We must count it an important advance that we are thus in a position
to reduce all the various embryonic phenomena in the different groups
of animals to these four principal forms of segmentation and
gastrulation. Of these four forms we must regard one only as the
original palingenetic, and the other three as cenogenetic and
derivative. The unequal, the discoid, and the superficial segmentation
have all clearly arisen by secondary adaptation from the primary
segmentation; and the chief cause of their development has been the
gradual formation of the food-yelk, and the increasing antithesis
between animal and vegetal halves of the ovum, or between ectoderm
(skin-layer) and entoderm (gut-layer).
(FIGURE 1.72. Gastrula of the placental mammal (epigastrula from the
rabbit), longitudinal section through the axis. e ectodermic cells
(sixty-four, lighter and smaller), i entodermic cells (thirty-two,
darker and larger), d central entodermic cell, filling the primitive
gut-cavity, o peripheral entodermic cell, stopping up the opening of
the primitive mouth (yelk-stopper in the Rusconian anus).)
(FIGURE 1.73. Gastrula of the rabbit. A as a solid, spherical cluster
of cells, B changing into the embryonic vesicle, bp primitive mouth,
ep ectoderm, hy entoderm.)
The numbers of careful studies of animal gastrulation that have been
made in the last few decades have completely established the views I
have expounded, and which I first advanced in the years 1872 to 1876.
For a time they were greatly disputed by many embryologists. Some said
that the original embryonic form of the metazoa was not the gastrula,
but the "planula"--a double-walled vesicle with closed cavity and
without mouth-aperture; the latter was supposed to pierce through
gradually. It was afterwards shown that this planula (found in several
sponges, etc.) was a later evolution from the gastrula. It was also
shown that what is called delamination--the rise of the two primary
germinal layers by the folding of the surface of the blastoderm (for
instance, in the Geryonidae and other medusae)--was a secondary
formation, due to cenogenetic variations from the original
invagination of the blastula. The same may be said of what is called
"immigration," in which certain cells or groups of cells are detached
from the simple layer of the blastoderm, and travel into the interior
of the blastula; they attach themselves to the inner wall of the
blastula, and form a second internal epithelial layer--that is to say,
the entoderm. In these and many other controversies of modern
embryology the first requisite for clear and natural explanation is a
careful and discriminative distinction between palingenetic
(hereditary) and cenogenetic (adaptive) processes. If this is properly
attended to, we find evidence everywhere of the biogenetic law.