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yolk sac
- a saclike vascular membrane containing yolk, an outgrowth from the ventral surface of very yolky vertebrate embryos, as those of fishes, birds, or reptiles, that supplies nourishment to the embryo
- a homologous organ in most mammalian embryos that contains no yolk and that becomes a vestige at an early embryonic stage
See yolk sac in American Heritage Dictionary 4
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