Allantois

[Gr. allantos - a sausage]. An extraembryonic membrane formed by an extension of the hindgut into the extraembryonic coelom and consisting of endoderm and splanchnic mesoderm. In birds and reptiles, it fuses with the chorion to form the chorioallantoic membrane which assists in respiration, and in addition it is the site of storage of nitrogenous wastes. In eutherian mammals, it forms the vessels of the umbilical cord and fuses with the chorion to form the embryonic portion of the placenta.