* (* The number of
segmentation-cells thus produced increases geometrically in the
original gastrulation, or the purest palingenetic form of cleavage.
However, in different animals the number reaches a different height,
so that the morula, and also the blastula, may consist sometimes of
thirty-two, sometimes of sixty-four, and sometimes of 128, or more,
cells.) The final result of this repeated cleavage is the formation of
a globular cluster of similar segmentation-cells, which we call the
mulberry-formation or morula. The cells are thickly pressed together
like the parts of a mulberry or blackberry, and this gives a lumpy
appearance to the surface of the sphere (Figure E).* (* The
segmentation-cells which make up the morula after the close of the
palingenetic cleavage seem usually to be quite similar, and to present
no differences as to size, form, and composition. That, however, does
not prevent them from differentiating into animal and vegetative
cells, even during the cleavage.)
When the cleavage is thus ended, the mulberry-like mass changes into a
hollow globular sphere. Watery fluid or jelly gathers inside the
globule; the segmentation-cells are loosened, and all rise to the
surface. There they are flattened by mutual pressure, and assume the
shape of truncated pyramids, and arrange themselves side by side in
one regular layer (Figures F, G). This layer of cells is called the
germinal membrane (or blastoderm); the homogeneous cells which compose
its simple structure are called blastodermic cells; and the whole
hollow sphere, the walls of which are made of the preceding, is called
the blastula or blastosphere.* (* The blastula of the lower animals
must not be confused with the very different blastula of the mammal,
which is properly called the gastrocystis or blastocystis. This
cenogenetic gastrocystis and the palingenetic blastula are sometimes
very wrongly comprised under the common name of blastula or vesicula
blastodermica.)
In the case of our coral, and of many other lower forms of animal
life, the young embryo begins at once to move independently and swim
about in the water. A fine, long, thread-like process, a sort of whip
or lash, grows out of each blastodermic cell, and this independently
executes vibratory movements, slow at first, but quicker after a time
(Figure F). In this way each blastodermic cell becomes a ciliated
cell. The combined force of all these vibrating lashes causes the
whole blastula to move about in a rotatory fashion. In many other
animals, especially those in which the embryo develops within enclosed
membranes, the ciliated cells are only formed at a later stage, or
even not formed at all. The blastosphere may grow and expand by the
blastodermic cells (at the surface of the sphere) dividing and
increasing, and more fluid is secreted in the internal cavity. There
are still to-day some organisms that remain throughout life at the
structural stage of the blastula--hollow vesicles that swim about by a
ciliary movement in the water, the wall of which is composed of a
single layer of cells, such as the volvox, the magosphaera, synura,
etc. We shall speak further of the great phylogenetic significance of
this fact in Chapter 2.19.
A very important and remarkable process now follows--namely, the
curving or invagination of the blastula (Figure H). The vesicle with a
single layer of cells for wall is converted into a cup with a wall of
two layers of cells (cf. Figures G, H, I). A certain spot at the
surface of the sphere is flattened, and then bent inward. This
depression sinks deeper and deeper, growing at the cost of the
internal cavity. The latter decreases as the hollow deepens. At last
the internal cavity disappears altogether, the inner side of the
blastoderm (that which lines the depression) coming to lie close on
the outer side. At the same time, the cells of the two sections assume
different sizes and shapes; the inner cells are more round and the
outer more oval (Figure I). In this way the embryo takes the form of a
cup or jar-shaped body, with a wall made up of two layers of cells,
the inner cavity of which opens to the outside at one end (the spot
where the depression was originally formed). We call this very
important and interesting embryonic form the "cup-embryo" or
"cup-larva" (gastrula, Figure 1.29, I longitudinal section, K external
view). I have in my Natural History of Creation given the name of
depula to the remarkable intermediate form which appears at the
passage of the blastula into the gastrula. In this intermediate stage
there are two cavities in the embryo--the original cavity (blastocoel)
which is disappearing, and the primitive gut-cavity (progaster) which
is forming.
I regard the gastrula as the most important and significant embryonic
form in the animal world. In all real animals (that is, excluding the
unicellular protists) the segmentation of the ovum produces either a
pure, primitive, palingenetic gastrula (Figure 1.29 I, K) or an
equally instructive cenogenetic form, which has been developed in time
from the first, and can be directly reduced to it. It is certainly a
fact of the greatest interest and instructiveness that animals of the
most different stems--vertebrates and tunicates, molluscs and
articulates, echinoderms and annelids, cnidaria and sponges--proceed
from one and the same embryonic form. In illustration I give a few
pure gastrula forms from various groups of animals (Figures 1.30 to
1.35, explanation given below each).
(FIGURES 1.30 TO 1.35. In each figure d is the primitive-gut cavity, o
primitive mouth, s segmentation-cavity, i entoderm (gut-layer), e
ectoderm (skin layer).
FIGURE 1.30. (A) Gastrula of a very simple primitive-gut animal or
gastraead (gastrophysema). (Haeckel.)
FIGURE 1.31. (B) Gastrula of a worm (Sagitta). (From Kowalevsky.)
FIGURE 1.32. (C) Gastrula of an echinoderm (star-fish, Uraster), not
completely folded in (depula). (From Alexander Agassiz.)
FIGURE 1.33. (D) Gastrula of an arthropod (primitive crab, Nauplius)
(as 32).
FIGURE 1.34. (E) Gastrula of a mollusc (pond-snail, Linnaeus). (From
Karl Rabl.)
FIGURE 1.35. (F) Gastrula of a vertebrate (lancelet, Amphioxus). (From
Kowalevsky.) (Front view.))
In view of this extraordinary significance of the gastrula, we must
make a very careful study of its original structure. As a rule, the
typical gastrula is very small, being invisible to the naked eye, or
at the most only visible as a fine point under very favourable
conditions, and measuring generally 1/500 to 1/250 of an inch (less
frequently 1/50 inch, or even more) in diameter. In shape it is
usually like a roundish drinking-cup. Sometimes it is rather oval, at
other times more ellipsoid or spindle-shaped; in some cases it is half
round, or even almost round, and in others lengthened out, or almost
cylindrical.
I give the name of primitive gut (progaster) and primitive mouth
(prostoma) to the internal cavity of the gastrula-body and its
opening; because this cavity is the first rudiment of the digestive
cavity of the organism, and the opening originally served to take food
into it. Naturally, the primitive gut and mouth change very
considerably afterwards in the various classes of animals. In most of
the cnidaria and many of the annelids (worm-like animals) they remain
unchanged throughout life. But in most of the higher animals, and so
in the vertebrates, only the larger central part of the later
alimentary canal develops from the primitive gut; the later mouth is a
fresh development, the primitive mouth disappearing or changing into
the anus. We must therefore distinguish carefully between the
primitive gut and mouth of the gastrula and the later alimentary canal
and mouth of the fully developed vertebrate.* (* My distinction (1872)
between the primitive gut and mouth and the later permanent stomach
(metagaster) and mouth (metastoma) has been much criticised; but it is
as much justified as the distinction between the primitive kidneys and
the permanent kidneys. Professor E. Ray-Lankester suggested three
years afterwards (1875) the name archenteron for the primitive gut,
and blastoporus for the primitive mouth.)
(FIGURE 1.36. Gastrula of a lower sponge (olynthus). A external view,
B longitudinal section through the axis, g primitive-gut cavity, a
primitive mouth-aperture, i inner cell-layer (entoderm, endoblast,
gut-layer), e external cell-layer (outer germinal layer, ectoderm,
ectoblast, or skin-layer).
The two layers of cells which line the gut-cavity and compose its wall
are of extreme importance. These two layers, which are the sole
builders of the whole organism, are no other than the two primary
germinal layers, or the primitive germ-layers. I have spoken in the
introductory section (Chapter 1.3.) of their radical importance. The
outer stratum is the skin-layer, or ectoderm (Figures 1.30 to 1.35 e);
the inner stratum is the gut-layer, or entoderm (i). The former is
often also called the ectoblast, or epiblast, and the latter the
endoblast, or hypoblast. FROM THESE TWO PRIMARY GERMINAL LAYERS ALONE
IS DEVELOPED THE ENTIRE ORGANISM OF ALL THE METAZOA OR MULTICELLULAR
ANIMALS. The skin-layer forms the external skin, the gut-layer forms
the internal skin or lining of the body. Between these two germinal
layers are afterwards developed the middle germinal layer (mesoderma)
and the body-cavity (coeloma) filled with blood or lymph.
The two primary germinal layers were first distinguished by Pander in
1817 in the incubated chick. Twenty years later (1849) Huxley pointed
out that in many of the lower zoophytes, especially the medusae, the
whole body consists throughout life of these two primary germinal
layers. Soon afterwards (1853) Allman introduced the names which have
come into general use; he called the outer layer the ectoderm
("outer-skin"), and the inner the entoderm ("inner-skin"). But in 1867
it was shown, particularly by Kowalevsky, from comparative
observation, that even in invertebrates, also, of the most different
classes--annelids, molluscs, echinoderms, and articulates--the body is
developed out of the same two primary layers. Finally, I discovered
them (1872) in the lowest tissue-forming animals, the sponges, and
proved in my gastraea theory that these two layers must be regarded as
identical throughout the animal world, from the sponges and corals to
the insects and vertebrates, including man. This fundamental "homology
[identity] of the primary germinal layers and the primitive gut" has
been confirmed during the last thirty years by the careful research of
many able observers, and is now pretty generally admitted for the
whole of the metazoa.
As a rule, the cells which compose the two primary germinal layers
show appreciable differences even in the gastrula stage. Generally (if
not always) the cells of the skin-layer or ectoderm (Figures 1.36 c
and 1.37 e) are the smaller, more numerous, and clearer; while the
cells of the gut-layer, or entoderm (i), are larger, less numerous,
and darker. The protoplasm of the ectodermic (outer) cells is clearer
and firmer than the thicker and softer cell-matter of the entodermic
(inner) cells; the latter are, as a rule, much richer in yelk-granules
(albumen and fatty particles) than the former. Also the cells of the
gut-layer have, as a rule, a stronger affinity for colouring matter,
and take on a tinge in a solution of carmine, aniline, etc., more
quickly and appreciably than the cells of the skin-layer. The nuclei
of the entoderm-cells are usually roundish, while those of the
ectoderm-cells are oval.
When the doubling-process is complete, very striking histological
differences between the cells of the two layers are found (Figure
1.37). The tiny, light ectoderm-cells (e) are sharply distinguished
from the larger and darker entoderm-cells (i). Frequently this
differentiation of the cell-forms sets in at a very early stage,
during the segmentation-process, and is already very appreciable in
the blastula.
We have, up to the present, only considered that form of segmentation
and gastrulation which, for many and weighty reasons, we may regard as
the original, primordial, or palingenetic form. We might call it
"equal" or homogeneous segmentation, because the divided cells retain
a resemblance to each other at first (and often until the formation of
the blastoderm). We give the name of the "bell-gastrula," or
archigastrula, to the gastrula that succeeds it. In just the same form
as in the coral we considered (Monoxenia, Figure 1.29), we find it in
the lowest zoophyta (the gastrophysema, Figure 1.30), and the simplest
sponges (olynthus, Figure 1.36); also in many of the medusae and
hydrapolyps, lower types of worms of various classes (brachiopod,
arrow-worm, Figure 1.31), tunicates (ascidia), many of the echinoderms
(Figure 1.32), lower articulates (Figure 1.33), and molluscs (Figure
1.34), and, finally, in a slightly modified form, in the lowest
vertebrate (the amphioxus, Figure 1.35).
(FIGURE 1.37. Cells from the two primary germinal layers of the mammal
(from both layers of the blastoderm). i larger and darker cells of the
inner stratum, the vegetal layer or entoderm. e smaller and clearer
cells from the outer stratum, the animal layer or ectoderm.
FIGURE 1.38. Gastrulation of the amphioxus, from Hatschek (vertical
section through the axis of the ovum). A, B, C three stages in the
formation of the blastula; D, E curving of the blastula; F complete
gastrula. h segmentation-cavity. g primitive gut-cavity.))
The gastrulation of the amphioxus is especially interesting because
this lowest and oldest of all the vertebrates is of the highest
significance in connection with the evolution of the vertebrate stem,
and therefore with that of man (compare Chapters 2.16 and 2.17). Just
as the comparative anatomist traces the most elaborate features in the
structures of the various classes of vertebrates to divergent
development from this simple primitive vertebrate, so comparative
embryology traces the various secondary forms of vertebrate
gastrulation to the simple, primary formation of the germinal layers
in the amphioxus. Although this formation, as distinguished from the
cenogenetic modifications of the vertebrate, may on the whole be
regarded as palingenetic, it is nevertheless different in some
features from the quite primitive gastrulation such as we have, for
instance, in the Monoxenia (Figure 1.29) and the Sagitta. Hatschek
rightly observes that the segmentation of the ovum in the amphioxus is
not strictly equal, but almost equal, and approaches the unequal. The
difference in size between the two groups of cells continues to be
very noticeable in the further course of the segmentation; the smaller
animal cells of the upper hemisphere divide more quickly than the
larger vegetal cells of the lower (Figure 1.38 A, B). Hence the
blastoderm, which forms the single-layer wall of the globular blastula
at the end of the cleavage-process, does not consist of homogeneous
cells of equal size, as in the Sagitta and the Monoxenia; the cells of
the upper half of the blastoderm (the mother-cells of the ectoderm)
are more numerous and smaller, and the cells of the lower half (the
mother-cells of the entoderm) less numerous and larger. Moreover, the
segmentation-cavity of the blastula (Figure 1.38 C, h) is not quite
globular, but forms a flattened spheroid with unequal poles of its
vertical axis. While the blastula is being folded into a cup at the
vegetal pole of its axis, the difference in the size of the
blastodermic cells increases (Figure 1.38 D, E); it is most
conspicuous when the invagination is complete and the
segmentation-cavity has disappeared (Figure 1.38 F). The larger
vegetal cells of the entoderm are richer in granules, and so darker
than the smaller and lighter animal cells of the ectoderm.
But the unequal gastrulation of the amphioxus diverges from the
typical equal cleavage of the Sagitta, the Monoxenia (Figure 1.29),
and the Olynthus (Figure 1.36), in another important particular. The
pure archigastrula of the latter forms is uni-axial, and it is round
in its whole length in transverse section. The vegetal pole of the
vertical axis is just in the centre of the primitive mouth. This is
not the case in the gastrula of the amphioxus. During the folding of
the blastula the ideal axis is already bent on one side, the growth of
the blastoderm (or the increase of its cells) being brisker on one
side than on the other; the side that grows more quickly, and so is
more curved (Figure 1.39 v), will be the anterior or belly-side, the
opposite, flatter side will form the back (d). The primitive mouth,
which at first, in the typical archigastrula, lay at the vegetal pole
of the main axis, is forced away to the dorsal side; and whereas its
two lips lay at first in a plane at right angles to the chief axis,
they are now so far thrust aside that their plane cuts the axis at a
sharp angle. The dorsal lip is therefore the upper and more forward,
the ventral lip the lower and hinder. In the latter, at the ventral
passage of the entoderm into the ectoderm, there lie side by side a
pair of very large cells, one to the right and one to the left (Figure
1.39 p): these are the important polar cells of the primitive mouth,
or "the primitive cells of the mesoderm." In consequence of these
considerable variations arising in the course of the gastrulation, the
primitive uni-axial form of the archigastrula in the amphioxus has
already become tri-axial, and thus the two-sidedness, or bilateral
symmetry, of the vertebrate body has already been determined. This has
been transmitted from the amphioxus to all the other modified
gastrula-forms of the vertebrate stem.
Apart from this bilateral structure, the gastrula of the amphioxus
resembles the typical archigastrula of the lower animals (Figures 1.30
to 1.36) in developing the two primary germinal layers from a single
layer of cells. This is clearly the oldest and original form of the
metazoic embryo. Although the animals I have mentioned belong to the
most diverse classes, they nevertheless agree with each other, and
many more animal forms, in having retained to the present day, by a
conservative heredity, this palingenetic form of gastrulation which
they have from their earliest common ancestors. But this is not the
case with the great majority of the animals. With these the original
embryonic process has been gradually more or less altered in the
course of millions of years by adaptation to new conditions of
development. Both the segmentation of the ovum and the subsequent
gastrulation have in this way been considerably changed. In fact,
these variations have become so great in the course of time that the
segmentation was not rightly understood in most animals, and the
gastrula was unrecognised. It was not until I had made an extensive
comparative study, lasting a considerable time (in the years 1866 to
1875), in animals of the most diverse classes, that I succeeded in
showing the same common typical process in these apparently very
different forms of gastrulation, and tracing them all to one original
form. I regard all those that diverge from the primary palingenetic
gastrulation as secondary, modified, and cenogenetic. The more or less
divergent form of gastrula that is produced may be called a secondary,
modified gastrula, or a metagastrula. The reader will find a scheme of
these different kinds of segmentation and gastrulation at the close of
this chapter.
By far the most important process that determines the various
cenogenetic forms of gastrulation is the change in the nutrition of
the ovum and the accumulation in it of nutritive yelk. By this we
understand various chemical substances (chiefly granules of albumin
and fat-particles) which serve exclusively as reserve-matter or food
for the embryo. As the metazoic embryo in its earlier stages of
development is not yet able to obtain its food and so build up the
frame, the necessary material has to be stored up in the ovum. Hence
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).