The primary yolk sac is formed in human embryos by the extraembryonic mesoderm which forms the outer layer of the primary yolk sac in the beginning of the second week of development. During the second half of week 2, hypoblast cells (primitive endoderm) migrate and line the blastoceolic cavity of the blastocyst, forming the inner layer of the primary yolk sac and the Heuser's membrane. Although both the murine perietal yolk sac and the human primary yolk sac are transient structures of hypoblast origin, they are not developmental equivalents. A second wave of hypoblast cells migrates to form the definitive yolk sac (equivalent to the murine visceral yolk sac), which pinches off the lower half of the primary yolk sac. The primary yolk sac breaks up into small vesicles that can persist at the abembryonic pole of the extraembryonic coelom (exocoelomic cyst).