Amniotic fluid is in the amniotic sac. Amniotic fluid is present from the formation of the gestational sac. Cord prolapse is more common in women who have had rupture of their amniotic sac. The immature ova originate from cells from the dorsal endoderm of the yolk sac. Hematopoiesis first takes place in the yolk sac. Each young chameleon is born within the sticky transparent membrane of its yolk sac. The hypoblast is pushed down and forms the yolk sac (exocoelomic cavity) lining. The pericardium is a tough double layered fibroelastic sac which covers the heart. The splanchnic depends on a continuous layer that covers the yolk sac. Half of the people with yolk sac tumors are diagnosed in stage I. The umbilical cord develops from and contains remnants of the yolk sac and allantois. There is no clinical significance to a residual external yolk sac. The developing embryos are at first sustained by a yolk sac. Pericarditis is inflammation of the pericardium (the fibrous sac surrounding the heart). Two polar nuclei are left in the central cell of the embryo sac.