allantois

from WordNet (r) 3.0 (2006)

allantois
    n 1: the vascular fetal membrane that lies below the chorion and
         develops from the hindgut in many embryonic higher
         vertebrates (reptiles, birds and mammals)

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]