yolk sac

noun Embryology.

1. an extraembryonic membrane that encloses the yolk of eggs in birds, reptiles, and marsupials and that circulates nourishment from the yolk to the developing embryo.
2. a similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.

noun zoology

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

n.

1. A membranous sac attached to an embryo, enclosing food yolk.umbilical vesicle

1. 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.