24 SCIENCE PROGRESS 



unimportant. Division being merely the end phenomenon in 

 the growth of the individual cell, what requires study is not 

 the manner in which the cells divide, but the mechanism of 

 cell assimilation and its relation to nutritional environment. 

 These problems have been attacked experimentally by 

 altering the conditions under which cancer is artificially pro- 

 pagated, and, as I have pointed out above, the nutritional 

 environment can be so altered that the cancer cell is unable 

 to grow. The study of the growth of cancer further permits 

 inferences as to the nature of the changes, in cells and in 

 organisms, leading to the inception of the continuous assimilation 

 of which cancer cells are capable. 



Murray and myself pointed out the importance of separately 

 considering two factors in studying the growth of cancer — 

 viz. the conditions of origin and the conditions of growth — 

 and we pointed out that artificial propagation enabled us to 

 study the conditions of growth experimentally. Ehrlich has 

 since penetrated more deeply into the relation of these factors 

 by differentiating the primary cell changes in a circumscribed 

 area from the conditions permitting the cells in such an area 

 to grow into a tumour, and by assuming the importance of 

 constitutional conditions favourable to growth. We have 

 pointed out, with reference to carcinoma mammae in the mouse, 

 that the differences revealed and shown to be retained in 

 artificial propagation indicate primary differences inherent in 

 the cells of different tumours, and that they can be conceived 

 as due either to a variety of causes or to one causative factor 

 acting in varying degree. As our work proceeded, evidence 

 accumulated pointing more and more definitely to the assump- 

 tion that one primary change occurring in varying degree 

 explained the different behaviour of apparently similar and 

 nearly related tumours ; but we are ignorant of how the 

 primary change is elicited. If histological differences be 

 ignored for the present, the only fundamental differences, from 

 the standpoint of growth, are in the energy of assimilation of 

 the cells, and in the chemiotactic influences they exert on the 

 connective tissues of the host which convey the food suppty. 

 In accordance with this conception what are abstractly called 

 variations or differences in malignancy between tumours of 

 the same histological type are concretely expressed as differ- 

 ences in rate of growth. The injury to the mouse in which 



