Medical Definition of Yolk sac
Yolk sac: Not all yolk has to do with birds' eggs. Human embryos have a yolk sac, too. The human yolk sac is a membrane outside the embryo that is connected by a tube (the yolk stalk) though the umbilical opening to the embryo's midgut. The yolk sac serves as an early site for the formation of blood and in time, is incorporated into the primitive gut of the embryo.
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