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