Epiblast

In embryonic development, differentiation of mesenchyme into epithelium occurs in the mesenchymal cell of the outer germ layer, called epiboly or epibolus (from Latin “above the top”, “above the surface”). Epiboly, consisting of flat polygonal monolayer cells in comparison with other areas, is formed by mesenchytic cells of the middle germ layer. This process leads to the formation of a primitive organ called the epibolus. It forms the basis of specialized tissue of embryogenesis - endoderm (internal germ layer). The epibolus, as an internal position remote from the body cavity, acts as the primary rudiment of the coelomia, forming the anterior (boarding) wall of the coelomia or the parietal serous anlage of the coelium (parietablasts).

The second rudiment of the coelomia is located symmetrically with respect to the epibolyum - the slit-shaped gonad (ovulation) or the mesobolus gonad anlage (mesoblast). The latter lies to the internal goblet angle of the preceding cells and contributes to the formation of the genitourinary system. During the next stage of development, the lining of the gonad or the primary urogenital tube is laid in the mesobolus cells and forms a closed lumen.