Initially, the thin sheath of mesodermal layer proliferates and forms the paraxial mesoderm medially and lateral plate laterally. They are connected by the intermediate mesoderm. The lateral plate divides into two layers: somatic (parietal) and splanchnic (visceral) mesoderm. The paraxial mesoderm breaks into segmented blocks, the somites (42-44 pairs).
The epithelial cells forming the somites lose their epithelial shape and migrate in the direction of the notochord and the spinal cord to form the sclerotome (future vertebral column). The dorsal wall of the somite differentiate into the myotome (furute muscles) and the dermatome (future dermis).
The intermediate mesoderm forms nephrotomes cranially and nephrogenic cord caudally, both developing into the excretory units of kidneys, gonads, ducts and accessory glands.