It is a hollow sac-like structure filled with clear fluid that forms part of the developing conceptus in an amniote that helps the embryo exchange gases and handle liquid waste.

The allantois, along with the amnion, chorion, and yolk sac (other extraembryonic membranes), identify humans and other mammals, birds, and reptiles as amniotes.

This sac-like structure, whose name is Greek for sausage (from ἀλλαντοειδής allantoeidḗs, in reference to its shape when first formed)[3] is primarily involved in nutrition and excretion, and is webbed with blood vessels.

In mammals, except the egg-laying monotremes, the allantois is one of the fetal membranes, and is part of and forms an axis for the development of the umbilical cord.

[4] The human allantois is a caudal out-pouching of the yolk sac, which becomes surrounded by the mesodermal connecting stalk known as the body-stalk.

The mouse allantois consists of mesodermal tissue, which undergoes vasculogenesis to form the mature umbilical artery and vein.

Because the allantois can be cultured ex vivo, it has utility as a model system for studying the formation of blood vessels as well as considerable usefulness in drug screening.