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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


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