The muscles of the back develop from the epaxial primordium (epaxial part of the myotome) and are supplied by the posterior branches of the spinal nerve. The muscles of the ventro-lateral trunk and the limbs come from the hypaxial primordium (hypaxial part of the myotome) and are innervated by the anterior branches of the spinal nerve. The pioneer cells of the hypaxial part form a premuscular mass that is subdivided by invading somatopleural cells on each side into four abdominal muscle blastemata: M. obliquus externus, m. obliquus internus, m. transversus and m. rectus abdominis. These muscle blastemata initially lie on the dorsal aspect of the abdominal wall, before shifting ventrally to their end position. The connective tissue between the blastemata of both sides condenses to form the linea alba. In the thoracic region, the intercostal muscles remain segmentally separated. They also grow in a dorsal to ventral direction.