Noun: yolk sac
- Membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
"The yolk sac provides nutrients to the developing embryo";
- vitelline sac, umbilical vesicle, vesicula umbilicus
- Membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo
"The chick embryo's yolk sac provides nourishment during development"
Derived forms: yolk sacs
Type of: sac
Encyclopedia: Yolk sac