Allantois is an extra-embryonic membrane which is found in birds, reptiles, and mammals. It arises from the hindgut as a sac or pouch. It is one of the four membranes surrounding the embryo. The other three are amnion, chorion, and yolk sac. Allantois lies between amnion and chorion. Chorion is the outermost layer and amnion is the innermost layer. Amnion covers and protects the developing embryo while chorion covers the embryo as well as all the extraembryonic membranes. The fourth structure is called the umbilical vesicle in mammals or the yolk sac in birds and reptiles. Together these four membranes protect the embryo from drying out as well as function in the gaseous exchange. Allantois is a vital structure in all vertebrates except fish and amphibians which do not possess an allantois during development.
Functions of Allantois
The allantois functions as a storehouse of embryonic/fetal wastes. It also functions as a temporary respiratory organ and performs the exchange of gases used by the embryo. It is webbed with blood vessels. It functions together with chorion.
In placental mammals, between the fifth and seventh week of gestation, the allantois changes into urachus which drains the urinary bladder. The urachus runs through the umbilical cord which is a connection between the placenta and embryo/fetus.