- yolk sac, vitelline sac, umbilical vesicle, vesicula umbilicus - membranous structure that functions as the circulatory system in mammal embryos until the heart becomes functional
--1 is a kind of sac
- yolk sac - membranous structure enclosing the yolk of eggs in birds reptiles marsupials and some fishes; circulates nutrients to the developing embryo
--2 is a kind of sac