MEANING OF YOLK SAC
yolk sac (vitelline sac) 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.