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Yolk sac

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noun, plural: yolk sacs

A membranous sac formed by the hypoblast and encases the yolk of eggs in bony fishes, sharks, reptiles, birds, and primitive mammals, or a hollow cavity in placental mammals


Fetal membranes are the membranes surrounding the embryo or fetus. These membranes are the amnion, the chorion, the allantois, and the yolk sac. The yolk sac is the membrane that encloses the yolk eggs in bony fishes, sharks, reptiles, birds, and primitive mammals. In placental mammals, the yolk sac encloses a hollow cavity and loses its nutritive function as the placenta develops.1

The yolk sac is formed by the hypoblast cells near the embryonic disk. Since it encases the yolk in the eggs of certain animals, the yolk sac is therefore associated with the provision of nourishment to the growing embryo.

Other animals, such as placental mammals, the yolk sac loses this function since the placenta acts as a site for nutrient exchange from the mother to the offspring in utero. The yolk sac is also associated with the formation of the primordial gut during the fourth week of development.2


  • umbilical vesicle

See also:

1 yolk sac. (n.d.). Unabridged. Retrieved from website [[1]]
2 The Developing Human: Clinically Oriented Anatomy: Chapter 7.