HISTOGENESIS OF STRIATED MUSCLE 



mu c 



consequence, pushed out to the outer edge, which position they occupy 

 for a long time (Fig. 88). 



The fibrils are laid down one after another, an outer cell always 

 having one or two more fibrils than a cell of equal size just inside from 

 it. This accumulation of fibrils is continued until each fiber seems to be 

 a mass of fibrils with a little sarcoplasm clinging to it, rather than a cell 

 containing a certain number of 

 fibrils. For an example of a 

 muscle cell that never acquires 

 many fibrils, see the heart of an 

 adult lobster (Fig. 92). While 

 the fibrils are appearing one by 

 one in the outer cells, the con- 

 nective-tissue cells lying between 

 the epithelium and the myotome 

 begin to migrate slowly in be- 

 tween the myotomes where the 

 connective-tissue septum is found 

 in the adult. After they are well 

 introduced into the septum, they 

 send cells down between the sar- 

 coblasts, or muscle cells, as we 

 must call them now, into the 

 myotome (Fig. 89). The blood 

 capillaries follow this connective 

 tissue later; whether from the 

 inner or outer edge was not de- 

 termined. 



At this time the muscle cell is 

 fully formed, and future changes 

 depend only upon its growth and 

 the addition of more muscle 



fibrils. It is possible that these cells or sarcoblasts divide. If so, the 

 division is an unequal, longitudinal splitting of the cell accompanied by 

 a proportional division of the nucleus and the muscle fibrils. Figure 82 

 shows, in transverse section, a possible division of this sort, where the 

 smallest fiber seems to have recently split from the next largest. In 

 fact the whole group of fibers, five in number, seem to have come 

 from one original fiber. 



Technic. It is comparatively easy to secure good preparations of this 

 tissue in its earlier stages. The tissues are soft and yield to the best fixa- 

 tives in a very satisfactory manner. The staining is also easy. Flem- 

 ming's fluid and two or three of the other best methods should be tried. 



mus. 



FIG. 88. Transverse section of a bit of periph- 

 eral body muscle of the sucker, Catostomus. 

 A little further developed than in Fig. 87. 

 mu.c., mucous cells of skin; mus.f., muscle 

 fibrils in groups in the proximal sarcoplasm of 

 the muscle cells. 



