Allantois

**Definition:** An extra-embryonic membranous sac derived from the YOLK SAC of REPTILES; BIRDS; and MAMMALS. It lies between two other extra-embryonic membranes, the AMNION and the CHORION. The allantois serves to store urinary wastes and mediate exchange of gas and nutrients for the developing embryo.

**Tree numbers:** - A10.615.284.147 - A16.254.750.147

**Synonyms:** - Allantoic Membrane