2Q2 HISTOGENESIS 



multiplication they form a mesial myotome, while the lateral cells of the 

 original mesodermal segment persist as a dermatome and give rise only to 

 the connective tissue of the corium (Fig. 223). The dermatome lies 

 lateral to the myotome (Fig. 47) and the two together constitute the 

 der mo -myotome. 



As to the manner of origin of the individual muscle fibers, there is also 

 a difference of opinion. It is generally believed that the myoblasts 

 elongate, and, by the repeated mitotic division of their nuclei, become 

 multinucleated. Godlewski, (1902), however, holds that several myo- 

 blasts 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 myofibrillce, which 

 increase in number by longitudinal splitting (Fig. 298 B, C). The myo- 

 fibrillae soon differentiate alternating dark and light bands, due to dif- 

 ferences 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 myofibrillae are arranged in bundles, or muscle columns 

 (sarcostyles). The fibrils of each column are said to arise by the longitu- 

 dinal splitting of single, primitive myofibrils. 



While smooth muscle forms a syncytium and the enveloping connec- 

 tive-tissue fibers are developed directly from the muscle cells, in the case 

 of striated skeletal muscle each fiber is a multinucleated entity which is 

 bound to others by connective tissue of independent origin. 



According to Baldwin (1912), the nucleus and perinuclear sarcoplasm is separated 

 from the rest of the muscle fiber by the sarcolemma. With Apathy (1888), he would 

 therefore regard the myofibrillag as a differentiated product of the muscle cells, to be homo- 

 logized with connective-tissue fibers. There is little to support this view. 



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

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



Striated Cardiac Muscle. This is developed from the splanchnic 

 mesoderm that forms both the epicardium and the myocardium (Fig. 

 255). The cells of he myocardium at first form a syncytium in which myo- 

 fibrillae develop from the linear union of cytoplasmic granules. The myo- 

 fibrillae 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, 

 probably appear in the early months of fetal life. 



