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yolk sac

n :  a membranous sac that is attached to an embryo and encloses food yolk, that is continuous in most forms through the omphalomesenteric duct with the intestinal cavity of the embryo, that being abundantly supplied with blood vessels is throughout embryonic life and in some forms later the chief organ of nutrition, and that in placental mammals is nearly vestigial and functions chiefly prior to the formation of the placenta .

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