yolk sac

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yolk sac In the embryos of vertebrates, a sac arising as a diverticulum of the alimentary canal with which, except in Teleostei, it communicates. In Reptilia, Aves, and Monotremata it contains yolk which nourishes the embryo, and as the yolk is consumed the sac merges with the embryo. In Marsupialia, the sac contains no yolk, but absorbs nutrient from the uterine wall and in some cases forms a temporary placenta. In Eutheria the sac contains no yolk and most of it is cut off from the embryo at birth.

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