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Dictionary

yolk' sac"

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.

yolk

yolk stalk

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