The sole-shaped germinal shields of all the amniotes are still, at the stage of construction which Fig. 134 illustrates in the rabbit and Fig. 135 in the opossum, so like each other that we can either not distinguish them at all or only by means of quite subordinate peculiarities in the size of the various parts. Moreover, the human sandal-shaped embryo cannot at this stage be distinguished from those of other mammals, and it particularly resembles that of the rabbit. On the other hand, the outer form of these flat sandal-shaped embryos is very different from the corresponding form of the lower animals, especially the acrania (amphioxus). Nevertheless, the body is just the same in the essential features of its structure as that we find in the chordula of the latter (Figs. 83–86), and in the embryonic forms which immediately develop from it. The striking external difference is here again due to the fact that in the palingenetic embryos of the amphioxus (Figs. 83, 84) and the amphibia (Figs. 85, 86) the gut-wall and body-wall form closed tubes from the first, whereas in the cenogenetic embryos of the amniotes they are forced to expand leaf-wise on the surface owing to the great extension of the food-yelk. It is all the more notable that the early separation of dorsal and ventral halves takes place in the same rigidly hereditary fashion in all the vertebrates. In both the acrania and the craniota the dorsal body is about this period separated from the ventral body. In the middle part of the body this division has already taken place by the construction of the chorda between the dorsal nerve-tube and the ventral canal. But in the outer or lateral part of the body it is only brought about by the division of the coelom-pouches into two sections—a dorsal _episomite_ (dorsal segment or provertebra) and a ventral _hyposomite_ (or ventral segment) by a frontal constriction. In the amphioxus each of the former makes a muscular pouch, and each of the latter a sex-pouch or gonad. Fig.131. Embryo of the opossum, sixty hours old. Fig. 132. Sandal-shaped embryonic shield of a rabbit of eight days. Fig. 131—Embryo of the opossum, sixty hours old, one-sixth of an inch in diameter. (From _Selenka_) _b_ the globular embryonic vesicle, _a_ the round germinative area, _b_ limit of the ventral plates, _r_ dorsal shield, _v_ its fore part, _u_ the first primitive segment, _ch_ chorda, _chr_ its fore-end, _pr_ primitive groove (or mouth). Fig. 132—Sandal-shaped embryonic shield of a rabbit of eight days, with the fore part of the germinative area (_ao_ opaque, _ap_ pellucid area). (From _Kölliker._) _rf_ dorsal furrow, in the middle of the medullary plate, _h, pr_ primitive groove (mouth), _stz_ dorsal (stem) zone, _pz_ ventral (parietal) zone. In the narrow middle part the first three primitive segments may be seen.