The Allantois, or urinary vesicle, in the chick is of importance, as the vessels developed in it are used for respiratory purposes, being spread out beneath the porous shell. In the mammalian embryo it is still more important, as it is the seat of the circulation, which performs the chief function of the foetal placenta. The allantois arises at the tail of the embryo, as a budding outward of a portion of the splanchnopleure forming the wall of the primitive intestine. It is lined by hypoblast, and projects into the pleuro-peritoneal cavity. As it grows away from the embryo it extends between the layers of the true and false amnion and approaches toward the vitelline membrane, but remains connected to the intestine by a narrow tube. When it reaches the periphery of the ovum, it spreads over the chorion as a complex lining, and sends processes into the villi of that organ. It becomes chiefly developed at that part of the chorion which is opposite the decidua serotina of the mother. In the mesoblastic laver of the allantois blood vessels arise which are connected with large trunks, proceeding from the primitive aortae, called the umbilical arteries; these will, however, be further described when treating of the foetal placenta.

Diagram of an embryo, showing the relationship of the vascular allantois to the villi of the chorion.

Fig. 274. Diagram of an embryo, showing the relationship of the vascular allantois to the villi of the chorion. (Cadiat).

a. Lies in the cavity of the amnion under the embryo, b. Yolk sac. c. Allantois. d. Vessels of the allantois dipping into the villi of the chorion, e. Chorion.

As the foetus becomes developed, the part of the allantois in connection with the body becomes gradually obliterated. A part of it remains as the urinary bladder, and the rest forms a fibrous cord, which runs from the apex of the bladder to the umbilicus, and is known as the urachus.