At the time of the folding in of the hinder end of the mesenteron the splitting of the mesoblast into somatopleure and splanchnopleure has extended up to the border of the hinder division of the primitive streak. As has been already mentioned, the ventral wall of the postanal section of the alimentary tract is formed by the primitive streak. Immediately in front of this is the involution which forms the proctodæum; while the wall of the hindgut in front of the anus owes its origin to a folding in of the splanchnopleure. The allantois first appears as a protuberance of the splanchnopleure just in front of the anus. This protuberance arises, however, before the splanchnopleure has begun to be tucked in so as to form the ventral wall of the hindgut; and it then forms a diverticulum (fig. 123 A, _All_) the open end of which is directed forward, while its blind end points somewhat upwards and towards the peritoneal space behind the embryo. [FIG. 123. TWO LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EMBRYO CHICK TO SHEW THE ORIGIN OF THE ALLANTOIS. A AT THE BEGINNING OF THE THIRD DAY; B AT THE MIDDLE OF THE THIRD DAY. (After Dobrynin.) _t._ the tail; _m._ the mesoblast of the body, about to form the mesoblastic somites; _x´._ the roof of _x´´._ the neural canal; _Dd._ the hind end of the hindgut; _So._ somatopleure; _Spl._ splanchnopleure; _u._ the mesoblast of the splanchnopleure carrying the vessels of the yolk-sack; _pp._ pleuroperitoneal cavity; _Df._ the epithelium lining the pleuroperitoneal cavity; _All._ the commencing allantois; _w._ projection formed by anterior and posterior divisions of the primitive streak; _y._ hypoblast which will form the ventral wall of the hindgut; _v._ anal invagination; _G._ cloaca.] As the hindgut becomes folded in the allantois shifts its position, and forms (figs. 123 B and 124) a rather wide vesicle lying immediately below the hind end of the digestive canal, with which it communicates freely by a still considerable opening; its blind end projects into the pleuroperitoneal cavity below.