Somite(redirected from Primitive segments)
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a paired metameric formation in the embryos of such invertebrates as annelids and insects and in the embryos of man and all other chordates. The middle germ layer, or mesoderm, is segmented into somites during embryonic development. The somites are arranged along the longitudinal axis of the body alongside the neural tube and the chorda. The entire mesoderm is segmented in invertebrates, whereas in man and other chordates only its dorsal section is segmented, with the remaining mesoderm forming lateral plates, or splanchnotomes. Segmentation begins from the anterior section of the body and gradually extends to the posterior section. In the process of development, each somite decomposes into a myotome, a sclerotome, and a dermatome, from which are formed, respectively, the truncal musculature, the axial skeleton (in fish, also the skeleton of the fins), and the connective-tissue portion of the skin and its derivatives.
T. A. DETLAF