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Ernst Heinrich Philipp August Haeckel.

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A. First Stage: Primary (palingenetic) embryonic process.

The germinal layers form from the first closed tubes, the one-layered
blastula being converted into the two-layered gastrula by
invagination. No food-yelk. (Amphioxus.)

B. Second Stage: Secondary (cenogenetic) embryonic process.

The germinal layers spread out leaf-wise, food-yelk gathering in the
ventral entoderm, and a large yelk-sac being formed from the middle of
the gut-tube. (Amphibia.)

C. Third Stage: Tertiary (cenogenetic) embryonic process.

The germinal layers form a flat germinal disk, the borders of which
join together and form closed tubes, separating from the central
yelk-sac. (Amniotes.)

As this theory, a logical conclusion from the gastraea theory, has
been fully substantiated by the comparative study of gastrulation in
the last few decades, we must exactly reverse the hitherto prevalent
mode of treatment. The yelk-sac is not to be treated, as was done
formerly, as if it were originally antithetic to the embryo, but as an
essential part of it, a part of its visceral tube. The primitive gut
of the gastrula has, on this view, been divided into two parts in the
higher animals as a result of the cenogenetic formation of the
food-yelk - the permanent gut (metagaster), or permanent alimentary
canal, and the yelk-sac (lecithoma), or umbilical vesicle. This is
very clearly shown by the comparative ontogeny of the fishes and
amphibia. In these cases the whole yelk undergoes cleavage at first,
and forms a yelk-gland, composed of yelk-cells, in the ventral wall of
the primitive gut. But it afterwards becomes so large that a part of
the yelk does not divide, and is used up in the yelk-sac that is cut
off outside.

(FIGURE 1.106. The visceral embryonic vesicle (blastocystis or
gastrocystis) of a rabbit (the "blastula" or vesicula blastodermica of
other writers), a outer envelope (ovolemma), b skin-layer or ectoderm,
forming the entire wall of the yelk-vesicle, c groups of dark cells,
representing the visceral layer or entoderm.

FIGURE 1.107. The same in section. Letters as above. d cavity of the
vesicle. (From Bischoff.))

When we make a comparative study of the embryology of the amphioxus,
the frog, the chick, and the rabbit, there cannot, in my opinion, be
any further doubt as to the truth of this position, which I have held
for thirty years. Hence in the light of the gastraea theory we must
regard the features of the amphioxus as the only and real primitive
structure among all the vertebrates, departing very little from the
palingenetic embryonic form. In the cyclostoma and the frog these
features are, on the whole, not much altered cenogenetically, but they
are very much so in the chick, and most of all in the rabbit. In the
bell-gastrula of the amphioxus and in the hooded gastrula of the
lamprey and the frog the germinal layers are found to be closed tubes
or vesicles from the first. On the other hand, the chick-embryo (in
the new laid, but not yet hatched, egg) is a flat circular disk, and
it was not easy to recognise this as a real gastrula. Rauber and
Goette have, however, achieved this. As the discoid gastrula grows
round the large globular yelk, and the permanent gut then separates
from the outlying yelk-sac, we find all the processes which we have
shown (diagrammatically) in Figure 1.108 - processes that were hitherto
regarded as principal acts, whereas they are merely secondary.

The oldest, oviparous mammals, the monotremes, behave in the same way
as the reptiles and birds. But the corresponding embryonic processes
in the viviparous mammals, the marsupials and placentals, are very
elaborate and distinctive. They were formerly quite misinterpreted; it
was not until the publication of the studies of Edward van Beneden
(1875) and the later research of Selenka, Kuppfer, Rabl, and others,
that light was thrown on them, and we were in a position to bring them
into line with the principles of the gastraea theory and trace them to
the embryonic forms of the lower vertebrates. Although there is no
independent food-yelk, apart from the formative yelk, in the mammal
ovum, and although its segmentation is total on that account,
nevertheless a large yelk-sac is formed in their embryos, and the
"embryo proper" spreads leaf-wise over its surface, as in the reptiles
and birds, which have a large food-yelk and partial segmentation. In
the mammals, as well as in the latter, the flat, leaf-shaped germinal
disk separates from the yelk-sac, and its edges join together and form
tubes.

How can we explain this curious anomaly? Only as a result of very
characteristic and peculiar cenogenetic modifications of the embryonic
process, the real causes of which must be sought in the change in the
rearing of the young on the part of the viviparous mammals. These are
clearly connected with the fact that the ancestors of the viviparous
mammals were oviparous amniotes like the present monotremes, and only
gradually became viviparous. This can no longer be questioned now that
it has been shown (1884) that the monotremes, the lowest and oldest of
the mammals, still lay eggs, and that these develop like the ova of
the reptiles and birds. Their nearest descendants, the marsupials,
formed the habit of retaining the eggs, and developing them in the
oviduct; the latter was thus converted into a womb (uterus). A
nutritive fluid that was secreted from its wall, and passed through
the wall of the blastula, now served to feed the embryo, and took the
place of the food-yelk. In this way the original food-yelk of the
monotremes gradually atrophied, and at last disappeared so completely
that the partial ovum-segmentation of their descendants, the rest of
the mammals, once more became total. From the discogastrula of the
former was evolved the distinctive epigastrula of the latter.

It is only by this phylogenetic explanation that we can understand the
formation and development of the peculiar, and hitherto totally
misunderstood, blastula of the mammal. The vesicular condition of the
mammal embryo was discovered 200 years ago (1677) by Regner de Graaf.
He found in the uterus of a rabbit four days after impregnation small,
round, loose, transparent vesicles, with a double envelope. However,
Graaf's discovery passed without recognition. It was not until 1827
that these vesicles were rediscovered by Baer, and then more closely
studied in 1842 by Bischoff in the rabbit (Figures 1.106 and 1.107).
They are found in the womb of the rabbit, the dog, and other small
mammals, a few days after copulation. The mature ova of the mammal,
when they have left the ovary, are fertilised either here or in the
oviduct immediately afterwards by the invading sperm-cells.* (* In man
and the other mammals the fertilisation of the ova probably takes
place, as a rule, in the oviduct; here the ova, which issue from the
female ovary in the shape of the Graafian follicle, and enter the
inner aperture of the oviduct, encounter the mobile sperm-cells of the
male seed, which pass into the uterus at copulation, and from this
into the external aperture of the oviduct. Impregnation rarely takes
place in the ovary or in the womb.) (As to the womb and oviduct see
Chapter 2.29.) The cleavage and formation of the gastrula take place
in the oviduct. Either here in the oviduct or after the mammal
gastrula has passed into the uterus it is converted into the globular
vesicle which is shown externally in Figure 1.106, and in section in
Figure 1.107. The thick, outer, structureless envelope that encloses
it is the original ovolemma or zona pellucida, modified, and clothed
with a layer of albumin that has been deposited on the outside. From
this stage the envelope is called the external membrane, the primary
chorion or prochorion (a). The real wall of the vesicle enclosed by it
consists of a simple layer of ectodermic cells (b), which are
flattened by mutual pressure, and generally hexagonal; a light nucleus
shines through their fine-grained protoplasm (Figure 1.108). At one
part (c) inside this hollow ball we find a circular disc, formed of
darker, softer, and rounder cells, the dark-grained entodermic cells
(Figure 1.109).

(FIGURE 1.108. Four entodermic cells from the embryonic vesicle of the
rabbit.

FIGURE 1.109. Two entodermic cells from the embryonic vesicle of the
rabbit.)

The characteristic embryonic form that the developing mammal now
exhibits has up to the present usually been called the "blastula"
(Bischoff), "sac-shaped embryo" (Baer), "vesicular embryo" (vesicula
blastodermica, or, briefly, blastosphaera). The wall of the hollow
vesicle, which consists of a single layer of cells, was called the
"blastoderm," and was supposed to be equivalent to the cell-layer of
the same name that forms the wall of the real blastula of the
amphioxus and many of the invertebrates (such as Monoxenia, Figure
1.29 F, G). Formerly this real blastula was generally believed to be
equivalent to the embryonic vesicle of the mammal. However, this is by
no means the case. What is called the "blastula" of the mammal and the
real blastula of the amphioxus and many of the invertebrates are
totally different embryonic structures. The latter (blastula) is
palingenetic, and precedes the formation of the gastrula. The former
(blastodermic vesicle) is cenogenetic, and follows gastrulation. The
globular wall of the blastula is a real blastoderm, and consists of
homogeneous (blastodermic) cells; it is not yet differentiated into
the two primary germinal layers. But the globular wall of the mammal
vesicle is the differentiated ectoderm, and at one point in it we find
a circular disk of quite different cells - the entoderm. The round
cavity, filled with fluid, inside the real blastula is the
segmentation-cavity. But the similar cavity within the mammal vesicle
is the yelk-sac cavity, which is connected with the incipient
gut-cavity. This primitive gut-cavity passes directly into the
segmentation-cavity in the mammals, in consequence of the peculiar
cenogenetic changes in their gastrulation, which we have considered
previously (Chapter 1.9). For these reasons it is very necessary to
recognise the secondary embryonic vesicle in the mammal (gastrocystis
or blastocystis) as a characteristic structure peculiar to this class,
and distinguish it carefully from the primary blastula of the
amphioxus and the invertebrates.

(FIGURE 1.110. Ovum of a rabbit from the uterus, one sixth of an inch
in diameter. The embryonic vesicle (b) has withdrawn a little from the
smooth ovolemma (a). In the middle of the ovolemma we see the round
germinal disk (blastodiscus, c), at the edge of which (at d) the inner
layer of the embryonic vesicle is already beginning to expand.
(Figures 1.110 to 1.114 from Bischoff.)

FIGURE 1.111. The same ovum, seen in profile. Letters as in Figure
1.110.

FIGURE 1.112. Ovum of a rabbit from the uterus, one-fourth of an inch
in diameter. The blastoderm is already for the most part two-layered
(b). The ovolemma, or outer envelope, is tufted (a).

FIGURE 1.113. The same ovum, seen in profile. Letters as in Figure
1.112.

FIGURE 1.114. Ovum of a rabbit from the uterus, one-third of an inch
in diameter. The embryonic vesicle is now nearly everywhere
two-layered (k) only remaining one-layered below (at d).

FIGURE 1.115. Round germinative area of the rabbit, divided into the
central light area (area pellucida) and the peripheral dark area (area
opaca). The light area seems darker on account of the dark ground
appearing through it.)

The small, circular, whitish, and opaque spot which the gastric disk
(Figure 1.106) forms at a certain part of the surface of the clear and
transparent embryonic vesicle has long been known to science, and
compared to the germinal disk of the birds and reptiles. Sometimes it
has been called the germinal disk, sometimes the germinal spot, and
usually the germinative area. From the area the further development of
the embryo proceeds. However, the larger part of the embryonic vesicle
of the mammal is not directly used for building up the later body, but
for the construction of the temporary umbilical vesicle. The embryo
separates from this in proportion as it grows at its expense; the two
are only connected by the yelk-duct (the stalk of the yelk-sac), and
this maintains the direct communication between the cavity of the
umbilical vesicle and the forming visceral cavity (Figure 1.105).

The germinative area or gastric disk of the animal consists at first
(like the germinal disk of birds and reptiles) merely of the two
primary germinal layers, the ectoderm and entoderm. But soon there
appears in the middle of the circular disk between the two a third
stratum of cells, the rudiment of the middle layer or fibrous layer
(mesoderm). This middle germinal layer consists from the first, as we
have seen in Chapter 1.10, of two separate epithelial plates, the two
layers of the coelom-pouches (parietal and visceral). However, in all
the amniotes (on account of the large formation of yelk) these thin
middle plates are so firmly pressed together that they seem to
represent a single layer. It is thus peculiar to the amniotes that the
middle of the germinative area is composed of four germinal layers,
the two limiting (or primary) layers and the middle layers between
them (Figures 1.96 and 1.97). These four secondary germinal layers can
be clearly distinguished as soon as what is called the sickle-groove
(or "embryonic sickle") is seen at the hind border of the germinative
area. At the borders, however, the germinative area of the mammal only
consists of two layers. The rest of the wall of the embryonic vesicle
consists at first (but only for a short time in most of the mammals)
of a single layer, the outer germinal layer.

(FIGURE 1.116. Oval area, with the opaque whitish border of the dark
area without.)

From this stage, however, the whole wall of the embryonic vesicle
becomes two-layered. The middle of the germinative area is much
thickened by the growth of the cells of the middle layers, and the
inner layer expands at the same time, and increases at the border of
the disk all round. Lying close on the outer layer throughout, it
grows over its inner surface at all points, covers first the upper and
then the lower hemisphere, and at last closes in the middle of the
inner layer (Figures 1.110 to 1.114). The wall of the embryonic
vesicle now consists throughout of two layers of cells, the ectoderm
without and the entoderm within. It is only in the centre of the
circular area, which becomes thicker and thicker through the growth of
the middle layers, that it is made up of all four layers. At the same
time, small structureless tufts or warts are deposited on the surface
of the outer ovolemma or prochorion, which has been raised above the
embryonic vesicle (Figures 1.112 to 1.114 a).

(FIGURE 1.117. Oval germinal disk of the rabbit, magnified about ten
times. As the delicate, half-transparent disk lies on a black ground,
the pellucid area looks like a dark ring, and the opaque area (lying
outside it) like a white ring. The oval shield in the centre also
looks whitish, and in its axis we see the dark medullary groove. (From
Bischoff.))

We may now disregard both the outer ovolemma and the greater part of
the vesicle, and concentrate our attention on the germinative area and
the four-layered embryonic disk. It is here alone that we find the
important changes which lead to the differentiation of the first
organs. It is immaterial whether we examine the germinative area of
the mammal (the rabbit, for instance) or the germinal disk of a bird
or a reptile (such as a lizard or tortoise). The embryonic processes
we are now going to consider are essentially the same in all members
of the three higher classes of vertebrates which we call the amniotes.
Man is found to agree in this respect with the rabbit, dog, ox, etc.;
and in all these animals the germinative area undergoes essentially
the same changes as in the birds and reptiles. They are most
frequently and accurately studied in the chick, because we can have
incubated hens' eggs in any quantity at any stage of development.
Moreover, the round germinal disk of the chick passes immediately
after the beginning of incubation (within a few hours) from the
two-layered to the four-layered stage, the two-layered mesoderm
developing from the median primitive groove between the ectoderm and
entoderm (Figures 1.82 to 1.95).

The first change in the round germinal disk of the chick is that the
cells at its edges multiply more briskly, and form darker nuclei in
their protoplasm. This gives rise to a dark ring, more or less sharply
set off from the lighter centre of the germinal disk (Figure 1.115).
From this point the latter takes the name of the "light area" (area
pellucida), and the darker ring is called the "dark area" (area
opaca). (In a strong light, as in Figures 1.115 to 1.117, the light
area seems dark, because the dark ground is seen through it; and the
dark area seems whiter). The circular shape of the area now changes
into elliptic, and then immediately into oval (Figures 1.116 and
1.117). One end seems to be broader and blunter, the other narrower
and more pointed; the former corresponds to the anterior and the
latter to the posterior section of the subsequent body. At the same
time, we can already trace the characteristic bilateral form of the
body, the antithesis of right and left, before and behind. This will
be made clearer by the "primitive streak," which appears at the
posterior end.

(FIGURE 1.118. Pear-shaped germinal shield of the rabbit (eight days
old), magnified twenty times. rf medullary groove. pr primitive groove
(primitive mouth). (From Kolliker.)

FIGURE 1.119. Median longitudinal section of the gastrula of four
vertebrates. (From Rabl.) A discogastrula of a shark (Pristiurus). B
amphigastrula of a sturgeon (Accipenser). C amphigastrula of an
amphibium (Triton). D epigastrula of an amniote (diagram). a ventral,
b dorsal lip of the primitive mouth.)

At an early stage an opaque spot is seen in the middle of the clear
germinative area, and this also passes from a circular to an oval
shape. At first this shield-shaped marking is very delicate and barely
perceptible; but it soon becomes clearer, and now stands out as an
oval shield, surrounded by two rings or areas (Figure 1.117). The
inner and brighter ring is the remainder of the pellucid area, and the
dark outer ring the remainder of the opaque area; the opaque
shield-like spot itself is the first rudiment of the dorsal part of
the embryo. We give it briefly the name of embryonic shield or dorsal
shield. In most works this embryonic shield is described as "the first
rudiment or trace of the embryo," or "primitive embryo." But this is
wrong, though it rests on the authority of Baer and Bischoff. As a
matter of fact, we already have the embryo in the stem-cell, the
gastrula, and all the subsequent stages. The embryonic shield is
simply the first rudiment of the dorsal part, which is the earliest to
develop. As the older names of "embryonic rudiment" and "germinative
area" are used in many different senses - and this has led to a fatal
confusion in embryonic literature - we must explain very clearly the
real significance of these important embryonic parts of the amniote.
It will be useful to do so in a series of formal principles: -

1. The so-called "first trace of the embryo" in the amniotes, or the
embryonic shield, in the centre of the pellucid area, consists merely
of an early differentiation and formation of the middle dorsal parts.

2. Hence the best name for it is "the dorsal shield," as I proposed
long ago.

3. The germinative area, in which the first embryonic blood-vessels
appear at an early stage, is not opposed as an external area to the
"embryo proper," but is a part of it.

4. In the same way, the yelk-sac or the umbilical vesicle is not a
foreign external appendage of the embryo, but an outlying part of its
primitive gut.

5. The dorsal shield gradually separates from the germinative area and
the yelk-sac, its edges growing downwards and folding together to form
ventral plates.

6. The yelk-sac and vessels of the germinative area, which soon spread
over its whole surface, are, therefore, real embryonic organs, or
temporary parts of the embryo, and have a transitory importance in
connection with the nutrition of the growing later body; the latter
may be called the "permanent body" in contrast to them.

The relation of these cenogenetic features of the amniotes to the
palingenetic structures of the older non-amniotic vertebrates may be
expressed in the following theses: The original gastrula, which
completely passes into the embryonic body in the acrania, cyclostoma,
and amphibia, is early divided into two parts in the amniotes - the
embryonic shield, which represents the dorsal outline of the permanent
body; and the temporary embryonic organs of the germinative area and
its blood-vessels, which soon grow over the whole of the yelk-sac. The
differences which we find in the various classes of the vertebrate
stem in these important particulars can only be fully understood when
we bear in mind their phylogenetic relations on the one hand, and, on
the other, the cenogenetic modifications of structure that have been
brought about by changes in the rearing of the young and the variation
in the mass of the food-yelk.

We have already described in Chapter 1.9 the changes which this
increase and decrease of the nutritive yelk causes in the form of the
gastrula, and especially in the situation and shape of the primitive
mouth. The primitive mouth or prostoma is originally a simple round
aperture at the lower pole of the long axis; its dorsal lip is above
and ventral lip below. In the amphioxus this primitive mouth is a
little eccentric, or shifted to the dorsal side (Figure 1.39). The
aperture increases with the growth of the food-yelk in the cyclostoma
and ganoids; in the sturgeon it lies almost on the equator of the
round ovum, the ventral lip (a) in front and the dorsal lip (b) behind
(Figure 1.119 b). In the wide-mouthed, circular discoid gastrula of
the selachii or primitive fishes, which spreads quite flat on the
large food-yelk, the anterior semi-circle of the border of the disk is
the ventral, and the posterior semicircle the dorsal lip (Figure 1.119
A). The amphiblastic amphibia are directly connected with their
earlier fish-ancestors, the dipneusts and ganoids, and further the
oldest selachii (Cestracion); they have retained their total unequal
segmentation, and their small primitive mouth (Figure 1.119 C, ab),
blocked up by the yelk-stopper, lies at the limit of the dorsal and
ventral surface of the embryo (at the lower pole of its equatorial
axis), and there again has an upper dorsal and a lower ventral lip (a,
b). The formation of a large food-yelk followed again in the
stem-forms of the amniotes, the protamniotes or proreptilia, descended
from the amphibia (Figure 1.119 D). But here the accumulation of the
food-yelk took place only in the ventral wall of the primitive-gut, so
that the narrow primitive mouth lying behind was forced upwards, and
came to lie on the back of the discoid "epigastrula" in the shape of
the "primitive groove"; thus (in contrast to the case of the selachii,
Figure 1.119 A) the dorsal lip (b) had to be in front, and the ventral
lip (a) behind (Figure 1.119 D). This feature was transmitted to all
the amniotes, whether they retained the large food-yelk (reptiles,
birds, and monotremes), or lost it by atrophy (the viviparous
mammals).

This phylogenetic explanation of gastrulation and coelomation, and the
comparative study of them in the various vertebrates, throw a clear
and full light on many ontogenetic phenomena, as to which the most
obscure and confused opinions were prevalent thirty years ago. In this
we see especially the high scientific value of the biogenetic law and
the careful separation of palingenetic from cenogenetic processes. To
the opponents of this law the real explanation of these remarkable
phenomena is impossible. Here, and in every other part of embryology,
the true key to the solution lies in phylogeny.


CHAPTER 1.13. DORSAL BODY AND VENTRAL BODY.

The earliest stages of the human embryo are, for the reasons already
given, either quite unknown or only imperfectly known to us. But as
the subsequent embryonic forms in man behave and develop just as they
do in all the other mammals, there cannot be the slightest doubt that
the preceding stages also are similar. We have been able to see in the


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