Skeletal (voluntary) system.

      The dermomyotome further differentiates into the myotome and the dermatome.

      Cells of the myotome migrate ventrally to surround the intraembry-onic coelom and the somatic mesoderm of the ventrolateral body wall. These myoblasts elongate, become spindle-shaped, and fuse to form multinucleated muscle fibers.

      Myofibrils appear in the cytoplasm, and, by the third month, cross-striations appear. Individual muscle fibers increase in diameter as myofibrils multiply and become arranged in groups surrounded by mesenchyme.

      Individual muscles form, as well as tendons that connect muscle to bone.

      Trunk musculature: By the end of the fifth week, body-wall musculature divides into a dorsal epimere, supplied by the dorsal primary ramus of the spinal nerve, and a ventral hypomere, supplied by the ventral primary ramus.

      Epimere muscles form the extensor muscles of the vertebral column, and hypomere muscles give rise to lateral and ven tral flexor musculature.

      The hypomere splits into three layers. In the thorax, the three layers form the external costal, internal intercostal, and transverse thoracic muscle.

      In the abdomen, the three layers form the external oblique, internal oblique, and transverse abdomii muscles. Head musculature.

      The extrinsic and intrinsic muscles of the tongue are thought to be derived from occipital myotomes that migrate forward.

      The extrinsic muscles of the eye may derive from preoptic myoto-mes that originally surround the prochordal plate.

      The muscles of mastication, facial expression, the pharynx, and the larynx are derived from different pharyngeal arches and maintain their innervation by the nerve of the arch of origin.

      Limb musculature originates in the seventh week from soma meso-derm that migrates into the limb bud. With time, the limb musculature splits into ventral flexor and dorsal extern groups.

      The limb is innervated by spinal nerves, which penetrate the limb bud mesodermal condensations. Segmental branches of the spinal nerves fuse to form large dorsal a ventral nerves.

      The cutaneous innervation of the limbs is also derived from spinal nerves and reflects the level at which the limbs arise.

      Smooth muscle: the smooth muscle coats of the gut, trachtea, bronchi, and blood vessels of the associated mesenteries are derived from splanchnic mesoderm surrounding the gastrointestinal tract. Vessels elsewhere in the body obtain their coat from local mesenchyme.

      Cardiac muscle, like smooth muscle, is derived from splanchnic mesoderm.