yolk sac, a structure that develops in the inner cell mass of the embryo and expands into a vesicle with a thick part that becomes the primitive gut and a thin part that grows into the cavity of the chorion. The cells of the extraembryonic mesoderm differentiate to develop endothelium, primitive blood plasma, and hemoglobin. The yolk sac usually disappears during the seventh week of pregnancy. See also allantois, Meckel’s diverticulum.
