THE HISTOGENESIS OF MUSCLE 301 



As to the origin of the striated voluntary muscle fibers, there is also a differ- 

 ence of opinion. It is generally believed that the myoblasts elongate and, by 

 the repeated mitotic division of their nuclei, become multinucleated. Godlewski 

 holds that several myoblasts unite to form a single muscle fiber. The nuclei lie 

 at first centrally, surrounded by the granular sarcoplasm in which myofibrils 

 differentiate peripherally. The myofibrils become striated like those of cardiac 

 muscle. During development the muscle fibers increase enormously in size, the 

 nuclei migrate to the surface and the myofibrillae are arranged in bundles or 

 muscle columns (sarcostyles) . This arrangement of the fibrillae may, however, 

 be due to shrinkage in the preparation of the sections observed. 



According to Baldwin (Zeitschr. f. allg. Physiol., vol. 14, 1912), the nucleus and perinu- 

 clear sarcoplasm is separated from the rest of the muscle fiber by the sarcolemma. With Apathy, 

 he would therefore regard the myofibrillae as a differentiated product of the muscle cells and to be 

 homologized with connective tissue fibers. The extrusion of the muscle cell from the muscle 

 fiber may be compared to the extrusion of cartilage cells from the precartilage matrix, as de- 

 scribed by Mall (see p. 294). 



During the later stages in the development of striated voluntary muscle, there is, according 

 to many observers, an active degeneration of the muscle fibers. 



While smooth muscle fibers form a syncytium and the enveloping connective 

 tissue is developed directly from the muscle cells, in the case of striated voluntary 

 muscle each fiber is a multinucleated entity which is bound together with others 

 by connective tissue of independent origin. 



Morphogenesis of the Muscles. The development of the individual muscles 

 of the human body has been described in detail by W. Lewis (in Keibel and Mall, 

 vol. i, p. 473) and to this work the student is referred. We may state briefly 

 here the origin of the muscles of the trunk, limbs and head. 



The muscles of the trunk. The deep muscles are derived from the myo- 

 tomes which extend ventrally and fuse with one another (Fig. 290). This fusion 

 is well advanced superficially in embryos of 9 to 10 mm. The deep portions of 

 the myotomes do not fuse but give rise to the intervertebral muscles, which thus 

 retain their primitive segmental arrangement. The various long muscles of the 

 back arise by longitudinal and tangential splitting. 



The thoraco-abdominal muscles arise as ventral extensions of the thoracic 

 myotomes into the somatopleure, growing in along with the ribs. 



The musculature of the extremities. It has been generally believed that the 

 muscles of the extremities were developed from buds of the myotomes which grew 

 into the anlages of the limbs. According to Lewis, "there are no observations 

 of distinct myotome buds extending into the limbs." A diffuse migration of 



