THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



277 



derived from the original myoblast nucleus by mitotic division. It was also 

 thought that the muscle fibrillae represented highly modified and specialized 

 parts of the cytoplasm, which arranged themselves longitudinally in the cell. 

 Some of the later researches indicate that a muscle fiber represents a number of 

 myoblasts fused together. This explanation is not, however, accepted by all 



investigators. 



In contrast with the above, there is a quite general consensus of opinion in 

 regard to the development of the internal structure of the muscle fiber. In the 



FlG. 236. Lateral view of a reconstruction of the muscles of the upper extremity of a human 

 embryo of 16 mm. (about 6 weeks). Lewis. 



The trapezius is the large muscle arising from the transverse processes of the vertebrae (at the right 

 of the figure) and converging to its insertion on the clavicle. Just below the insertion of the 

 trapezius is the deltoid, which partly hides the subscapular (on the right) and the pectoralis 

 major (on the left). Arising beneath the deltoid and running downward to the elbow is the 

 triceps. To the right of the triceps is the teres major (composed of two parts). The large 

 sheet of muscle extending down the forearm and sending divisions to the ad, 3d, 4th and 5th 

 digits is the extensor communis digitorum. 



cytoplasm of the myoblasts there appear granules which soon arrange them- 

 selves in parallel rows and unite to form slender thread-like fibrils (Fig. 238). 

 These fibrils are at first confined to one myoblast area. If several myoblasts 

 fuse, the fibrils probably extend in a short time from one myoblast area to 

 another. If one myoblast produces a fiber, the fibrils naturally are confined to 

 a single myoblast area throughout development. The fibrils are usually 

 formed first at the periphery of the cell and later in the interior (Figs. 239 



