When the yolk has become completely enveloped a postanal section of gut undoubtedly becomes formed; and although, owing to the solid condition of the central nervous system, a communication between the neural and alimentary canals cannot at first take place, yet the terminal vesicle of the postanal gut of Elasmobranchii is represented by a large vesicle, originally discovered by Kupffer (No. 68), which can easily be seen in the embryos of most Teleostei, but the relations of which have not been satisfactorily worked out (_vide_ fig. 34, _hyv_). As the tail end of the embryo becomes separated off from the yolk the postanal vesicle atrophies. General development of the Embryo. Attention has already been called to the fact that the embryo first appears as a thickening of the edge of the blastoderm which soon assumes a somewhat shield-like form (fig. 33 A). The hinder end of the embryo, which is placed at the edge of the blastoderm, is somewhat prominent, and forms the caudal swelling (_ts_). The axis of the embryo is marked by a shallow groove. [FIG. 33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON. (After His.) _ts._ tail swelling; _au.v._ auditory vesicle; _oc._ optic vesicle; _ce._ cerebral rudiment; _m.b._ mid-brain; _cb._ cerebellum; _md._ medulla oblongata; _m.so._ mesoblastic somite.] The body now rapidly elongates, and at the same time becomes considerably narrower, while the groove along the axis becomes shallower and disappears. The anterior, and at first proportionately a very large part, soon becomes distinguished as the cephalic region (fig. 33 B). The medullary cord in this region becomes very early divided into three indistinctly separated lobes, representing the fore, the mid, and the hind brains: of these the anterior is the smallest. With it are connected the optic vesicles (_oc_)--solid at first--which are pushed back into the region of the mid-brain. The trunk grows in the usual way by the addition of fresh somites behind. After the yolk has become completely enveloped by the blastoderm the tail becomes folded off, and the same process takes place at the front end of the embryo. The free tail end of the embryo continues to grow, remaining however closely applied to the yolk-sack, round which it curls itself to an extent varying with the species (_vide_ fig. 34).