yolk sac

yolk sac [′yōk ‚sak]

(embryology)

A distended extraembryonic extension, heavy-laden with yolk, through the umbilicus of the midgut of the vertebrate embryo.

---

Yolk sac

An extraembryonic membrane which extends through the umbilicus in vertebrates. In some elasmobranchs, birds, and reptiles, it is laden with yolk which serves as the nutritive source of embryonic development.

In mammals, as in birds, the yolk sac generally develops from extraembryonic splanchnopleure, and extends beneath the developing embryo. A blood vessel network develops in the mammalian yolk sac lining. Though these blood vessels are empty, they play an important role in absorbing nourishing food and oxygen from the mother. Thus, although the yolk sac in higher mammals may be considered an evolutionary vestige from its yolky-egged ancestors, it still serves important functions in the young embryo. As the embryo ages, the yolk sac shrinks in size, and the allantois takes over the role of nutrition. See Allantois