Until the fifth gestational week, the septum transversum is opposite the third, fourth, and fifth cervical somites. During the fifth week, however, the myoblasts that have formed from the somitic myotomes migrate into the primitive diaphragm. Along with these primitive muscle cells come their nerves, which migrate through the pleuropericardial membranes. Thus, the phrenic nerves are comprised of the third, fourth, and fifth cervical nerves.
The diaphragm migrates downward as a result of the dorsal part of the embryo developing more rapidly than the ventral part. The developing diaphragm is at the level of the thoracic somites by the sixth week and by the beginning of the eighth week is at the level of the first lumbar vertebra. All the while, the phrenic nerves have been migrating with the diaphragm and thus have been increasing in length to be the motor nerve supply to the diaphragm. The phrenic nerves also supply sensory data to the center of the diaphragm. The peripheral parts of the diaphragm, however, which developed from the body wall, receive their nerve supply from the lower six or seven intercostal nerves. As the various parts of the developing diaphragm fuse together, some mesenchyme of the septum transversum migrates into other parts of the diaphragm and forms the myoblasts that differentiate into the muscle of the diaphragm.