Source : WordNet®

yolk sac
     n 1: membranous structure that functions as the circulatory
          system in mammal embryos until the heart becomes
          functional [syn: {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