The three higher classes of vertebrates which we call the
amniotes--the mammals, birds, and reptiles--are notably distinguished
by a number of peculiarities of their development from the five lower
classes of the stem--the animals without an amnion (the anamnia). All
the amniotes have a distinctive embryonic membrane known as the amnion
(or "water-membrane"), and a special embryonic appendage--the
allantois. They have, further, a large yelk-sac, which is filled with
food-yelk in the reptiles and birds, and with a corresponding clear
fluid in the mammals. In consequence of these later-acquired
structures, the original features of the development of the amniotes
are so much altered that it is very difficult to reduce them to the
palingenetic embryonic processes of the lower amnion-less vertebrates.
The gastraea theory shows us how to do this, by representing the
embryology of the lowest vertebrate, the skull-less amphioxus, as the
original form, and deducing from it, through a series of gradual
modifications, the gastrulation and coelomation of the craniota.
It was somewhat fatal to the true conception of the chief embryonic
processes of the vertebrate that all the older embryologists, from
Malpighi (1687) and Wolff (1750) to Baer (1828) and Remak (1850),
always started from the investigation of the hen's egg, and
transferred to man and the other vertebrates the impressions they
gathered from this. This classical object of embryological research
is, as we have seen, a source of dangerous errors. The large round
food-yelk of the bird's egg causes, in the first place, a flat discoid
expansion of the small gastrula, and then so distinctive a development
of this thin round embryonic disk that the controversy as to its
significance occupies a large part of embryological literature.
(FIGURE 1.105. Severance of the discoid mammal embryo from the
yelk-sac, in transverse section (diagrammatic). A The germinal disk
(h, hf) lies flat on one side of the branchial-gut vesicle (kb). B In
the middle of the germinal disk we find the medullary groove (mr), and
underneath it the chorda (ch). C The gut-fibre-layer (df) has been
enclosed by the gut-gland-layer (dd). D The skin-fibre-layer (hf) and
gut-fibre-layer (df) divide at the periphery; the gut (d) begins to
separate from the yelk-sac or umbilical vesicle (nb). E The medullary
tube (mr) is closed; the body-cavity (c) begins to form. F The
provertebrae (w) begin to grow round the medullary tube (mr) and the
chorda (ch): the gut (d) is cut off from the umbilical vesicle (nb). H
The vertebrae (w) have grown round the medullary tube (mr) and chorda;
the body-cavity is closed, and the umbilical vesicle has disappeared.
The amnion and serous membrane are omitted. The letters have the same
meaning throughout: h horn-plate, mr medullary tube, hf
skin-fibre-layer, w provertebrae, ch chorda, c body-cavity or coeloma,
df gut-fibre-layer, dd gut-gland-layer, d gut-cavity, nb umbilical
vesicle.)
One of the most unfortunate errors that this led to was the idea of an
original antithesis of germ and yelk. The latter was regarded as a
foreign body, extrinsic to the real germ, whereas it is properly a
part of it, an embryonic organ of nutrition. Many authors said there
was no trace of the embryo until a later stage, and outside the yelk;
sometimes the two-layered embryonic disk itself, at other times only
the central portion of it (as distinguished from the germinative area,
which we will describe presently), was taken to be the first outline
of the embryo. In the light of the gastraea theory it is hardly
necessary to dwell on the defects of this earlier view and the
erroneous conclusions drawn from it. In reality, the first
segmentation-cell, and even the stem-cell itself and all that issues
therefrom, belong to the embryo. As the large original yelk-mass in
the undivided egg of the bird only represents an inclosure in the
greatly enlarged ovum, so the later contents of its embryonic yelk-sac
(whether yet segmented or not) are only a part of the entoderm which
forms the primitive gut. This is clearly shown by the ova of the
amphibia and cyclostoma, which explain the transition from the
yelk-less ova of the amphioxus to the large yelk-filled ova of the
reptiles and birds.
It is precisely in the study of these difficult features that we see
the incalculable value of phylogenetic considerations in explaining
complex ontogenetic facts, and the need of separating cenogenetic
phenomena from palingenetic. This is particularly clear as regards the
comparative embryology of the vertebrates, because here the
phylogenetic unity of the stem has been already established by the
well-known facts of paleontology and comparative anatomy. If this
unity of the stem, on the basis of the amphioxus, were always borne in
mind, we should not have these errors constantly recurring.
In many cases the cenogenetic relation of the embryo to the food-yelk
has until now given rise to a quite wrong idea of the first and most
important embryonic processes in the higher vertebrates, and has
occasioned a number of false theories in connection with them. Until
thirty years ago the embryology of the higher vertebrates always
started from the position that the first structure of the embryo is a
flat, leaf-shaped disk; it was for this reason that the cell-layers
that compose this germinal disk (also called germinative area) are
called "germinal layers." This flat germinal disk, which is round at
first and then oval, and which is often described as the tread or
cicatricula in the laid hen's egg, is found at a certain part of the
surface of the large globular food-yelk. I am convinced that it is
nothing else than the discoid, flattened gastrula of the birds. At the
beginning of germination the flat embryonic disk curves outwards, and
separates on the inner side from the underlying large yelk-ball. In
this way the flat layers are converted into tubes, their edges folding
and joining together (Figure 1.105). As the embryo grows at the
expense of the food-yelk, the latter becomes smaller and smaller; it
is completely surrounded by the germinal layers. Later still, the
remainder of the food-yelk only forms a small round sac, the yelk-sac
or umbilical vesicle (Figure 1.105 nb). This is enclosed by the
visceral layer, is connected by a thin stalk, the yelk-duct, with the
central part of the gut-tube, and is finally, in most of the
vertebrates, entirely absorbed by this (H). The point at which this
takes place, and where the gut finally closes, is the visceral navel.
In the mammals, in which the remainder of the yelk-sac remains without
and atrophies, the yelk-duct at length penetrates the outer ventral
wall. At birth the umbilical cord proceeds from here, and the point of
closure remains throughout life in the skin as the navel.
As the older embryology of the higher vertebrates was mainly based on
the chick, and regarded the antithesis of embryo (or formative-yelk)
and food-yelk (or yelk-sac) as original, it had also to look upon the
flat leaf-shaped structure of the germinal disk as the primitive
embryonic form, and emphasise the fact that hollow grooves were formed
of these flat layers by folding, and closed tubes by the joining
together of their edges.
This idea, which dominated the whole treatment of the embryology of
the higher vertebrates until thirty years ago, was totally false. The
gastraea theory, which has its chief application here, teaches us that
it is the very reverse of the truth. The cup-shaped gastrula, in the
body-wall of which the two primary germinal layers appear from the
first as closed tubes, is the original embryonic form of all the
vertebrates, and all the multicellular invertebrates; and the flat
germinal disk with its superficially expanded germinal layers is a
later, secondary form, due to the cenogenetic formation of the large
food-yelk and the gradual spread of the germ-layers over its surface.
Hence the actual folding of the germinal layers and their conversion
into tubes is not an original and primary, but a much later and
tertiary, evolutionary process. In the phylogeny of the vertebrate
embryonic process we may distinguish the following three stages:--
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