Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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