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| Yolk Sac Sac, Yolk | Sacs, Yolk | Yolk Sacs An embryonic membrane formed from endoderm and mesoderm. In reptiles and birds it incorporates the yolk into the digestive tract for nourishing the embryo. In placental mammals its nutritional function is vestigial; however, it is the source of most of the intestinal mucosa and the site of formation of the germ cells. It is sometimes called the vitelline sac, which should not be confused with the vitelline membrane of the egg. |