|1.||the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk|
|2.||the corresponding part in the embryo of mammals, which contains no yolk|
yolk sac n.
A membranous sac attached to an embryo, enclosing food yolk. Also called umbilical vesicle.
A sac that is attached to the gut of an embryo and encloses the yolk in bony fish, sharks, reptiles, mammals, and birds. In most mammals, the yolk sac functions as part of the embryo's circulatory system before the placenta develops.