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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.


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.


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


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 allantois, along with the amnion and chorion (other embryonic membranes), identify humans, and other mammals, as amniotes. Other amniotes include reptiles, dinosaurs, and birds. Of the vertebrates, only Ichthyopsidas ( fish and amphibians) lack this structure.