yolk sac

The yolk sac, also called the vitelline sac, is the membranous sac, composed of mesoderm lined with endoderm, that lies ventral to the embryo. It is one of the extraembryonic membranes. Its initially wide communication with the future gut is later reduced to a narrow duct passing through the umbilicus. It probably assists in transporting nutrients to the early embryo and is one of the first sites where blood cells are formed.