part of part of develops from develops from adjacent to adjacent to inner cell mass hypoblast (generic) bilaminar disc epiblast (generic) inner cell mass derived hypoblast A thin monolayer of small cuboidal cells that make up the lower layer of the bilaminar embryonic disc, beneath the epiblast. EHDAA:44 UBERON:0008776 BTO:0002123 Wikipedia:Hypoblast visceral endoderm hypoblast proximal endoderm UBERON:0000417 This layer can be distinguished from the overlying epiblast in the mouse embryo as early as day 4.5, and day 7 in human). The cells of the hypoblast are not thought to contribute to the developing embryo proper, but contribute to several extraembryonic structures. In rodents, the hypoblast cells form the parietal endoderm, parietal and visceral yolk sac as well as extraembryonic membranes, such as the extraembryonic endoderm, the yolk sac and the stalk that links it to the endodermal digestive tube. In human embryos, hypoblast cells migrate and line the blastocoelic cavity of the blastocyst, forming the primary yolk sac and Heuser's membrane. Although both the murine parietal yolk sac and the human primary yolk sac are transient structures, they are not developmental equivalents. A second wave of hypoblast cells migrates to form the definitive yolk sac which displaces the primary yolk sac and is equivalent to the murine visceral yolk sac. The primary yolk sac breaks up into small vesicles that can persist at the mural (abembryonic) pole. UBERON:0003225 UBERON:0003226 EMAPA:16051 EHDAA:79 primary endoderm EHDAA2:0000796 FMA:85520 primitive endoderm