yolk sac visceral yolk sac yolk sac FMA:296715 UBERON:0008852 Extraembryonic tissue membrane, formed from the visceral endoderm and the extraembryonic mesoderm, which is located ventral to the embryonic disc and is connected to the presumptive midgut of the embryo; the yolk that it contains is the site of embryonic hematopoiesis and vitelline circulation is involved in early embryonic circulation; it is the origin of the primordial germ cells. secondary yolk sac vitelline sac umbilical vesicle