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The inner cell mass (ICM) segregates into a bilaminar embryonic disc (bilaminar blastoderm) which consists of two epithelial layers, each of a distinct lineage: the external (dorsal) epiblast and the internal (ventral) hypoblast. The hypoblast is a transient structure which contributes to the development of the extraembryonic mesoderm and yolk sac and plays a key role in signaling to establish axial patterning in the embryo itself.
During gastrulation, the process in which the three germ layers of the trilaminar embryonic disc are formed, cells from the epiblast migrate, via the primitive streak, into the interior of the embryo, in a process termed ingression, a process which involves cellular epithelial-to-mesenchymal transition (EMT). The initial wave of migrating cells streams through the primitive streak, displacing the hypoblast cells to become definitive endoderm, which ultimately produces the future gut derivatives and gut linings.