During vertebrate evolution, the head underwent changes related to the development of the special senses. The anterior end of the nerve cord became a brain, and the nerves passing to and from the brain became the cranial nerves. In the prevertebrate amphioxus, which has no brain, muscle is present in the region of the mouth of the digestive system (see Plate 1-1). In the vertebrate fish, the gills have a branchial arch musculature that arises from the mesoderm associated with the developing pharyngeal region of the foregut. Therefore, this musculature can properly be called visceral musculature, even though it is voluntary and striated. A better term is branchial, or branchiomeric, musculature because it represents a serial division, or metamerism, of the lateral (gill or branchial) mesoderm that does not segment in its counterpart in the trunk.

In the human embryo, the branchial arches and their contained structures initially develop as if the aqueous gill-slit type of breathing apparatus were going to be retained. Instead of disappearing, most of the branchial arch structures are gradually modified and incorporated into the permanent acoustic and air-breathing respiratory systems. The branchiomeric musculature that develops from the mesoderm of the series of branchial arches on each side of the embryonic head becomes innervated by cranial nerves. Most of these muscles ultimately attach to the skull.

In addition to the skeletal muscles derived from the myotome and branchial arch, there are those that arise, in situ, directly from the local mesenchyme. Some of these locally derived muscles are the result of the slurring over of the sequence of evolutionary events during development, so that their derivation from myotome or branchial arch mesenchyme is obscured. Others, such as the limb muscles, appear relatively late in evolution and development. In the human embryo, the muscles of the limbs that evolved from fins appear after the myotomic and branchial arch musculature formation is well under way. The muscles of the pelvic diaphragm, perineum, and external genitalia also appear relatively late in development.

A developing skeletal muscle normally provides attractive forces that serve to guide a nerve to it. With only a few exceptions, the muscles retain their original innervation throughout life, no matter how far they may migrate from their site of origin during development; this is true whether a muscle is of myotomic origin and innervated by a spinal nerve or of branchial arch origin and innervated by a cranial nerve. Therefore, the innervation of adult muscles can be used as a clue to determine their embryonic origin. Embryonic muscle masses receive their motor innervation very early at or near their midpoint.

If a nerve supplies more than one muscle, it can be assumed that the muscles are subdivisions of an original myotome. Thus, the developmental histories of adult muscles formed by early fusion, splitting, migration, or other modifications can be reconstructed with considerable certainty.

Nearly all the skeletal muscles are present and, in essence, have their mature form in a fetus of 8 weeks with a crown-to-rump length of about 30 mm (see Plate 1-16). From the time the first myotomes begin to differentiate into skeletal muscles early in the fifth week, six fundamental processes that occur up to the eighth week are involved in the gross development of the muscles. Frequently, the formation of a muscle is the result of more than one of these processes.