As mentioned earlier, it has now been established that the limb muscles do not develop from the local meso-derm but are derived from the meso-derm of the somites related to the limb buds cells derived from the dermo-myotome region of the somites migrate into the developing limb buds during the 4th week of development. As long bones begin to develop, the cells of somatic region give rise to myoblasts which form masses of muscles tissue located ventral and dorsal to the developing bones. The ventral muscle mass gives rise to the flexors and pronators of the upper limb and flexors and adductors of the lower limb, whereas the dorsal muscle mass gives rise to the extensors and abductors of the lower limb. The limb muscles are innervated by branches of the ventral primary rami of the spinal nerves (Cs to T1 for the upper limb and L4 to S3 for the lower limb).
Muscles derived from the ventral muscles mass are innervated by anterior divisions of the ventral primary rami. Similarly, muscles originating in the dorsal muscles mass receive their nerve supply from the posterior divisions of the ventral primary rami.