The allantois is a fetal membrane that develops from a small vesicle at the hindgut of the very early embryo. Its function is to collect liquid waste from and to exchange gases used by the embryo. The fetal bladder is connected to the allantois. With advancing embryonic development, the size of the human allantois is decreased and becomes an elongated sac and part of the umbilical cord. The allantois is vestigial in humans.