yolk sac In the embryos of vertebrates, a sac arising as a diverticulum of the alimentary canal with which, except in Teleostei, it communicates. In Reptilia, Aves, and Monotremata it contains yolk which nourishes the embryo, and as the yolk is consumed the sac merges with the embryo. In Marsupialia, the sac contains no yolk, but absorbs nutrient from the uterine wall and in some cases forms a temporary placenta. In Eutheria the sac contains no yolk and most of it is cut off from the embryo at birth.

yolk sac Membranous sac-like structure in the eggs of most animals. It attaches directly to the ventral surface or gut of the developing embryo in the eggs of birds, reptiles, and some fish, and contains yolk. The term also refers to an analogous sac-like membrane that develops below the mammalian embryo. It contains no yolk but is connected to the umbilical cord.

yolk sac One of the protective membranes surrounding the embryos of birds, reptiles, and mammals (see extraembryonic membranes). The embryo derives nourishment from the yolk sac via a system of blood vessels. In birds and reptiles the yolk sac encloses the yolk; in most mammals a fluid replaces the yolk.

yolk sac (vitelline sac) (yohk) n. the membranous sac that lies ventral to the embryo. One of the extra-embryonic membranes, it probably assists in transporting nutrients to the early embryo and is one of the first sites where blood cells are formed.