THE FOUR LAWS OF AGE 219 



class of cells presents to us the curious spectacle of a 

 partial differentiation ; such are the muscle fibres 

 by which we accomplish our voluntary movements. 

 These fibres consisted originally only of protoplasm 

 with the appropriate nuclei, but, as they are differen- 

 tiated, part of the protoplasm changes into contractile 

 substance. Another part remains pure protoplasm 

 unaltered. If now the muscular or contractile por- 

 tion of the fibre be destroyed, the undifferentiated 

 part of the protoplasm then shows that it has still the 

 power of growth. It has only been held back by the 

 condition of organisation, and we perceive in the re- 

 generation of these fibres evidence of the fact that so 

 long as the protoplasm is undifferentiated it has the 

 power of growth, which, however, does not reveal 

 itself unless an opportunity is afforded. Third, we 

 come to the cells which are moderately differentiated ; 

 such, for instance, are the cells of the liver, and if 

 for any reason a portion of the liver be injured by ac- 

 cident or disease, we find that these partially differen- 

 tiated cells reveal at once that they have a limited 

 power of growth still left. If we pass on to the 

 fourth class, that in which differentiation is carried to 

 the highest extreme, we find that the cells do not 

 have the power of multiplication. Such are the nerve 

 cells by which the higher functions of the body are 

 carried on. They represent the extreme of cellular 

 differentiation, and almost never do we see these 

 cells multiplying after the differentiation is accom- 

 plished. Presented in this form, we then recognise, it 



