The somites, when first formed, are masses of mesodermal cells with a small cavity in the middle. The cells are arranged radially around the central cavity. With further development, the shape of the somites changes; they become extended in the dorsoventral direction and flattened mediolaterally. In the vertebrates with discoidal cleavage, this change in shape leads to the elevation of the dorsal parts of the embryo above the general level of the blastodisc.
The flattening of the somite is accompanied by a change in the shape of its central cavity (the myocoele). Instead of being spherical, the cavity becomes a narrow vertical slit. An inner wall and an outer wall become clearly distinguishable, corresponding to the parietal and visceral layers of the lateral plates. The inner wall of the somites becomes very much thicker than the outer wall.
The fate of the inner and outer walls of the somites is completely different. The outer wall contributes to the formation of the connective tissue layer of the skin and is therefore called the dermatome. The inner wall produces skeletogenous tissue and the voluntary striated muscles of the body. The skeletogenous tissue develops from the lower edge of the inner wall, and this part of the somite is therefore called the sclerotome.
The sclerotome breaks up into a mass of mesenchymal cells. The cells migrate into the spaces surrounding the notochord and the spinal cord, envelop these organs, and later differentiate into cartilage, thus forming the bodies and the neural arches of the vertebrae. The hemal arches in the tail region and the ribs are of the same origin.
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The dorsal part of the inner wall of the somite is the source of somatic muscle in the vertebrate’s body and is therefore called the myotome. The cells of the myotome rearrange themselves so that they become elongated in a longitudinal direction. These longitudinally elongated cells differentiate subsequently into the striated muscle fibers.
Originally each myotome becomes a muscle segment, separated from the one anterior and the one posterior to it by a connective tissue layer, the vertical myocomma. At one time in its development, the whole somatic musculature of the vertebrate consists of such segments arranged in linear order.
In the lower vertebrates, the myotome is the largest part of the somite, the sclerotomes being small and rather inconspicuous. In the Amniota, however, the sclerotomes are much larger. Only the upper edge of the inner wall of the somite adjoining the dermatome becomes the myotome. The size of this part increases rapidly; the myotomes grow downward to assume the same position, lateral to the neural tube and the notochord, that they occupy in the amphibians right from the start.
Since they develop from the somites, the myotomes are originally dorsal in position. In the course of subsequent development, the muscle segments spread downward in the space between the skin on the outside and the somatic layer of the lateral plate on the inside, until the muscle segments on the right and the left sides meet ventrally.
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With minor alterations, the somatic muscles persist in this condition in the fishes and in the aquatic larvae of amphibians. The segmentation of the lateral and ventral muscles, as well as of the dorsal muscles, is directly derived from the segmentation of the mesodermal mantle in somites.
In terrestrial vertebrates, the primitive segmentation of the somatic muscles is more or less obliterated in connection with a change in locomotion. The segmented lateral bands of muscle are adapted for locomotion by lateral inflections of the body and the tail. The locomotion by means of two pairs of legs requires a completely different organization of the muscle system. Consequently, only traces of the original muscle segmentation can be discovered in the terrestrial vertebrates.
In Amphioxus, muscle segments continue anteriorly almost to the tip of the snout. With the development of the brain and the skull in vertebrates, the head region is exempted from the process of propelling the body by lateral inflections. The somatic muscles become superfluous in the head region. Nevertheless, mesodermal somites are formed in the head region of the embryo.
The muscle segments derived from the somites lying posterior to the ear (the postotic somites) may persist and be linked with the muscle segments of the neck region. The somites lying anterior to the ear vesicle are always very transitory structures.
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Part of the cells of these somites, however, differentiates as somatic muscle. The muscles derived from this source do not serve for locomotion but are the six pairs of oculomotor muscles – the four rectus muscles and the two oblique muscles on each side.