membranous structure enclosing the yolk of eggs in birds reptiles marsupials and some fishes; circulates nutrients to the developing embryo membranous structure that functions as the circulatory system in mammal embryos until the heart becomes functional
thin membrane which surrounds the yolk of an egg and supplies nutrients to the embryo (in mammals, the yolk sac ceases to function when the placenta forms)
A membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals and functioning as the circulatory system of the human embryo before internal circulation begins