i 3 4 AGE, GROWTH, AND DEATH 



this evening is that the increase of the protoplasm, 

 together with its differentiation, is to be regarded as 

 the explanation (or should we say cause) of sen- 

 escence. Though protoplasm is the physical basis of 

 life, though it is the actual living substance of the 

 body, its undue increase beyond the growth of the 

 nucleus changes the proportions of the two, and that 

 change of proportion causes an alteration in the con- 

 ditions of the living cell itself, and that alteration I in- 

 terpret, as I shall explain more accurately later, as the 

 cause of senescence, as the fundamental cause of old 

 age. This slide (Fig. 46) also shows to us the early 

 development of the cells through those phases which 

 result in the multiplication of them. The nucleus 

 changes in appearance and becomes a very different 

 looking structure. These changes I need not now go 

 through again. Suffice it to say that after the com- 

 plicated alterations have completed their cycle, we get 

 in the place of a single cell, two, and each has its own 

 nucleus, and each its own protoplasm. Notice here 

 that the two cells (No. 10) which finally result are 

 smaller than the original cells from which they 

 sprang. These are by no means imaginary pictures, 

 but accurate microscopic drawings from real cells of 

 the salamander. The two cells which are thus pro- 

 duced from one parent cell are characterised by their 

 smaller size, and this smaller size applies not only to 

 the cell as a whole, but likewise to its nucleus. After 

 having been thus reduced in size, the nuclei and the 

 cells will both expand, and soon the daughter cells 



