Birds

• Reptiles)
• Vascular fetal membrane that develops from the hindgut in embryonic higher vertebrates (mammals

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Allantois \Al*lan"to*is\, Allantoid \Al*lan"toid\, ] n.. (Anat.) A membranous appendage of the embryos of mammals, birds, and reptiles, -- in mammals serving to connect the fetus with the parent; the urinary vesicle.

Wikipedia Word definitions in Wikipedia
Allantois (; plural allantoides or allantoises ) is a sac-like structure that forms part of a developing amniote 's conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste. The ...

WordNet Word definitions in WordNet
n. vascular fetal membrane that develops from the hindgut in embryonic higher vertebrates (reptiles, birds and mammals)

Wiktionary Word definitions in Wiktionary
n. (context anatomy English) A sac, having a number of functions, that develops in the alimentary canal of the embryos of mammals, birds and reptiles.