96 



MANUAL OF HISTOLOGY. 



structureless sheath. This contained substance is seen, with varying de- 

 grees of distinctness in different cases, to be marked by fine longitudinal 

 lines combined with transverse striation, and to be studded with nuclei 

 (d d) at short intervals; these nuclei are surrounded each with a small 

 amount of protoplasm. 



r , 



Fig. 97. 1. Fibre of striped muscle split up into 

 primitive ftbiillae, a; more distinctly striped 

 at b; longitudinal lines more visible at c; 

 nuclei, d d. 2. A fibre, 6 b; torn through, with 

 empty sheath partially separated, a. (Copied 

 from Bowman). 



Fig. 98. Stages of development in the for- 

 mative cell of the striped muscle fibre of 

 the frog. (After Remak.) 



Leberts and Remak's, as well as more recent investigations, have shown 

 that each of these fibres has its origin in a single cell. 



In the formative cells of frog's muscle (fig. 98), (the usual nucleated 

 elements, with granular protoplasm, of which the body of the embryo is 

 formed), segmentation may be recognised as elsewhere (a). By the growth 

 of the cell and multiplication of the nucleus by division, the whole struc- 

 ture assumes the appearance sketched in fig. 98, 6. Later on the dark 

 granules disappear from the elongated cell, and the characteristic trans- 

 verse streaking commences (c d e). Finally, by continuous elongation 

 and constant multiplication of the nuclei, the cells take on the form in- 

 dicated by/, where the longitudinal lines are commencing, and the muscle- 

 fibre has almost reached its full development. ' The origin of the nuclei 

 in fig. 97 (1), is thus cleared up. But it must not be supposed, as was 

 formerly the case, that the structureless sheath (&) corresponds to the cell- 

 membrane ; it is rather a matter deposited externally on the fibre. 



60 



From what we have seen of the mode of development of striped muscle- 

 fibres in the preceding section, it is clear that many cells may undergo 

 considerable transformation without in the least forfeiting their indivi- 

 duality. 



