CELLULAR DIFFERENTIATION 



59 



represents several incompletely separated cells, or a multi- 

 nucleate condition of a much-grown and metamorphosed single 

 cell. In both classes the fibres are made up of numerous minute 

 strands or fibrillcs which in the plain muscle are homogeneous 

 throughout, but in the cross-striated are made up of alternating 

 segments of lighter and darker optical appearance (Fig. 30, B). 



FIG. 29. 



FIG. 29. Plain muscle fibres, n, nucleus of muscle cell; p, undifferentiated cell piotoplasm; p' , 

 the differentiated contractile portion of the cell. 



Questions on the figure. What are the two principal portions of these cells? 

 How do very young muscle cells compare with older ones in the relative amount 

 of these portions in the cell? Which is the more highly differentiated portion? 

 Where are such tissues found in the animal body ? Why are muscle fibres elongated ? 



The undifferentiated protoplasmic remnant is often very small 

 in amount, and is collected about the nucleus (Figs. 29, 30). It 

 may be at the surface of the fibre or in the center, enveloped by 

 the contractile matter. A thin membrane (sarcolemma) binds 

 the fibrillae into fibres. The fibres are bound together by 

 strands of connective tissue into bundles, and of these bundles 

 the muscle is made up. 



