The allantois is a membranous sac that develops as an outgrowth of the embryonic hindgut. Its outer (mesodermal) layer carries blood vessels to the placenta and so forms part of the umbilical cord. Its cavity is small and becomes further reduced in size during fetal development.

The allantois occurs in the embryos of mammals, birds, and reptiles. It is involved in respiration and the absorption of nutrients in mammals, and acts as a bladder for the storage of waste products within the egg in birds and reptiles.