'yolk sac' definitions:
Definition of 'yolk sac'
(from WordNet)
noun
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]
noun
Membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo