Membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional.

synonyms: umbilical vesicle, vesicula umbilicus, vitelline sac.

Polish: pęcherzyk żółtkowy

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