Ultrastructure of developing human muscle

Striated muscle cells of ver tebrates are of mesodermal origin and particularly of the paraxial mesoderm by way of the segmentally a r ranged somites. Centain par t s of the voluntary musculature, e.g. those of the limbs, tongue and orbit appear to arise independently of the somites, and differentiate in situ in the mesenchyme l. The paraxial mesoderm consists of longitudinal masses on either side of the notochord and arises from the intra-embryonic mesoderm. In man, by the twenty-first day of development, the paraxial me soderm on either side of the notochordal process and, later, of the definitive notochord, begins to segment into paired cubical masses called somites. During the period of somite formation, which in man extends approximately from the twenty-first to thirtieth day of development, the embryo is said to be in the somite s tage. Altogether 42-44 pairs of somites are formed in the human embryo, and in embryos of 32 days (5mm C.R. length) each somite begins to differentiate into three por t ions: a myotome which gives origin to muscle, a dermotome which is concerned with the integumentary tissues and a sclerotome which i