DIFFERENTIATION AND REJUVENATION 143 



guished from another, but always the nucleus consists 

 of the same fundamental constants. There is the 

 membrane bounding it ; there is the sap or juice in the 

 interior, and there is the network of living threads 

 stretching across it. Here and there imbedded in 

 and connected with the network are spots of a special 

 substance, which we call chromatin. These four 

 things exist in the nuclei and are apparently always 

 present, and there is usually not to be seen in the 

 resting nucleus anything of change comparable, in 

 extent at least, with the change which goes on in the 

 protoplasm on the other hand, the protoplasm ac- 

 quires items of structure which were totally absent 

 from it before. The nucleus rearranges its parts 

 rather than changes them. This is a very important 

 fact, and shows us, if we confine our attention even to 

 these little organisms only, that the differentiation of 

 the protoplasm is quantitatively the more important 

 of the two the differentiation of the nucleus the less 

 important. 



We can now turn from a consideration of these 

 low organisms to the higher forms, among which we 

 ourselves of course are counted, in which the body 

 is formed by a very considerable number of cells. 

 Again I should like to take advantange of your kind- 

 ness and show you some of the pictures we have 

 already reviewed, in order to utilise the features 

 which they show as illustrations of the fundamental 

 principle that the conspicuous change is in the proto- 

 plasm. First we have nerve cells, Fig. 52. In the 



