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1. (n.) yolk sac
an extraembryonic membrane that encloses the yolk of eggs.
Etymology: (1860–65)
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| Definition of 'yolk sac' |
Princeton's WordNet |
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1. (noun) yolk sac, vitelline sac, umbilical vesicle, vesicula umbilicus
membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
2. (noun) yolk sac
membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo
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| Definition of 'yolk sac' |
U.S. National Library of Medicine |
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1. yolk sac
The first of four extra-embryonic membranes to form during EMBRYOGENESIS. In REPTILES and BIRDS, it arises from endoderm and mesoderm to incorporate the EGG YOLK into the DIGESTIVE TRACT for nourishing the embryo. In placental MAMMALS, its nutritional function is vestigial; however, it is the source of INTESTINAL MUCOSA; BLOOD CELLS; and GERM CELLS. It is sometimes called the vitelline sac, which should not be confused with the VITELLINE MEMBRANE of the egg.
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Definitions of yolk sac 
