When the blastula is fully formed, we have again in this case the important folding or invagination that determines gastrulation. The space between the skin-layer and the gut-layer (the remainder of the segmentation-cavity) remains full of food-yelk, which is gradually used up. This is the only material difference between our vesicular gastrula (_perigastrula_) and the original form of the bell-gastrula (_archigastrula_). Clearly the one has been developed from the other in the course of time, owing to the accumulation of food-yelk in the centre of the ovum.[23]
[23] On the reduction of all forms of gastrulation to the original palingenetic form see especially the lucid treatment of the subject in Arnold Lang’s _Manual of Comparative Anatomy_ (1888), Part I.
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.