SOMITES (Greek soma a body) — the dorsal part of a mesoderm of a germ lying on both sides of a chord and a neurotubule, which is dismembered, or segmented in the kranio-caudal direction. In the course of segmentation (differentiation) nephrotomes and splanchnotomes are formed of a mesoderm of vertebrata along with S. (see. Germ , Mesoderm ).
Pages at a germ of the person appear on 21 — the 23rd days of pre-natal development. Process of segmentation continues throughout 4 — the 5th week of development and by 35th day comes to the end, at the same time 43 — 44 couples are formed S. Srok of education at animals is various: e.g., at a chicken germ segmentation begins at the end of the first days of an incubation.
Cellular material C. has an appearance of homogeneous epithelial-like column cells in the beginning, kernels to-rykh are located at the different levels. Afterwards there is a differentiation of this material and its division on the dorsolateral site — the dermatome spreading an ectoderm, the medio-ventral site — the sclerotome adjoining a neurotubule and a chord and the site, intermediate between them — a myotome.
Cells of a dermatome form mesenchyma (see), from a cut the connective tissue basis of skin develops; the sclerotome forms the skeletogenous mesenchyma which later is differentiated on cartilaginous and bone tissues of an axial skeleton; the myotome makes a rudiment of skeletal (somatic) muscles, its cells early become myoblasts. The dorsomedial edge of a dermatome most longer keeps an epithelial-like arrangement of cells, to-rye, proliferating, provide replenishment of cellular structure of a dermatome and myotome. At mammals replenishment of a myotome and at the expense of cells of a sclerotome is possible.
Two first head segments of a mesoderm (premandibulyarny and mandibular) are not subdivided into S., oculomotor muscles and partially — connecting fabric are formed of them.
Bibliography: Belousov L. V. Introduction to the general embryology, M., 1980; Knorre A. G. Embryonal histogenesis, L., 1971; Konstantinov A. V. Biology of an ontogeny, Minsk, 1978.
O. V. Volkova.