Allantois

allantois During the embryonic stage of mammals, birds, and reptiles, the allantois is a small sac. It is one of four extra-embryonic membranes (along with amnion, yolk sac, chorion) and serves several functions. Such as a repository for the embryo’s nitrogenous waste (chiefly uric acid) in reptiles and birds (in the egg). The allan­tois provides oxygen to mammals, birds, and reptiles, as well as food in mammals (via the placenta).

The membrane of the allantois works with the chorion in respiratory functions, allowing the exchange of gases between the embryo and surrounding air. In humans, it is involved in the development of the uri­nary bladder.