1. An elongated bladder developing from the hindgut of the fetus in mammals, birds, and reptiles. In mammals, it contributes to the development of the umbilicus and plancenta. In birds and reptiles, it provides for the exchange of gases through the shell.
2. A membranous sac in the embryos of mammals, birds, and reptiles.
3. The fetal membrane (so named by ancient anatomists from its form in a calf) found only in mammals, birds, and reptiles, which lies between the amnion and chorion, and forms a means of communication between the fetal and maternal blood.