The ventromedial part of each somite is induced to become a sclerotome by factors secreted by the ventral neural tube and notochord. The sclerotomes will form the vertebrae and intervertebral discs.
Factors secreted by the dorsal neural tube induce the dorsomedial part of the somite to become the epimere which will develop into the epaxial musculature, ie. deep muscles of the back (erector spinae).
Factors produced by the surface ectoderm and the somatic layer of the lateral plate mesoderm induce the formation of the hypomere from the lateral ventral part of the myotome.
The dorsal part of the neural tube induces the formation of the dermatome to form the dermis of the skin in each specific body segment.
In order to add confusion to the process, embryologists have now come up with new designations for the origins of muscles.
The primaxial domain refers to mesoderm derived exclusively from the somite mesoderm, the myotome. Muscles derived from the primaxial domain include the epaxial muscles or erector spinae and related deep back muscles innervated by the dorsal primary rami of spinal nerve and also some of the muscles of the shoulder girdle and intercostal muscles, innervated by ventral primary rami of spinal nerves.
The abaxial domain refers to mesoderm of both somite and lateral plate origin, forming the infrahyoid muscles, abdominal wall muscles and the limb muscles. These muscles are all innervated by the ventral primary rami of spinal nerves.