yolk sac

Noun

• membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo

• membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional

Related words

• Synonyms: 
  vitelline sac , umbilical vesicle , vesicula umbilicus