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,