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). Fig.72. Gastrula of the placental mammal (epigastrula from the rabbit), longitudinal section through the axis. Fig. 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). 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 1872–76. 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. Fig.73. Gastrula of the rabbit. Fig. 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. 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 _Geryonidæ_ and other medusæ)—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.