THE HISTOGENESIS OF MUSCLE 293 



mesodermal segment persist as a dermatome and give rise only to the connective 

 tissue of the corium (Fig. 323). The dermatome lies lateral to the myotome 

 (Fig. 47) and the two together constitute the dermo-myotome (WilHams). 



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

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

 the repeated mitotic division of their nuclei, become multinucleated. (God- 

 lewski, however, holds that several myoblasts unite to form a single muscle fiber.) 

 The nuclei lie at first centrally, surrounded by the granular sarcoplasm (Fig. 

 298 A). The sarcoplasmic granules become arranged in rows and constitute the 

 myofibrillcs which increase in number by longitudinal splitting (Fig. 298 B, C). 

 The myofibrillas soon differentiate alternating dark and light bands, due to differ- 

 ences in density, and the individual fibrillas become so grouped that their dark 

 and light bands coincide (Fig. 298 C). During development the muscle fibers 

 increase enormously in size, the nuclei migrate to the surface, and the myofibrillas 

 are arranged in bundles or muscle columns (sarcostyles) . The fibrils of each col- 

 umn are said to arise by the longitudinal splitting of single primitive myofibrils. 



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 pre- 

 cartilage matrix, as described by Mall (see p. 287). 



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

 ing 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 skeletal 

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

 by connective tissue of independent origin. 



Striated Cardiac Muscle. — This is developed from the splanchnic mesoderm 

 which forms both the epicardium and the myocardium (Fig. 255). The cells of 

 the myocardium at first form a syncytium in which myoiibrilte develop from 

 chondrioconta, or cytoplasmic granules. The myofibrillae are developed at the 

 periphery of the syncytial strands of cytoplasm and extend long distances in 

 the syncytium. They multiply rapidly and form dark and light bands as in 

 skeletal muscle. The syncytial character of cardiac muscle persists in the adult 

 and the nuclei remain central in position. The intercalated discs, typical of adult 

 cardiac muscle, appear relatively late, just before birth in the guinea pig (Jordan 

 and Steele, 1912). 



