Dictionary of Human Evolution and Biology

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Allantois

Membranous sac outside the body of the embryo that develops out of the yolk sac. In oviparous animals it serves as a waste repository, but in mammals it functions in the formation of blood cells during the embryo stage. Later the blood vessels of the allantois become the umbilical blood vessels. The allantois also contributes to the formation of the urinary bladder.

See angiogenesis.

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