The muscles of the neck and trunk develop from the myotomes which are derived from the somites. During the 5th week, of development each myotome becomes divided into two parts, each re-. presenting a muscular mass. The subdivisions of the myotomes are: (1) a
smaller, dorsal part called epaxial division or epimere, and (2) a larger, ventral part called hypaxial division or hypomere. Each developing spinal nerve also divides into a posterior primary ramus that gives nerve supply to the epimere, and an anterior primary ramus that innervates the hypomere.