

THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



279 



characteristic cross striation. The nature of this differentiation is not known. 

 One investigator holds that both substances are derived from the original 

 granules that unite to form the fibrils, alternate granules being composed of like 

 substance and united by delicate strands of the other substance. 



While the fibrils are being formed, the nuclei of the myoblasts undergo rapid 

 mitotic division. When the cells are about filled with fibrils, the nuclei migrate 

 to the periphery where they are situated in the fully formed fiber (Fig. 278). 

 Each fiber thus possesses a number of nuclei, whether it is derived from one 

 myoblast or from several. 



2Sfes \\^^^<, // / P'U^e^y v*3*-v l 



FIG. 240 



FIG. 239. From a cross section of developing voluntary striated muscle in the leg of a pig embryo 



of 45 mm., showing fibril bundles at the periphery of the cells. MacCallum. 

 FIG. 240. From a cross section of developing voluntary striated muscle in the leg of a pig embryo 



of 75 mm., showing fibril bundles more numerous than in Fig. 239. A, Central vesicular 



nucleus; B, peripheral more compact nucleus. MacCallum. 



For some time at least, the number of fibers in a developing muscle increases 

 by division of those already formed. This process would produce a certain 

 degree of enlargement of the muscle as a whole. Later the increase in the 

 number of fibers ceases, and the muscle grows by enlargement of the individual 

 fibers. It is not certain at what period in development the increase in the num- 

 ber of fibers ceases. 



In many muscles development is further complicated by a retrograde proc- 

 essa degeneration of some of the fibers. This occurs quite regularly in the 

 extremities. A well fibrillated fiber first presents a homogeneous appearance, 

 then becomes vacuolated, the nuclei disintegrate, and finally the whole 

 structure disappears. Mesenchymal (or connective) tissue takes its place, and 

 the remaining fibers are thus grouped into bundles and the bundles into 

 muscles. This would account to a certain extent for the intermuscular con- 



