i 3 2 AGE, GROWTH, AND DEATH 



tion than one of growth alone, and that there occur, 

 as the years advance, changes in the structure of the 

 body. It is convenient to use one collective term for 

 all these phenomena of becoming old, and that term, 

 established by long usage, is senescence, the becoming 

 old. What, therefore, we have to search for at 

 present is a cause, a proximate cause at least, of 

 senescence. In order to make the view I am to bring 

 forward this evening quite clear to you, I must first 

 of all take advantage of your kindness and recapitu- 

 late briefly what I said in regard to cells, for you will 

 remember that the cell is the foundation and unit of 

 organic structure. With your permission I should 

 like to recall more exactly to your minds what I said 

 of the cells by having thrown upon the screen the 

 slide which we saw before and which we used as an 

 illustration of the cell. Here is the picture. Above 

 we see the typical cell (No. i) from the oral epithelium 

 of the salamander, and you remember in the centre 

 this more conspicuous body with a granular and 

 reticulated structure which we called the nucleus, 

 and surrounding it is this mass which we called the 

 body of the cell, or the protoplasm. Here (No. 2) is 

 another condition of a cell of the skin of the salaman- 

 der in which the nucleus presents a slightly different 

 appearance. Here also we have quite a body of proto- 

 plasm about the nucleus. Every cell consists of these 

 two essential and fundamental parts, the nucleus and 

 the protoplasm. Now the conclusion to which I shall 

 gradually bring you by the facts to be laid before you 



