part of part of contributes to morphology of luminal space of anatomical space extraembryonic structure lumen of epithelial sac visceral yolk sac visceral yolk sac cavity The closed space containing the yolk, formed by the fusion of the edges of the embryonic epiblast (hypoblast) and later surrounded by the visceral yolk sac (hypoblast and visceral yolk sac mesoderm). UBERON:0009743 extraembryonic cavity