Yolk sac:
Not all yolk has to do with birdsÆ eggs. Human embyros have a yolk sac, a membane outside the embryo but connected by a tube (the yolk stalk) though the umbilical opening to the embryoÆs 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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