PREFACE 13 



directly caused by chemical agents; we consider 

 that before a cell can divide it must absorb a 

 definite quantity of specific chemical substances. 

 As already mentioned, we arrived at this con- 

 clusion because we found that certain human 

 cells, which hitherto had never been seen to 

 divide, could be made to undergo division figures 

 on a microscope slide in response to chemical 

 agents ; a discovery which will be found fully 

 described, together with the methods employed, 

 in the book above referred to. In one of the 

 following papers the chemical agents are specified 

 and further investigated ; and judging by the 

 way in which these substances are set free in 

 the body, we maintain that the death of cells, as 

 caused by injury, for instance, gives rise to the 

 reproduction of their living neighbours, for the 

 exciting agents are set free by the death of 

 protoplasm. If this is correct, we have also 

 evidence to show that malignant proliferation may 

 be brought about by the addition to the normal 

 agents of other exciting substances, the combina- 

 tion of which causes abnormal or excessive cell- 

 multiplication, and which can be produced in an 

 injured site by the action of organisms. As 

 already mentioned, our work has reached the 

 stage at which it must be determined whether 

 the figures which we can induce in the cells, and 

 which we interpret as division figures, are really 

 the phenomena of cell-division or not ; an inter- 

 pretation which involves a point revolutionary 

 to commonly accepted cytological teaching. 



I do not wish to imply that we originated the 

 idea that cell-reproduction is affected by chemical 



