334 THE CHEMISTRY OF PROLIFERATION 



cells which have been removed for the purpose from 

 the body; and, as already pointed out, the cells in this 

 in-vitro experimentation are not by any means in 

 conditions similar to the natural ones under which they 

 normally exist. Still there is no question whatever 

 that the cells do divide in response to these agents ; and 

 if they will do so under detrimental experimental 

 conditions, it is obvious that they will be far more 

 likely to divide and respond to the same agents ; in their 

 normal conditions. The agents we know do not exist 

 in the body, and therefore it is practically a certainty 

 that these substances will cause proliferation there. 

 For reasons already given, on the microscope slide one 

 cannot induce more than one generation of cells by 

 chemical agents, because premature death cannot be 

 prevented; but in the body the premature death need 

 not necessarily occur, for its cause is absent, and hence, 

 provided the causes of cell-division are being constantly 

 supplied to cells, generation after generation must be 

 produced. 



On the microscope slide cells will not divide, so far 

 as can be seen, unless they absorb definite quantities 

 of the agents which cause cell-division. We do not 

 say that there are no other substances w r hich cause cell- 

 division besides those which have been mentioned 

 in fact, we know that there must be others; but what 

 we think is now becoming evident is the fact that cells 

 will not divide at all unless they receive some chemical 

 agent which makes them do so. That is to say, we 

 think that there is strong evidence in support of the 

 view that cell-division in the body is entirely caused by 



