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Noun: yolk sac
  1. Membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
    - vitelline sac, umbilical vesicle, vesicula umbilicus
  2. Membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo

Derived forms: yolk sacs

Type of: sac

Encyclopedia: Yolk sac