THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 307 



Histogenesis of Striated Voluntary Muscle Tissue. 



The majority of the striated voluntary muscles of the body are derived from 

 the myotomes. Some are derived from the mesenchymal tissue in the branchial 

 arches, some possibly from the mesenchymal tissue in the limb buds. The 

 primitive segments are at first composed of closely arranged, epithelial-like cells 

 that radiate from a small centrally placed cavity (Fig. 141). The cavity repre- 

 sents part of the ccelom and from this point of view it can be said that the muscle 

 is a derivative of the epithelial lining of the ccelom. A part of each primitive 



5th nerve 



Phrenic nerve 

 Brachial plexus 



Sympathetic 



Diaphragm 



Vertebra 



Border vein 



4th rib 



FIG. 273. Drawing from a reconstruction of the upper limb region of a human 



embryo of 9 mm. (4 J weeks) ; ventral view. Lewis. 



Inf. hy., infrahyoid; Lev. scap., levator scapulae; My., myotome mass; Rhom., 

 rhomboid; Trap., trapezius. 



segment becomes the sclerotome and cutis plate. The remaining part be- 

 comes the myotome or muscle plate (Fig. 261). 



The cells of the myotome are at first not essentially different from those of 

 the rest of the primitive segment. Soon, however, changes take place in them 

 and they become the so-called myoblasts or muscle-forming cells, which are 

 destined to give rise to the muscle fibers. Opinions differ as to the manner in 

 which the myoblasts produce the muscle fibers. It was once thought that each 

 myoblast gave rise to a single muscle fiber in which there were several nuclei, all 



