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Somites


- See: limb development

- Discussion:
    - skeleton and skeletal muscle develop from the mesoderm;
          - thin layer of cells in early embryo located between ectoderm & endoderm & interrupted in midline by notochord;
    - mesoderm on either side of notochord forms longitudinal mass, paraxial mesoderm;
            - at 21st day of development, paraxial mesoderm forms paired, cuboidal masses, somites;
    - somites first form first occipital somites, at cranial end of the notochord;
    - subsequent pairs of somites appear caudally next to previous pair;
    - total of some 42 to 44 pairs of somites are formed between the 21st and the 30th days of development;
    - lateral plate mesoderm:
          - remains largely unsegmented
          - remains attached to ventrolateral angle of each somite by intermediate mesoderm, which later forms the nephrogenic cord;
    - in neck & trunk, each somite develops into three recognizable portions
          - laterally a dermatome, destined to differentiate into dermis;
          - medially & ventrally sclerotome, from which develop vertebrae, intervertebral discs, & ligaments of vertebral column



The Ectodermal Thickening and Ridge in the Limbs of Staged Human Embryos.