WordNet (r) 3.0 (2006):

yolk sac
    n 1: membranous structure that functions as the circulatory
         system in mammalian embryos until the heart becomes
         functional [syn: yolk sac, vitelline sac, umbilical
         vesicle, vesicula umbilicus]
    2: membranous structure enclosing the yolk of eggs in birds,
       reptiles, marsupials, and some fishes; circulates nutrients
       to the developing embryo