An elongated bladder developing from the hindgut of the fetus in mammals, birds, and reptiles: In mammals, the allantois contributes to the development of the umbilicus and plancenta. In birds and reptiles, it provides for the exchange of gases through the shell.

A membranous sac in the embryos of mammals, birds, and reptiles: The allantois has various functions.

The fetal membrane was named by ancient anatomists from its form in a calf): The allantois is found only in mammals, birds, and reptiles, and lies between the amnion and chorion, and forms a means of communication between the fetal and maternal blood.