The allantois[67] is essentially a diverticulum of the alimentary tract into which it opens immediately in front of the anus. Its walls are formed of splanchnic mesoblast with blood-vessels, within which is a lining of hypoblast. It becomes a conspicuous object on the third day of incubation, but its first development takes place at an earlier period, and is intimately connected with the formation of the posterior section of the gut.
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.
Still later the allantois grows forward, and becomes a large spherical vesicle, still however remaining connected with the cloaca by a narrow canal which forms its neck or stalk ([fig. 121] G, al). From the first the allantois lies in the pleuroperitoneal cavity. In this cavity it grows forwards till it reaches the front limit of the hindgut, where the splanchnopleure turns back to enclose the yolk-sack. It does not during the third day project beyond this point; but on the fourth day begins to pass out beyond the body of the chick, along the as yet wide space between the splanchnic and somatic stalks of the embryo, on its way to the space between the external and internal folds of the amnion, which it will be remembered is directly continuous with the pleuroperitoneal cavity ([fig. 121] K). In this space it eventually spreads out over the whole body of the chick. On the first half of the fourth day the vesicle is still very small, and its growth is not very rapid. Its mesoblast wall still remains very thick. In the latter half of the day its growth becomes very rapid, and it forms a very conspicuous object in a chick of that date ([fig. 118], Al). At the same time its blood-vessels become important. It receives its supply of blood from two branches of the iliac arteries known as the allantoic arteries[68], and the blood is brought back from it by two allantoic veins which run along in the body walls ([fig. 119]) and after uniting into a single trunk fall into the vitelline vein close behind the liver.
