120 



HISTOLOGY 



of a mesenchymal or connective tissue network, thus differing from the 

 myoblasts of smooth muscle. The latter unite with one another through 

 protoplasmic or fibrous processes; the striated fibers are bound together 

 by connective tissue sheaths. In producing striated fibers, the myo- 

 blasts become greatly elongated cylindrical structures, with rounded or 

 blunt ends. Although according to Schafer they generally do not exceed 

 36mm. in length, they sometimes measure from 53-123 mm. (Stohr); 

 their diameter is o.oi-o.i mm. During the growth of the myoblast, 

 mitotic nuclear division takes place repeatedly, producing multi-nucleate 

 cells; and in the adult fibers, a further multiplication of nuclei through 

 amitosis has been reported. Each developing myoblast thus acquires 

 a row of centrally placed nuclei, imbedded in granular protoplasm. In 

 the outer part of the myoblasts coarse myofibrils develop, which, as seen 

 in cross section, form an encircling ring about the nuclei and axial core 

 of protoplasm (Fig. 109). The entire myoblast is surrounded by a mem- 

 brane, to the formation of which the adjacent mesenchyma contributes. 



Capillaries 



Bundles of fibrils 

 S (Co hnheim's areas) 



FIG. 109. CROSS SECTION OF MYOBLASTS 

 AND MESENCHYMAL CELLS FROM AN 

 i8-MM. PIG. 



mes., Mesenchymal cell; f., myofibril; n. 

 nucleus of a myoblast; s., sarcolemma. 



Connective tissue 



FIG. no. CROSS SECTION OF FOUR MUSCLE FIBERS OF THE 

 HUMAN VOCAL MUSCLE. X 590. 



The group of cells shown in Fig. 109 corresponds with a portion of the 

 myotome in Fig. 108, D. It is sectioned in the same plane, but represents 

 a later stage. In the adult, such an area of tissue as shown in Fig. 109 

 becomes a group of fibers as in Fig. no. The myoblasts have greatly 

 enlarged, and their protoplasm is filled with myofibrils which are often 

 arranged in "fields," known as Cohnheim's areas. These fields are cross 

 sections of longitudinal bundles of fibrils known as muscle columns, which 

 Schafer later named sarcostyles (i.e., muscle columns). The term sarco- 

 style is, however, often loosely applied to the separate myofibrils. It has 

 been supposed that the fibrils in a column arise by the longitudinal split- 

 ting of a primitive myofibril, but in sections it often appears that the areas 

 or columns are due to shrinkage. As the fibrils multiply, the nuclei, each 



