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Meaning of yolk sac

yolk' sac"

Pronunciation: [key]
— Embryol. Embryol.
  1. an extraembryonic membrane that encloses the yolk of eggs in birds, reptiles, and marsupials and that circulates nourishment from the yolk to the developing embryo.
  2. a similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.
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