Yolk Sac

 
Yolk sac: Not all yolk has to do with birds' eggs. Human embryos have a yolk sac, a membrane outside the embryo but 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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