Gastrulation is the process in which the three germ layers (ectoderm, mesoderm and endoderm) are formed by successive waves of epiblast cells migrating through the primitive streak. Formation of the primitive streak on day 15, marks the first event of gastrulation. Cells from the epiblast migrate into the interior of the embryo, via the primitive streak, in a process termed ingression, which involves a cellular epithelial-to-mesenchymal transition (EMT). The initial wave of migrating cells (day 16) streams through the primitive streak, displacing the hypoblast cells to become definitive endoderm, which ultimately produces the future gut derivatives and gut linings. The second wave of migrating cells populate a layer between the epiblast and the definitive endoderm, thereby forming the mesoderm layer.