During the 5th week of development the ventral rami of certain spinal nerves unite to form plexuses for limbs. The brachial plexus (Cs-8. T1) for the upper limb, and the lumbo sacral plexus (L.4, 5, Si-3) for the lower limb.
The main (named) nerves develop from these plexuses and extend progressively into the developing limbs. Due to mixing of nerve fibers in the plexuses, the main nerves of the limbs contain fibers from several spinal segments, (eg, Cs-Ti, in the radial nerve and C7-T1 in the ulnar nerve). The main nerves then give branches to the muscular groups and to the skin and by the end of the 7th week the pattern is similar to that in the adult.