THE STUDY OF CANCER 17 



obviously of importance to determine experimentally whether 

 or not all tumours tend to augment their energy of growth to 

 the maximum exhibited by this group, and at the same time to 

 assume the same histological garb. At present many tumours 

 appear to retain their own peculiarities, and although the 

 energy of growth may increase, it is not yet possible to deter- 

 mine whether or not the tumours of one organ ultimately 

 approximate to one form when growth is prolonged sufficiently. 



I may point out here that the first and second groups of 

 tumours behave on transplantation as embryonic and adult 

 tissues do, by exhibiting limited powers of proliferation, with 

 the difference between cancer and embryonic tissue, that the 

 former does not differentiate into an adult form. We are 

 therefore able to place a positive value on the negative efforts 

 which have been made to produce tumours by propagating 

 normal tissues, and we are able to establish all gradations from 

 the limited growth of normal tissues under experimental 

 conditions, up to the remarkable phenomenon exhibited by the 

 unlimited growth of the cells of some cancers under the same 

 conditions. Thus experiment has bridged over the gulf between 

 normal and cancerous tissue so far as their powers of growth 

 are concerned. The limited growth of normal tissues, when 

 transplanted, is independent of any extraneous organism. There 

 is no need to assume such a dependence when cancerous tissues 

 exhibit corresponding limitations ; and if not, why should it be 

 necessary to assume this intervention when the powers of 

 growth pass insensibly through all gradations to those of 

 unlimited amount? 



The possibility of an accelerated rate of proliferation may be 

 entertained when a tumour has been continually transplanted 

 during long periods, as in the case of Jensen's tumour. If I 

 understand Ehrlich rightly, this is what he means when he 

 states that the increase in " virulence " is due to the avidity of 

 the receptors having attained a maximum in consequence of 

 rapid passage from animal to animal. I shall show immediately 

 that there are natural fluctuations in the amount of pro- 

 liferation. In the case of rapidly growing tumours all the 

 inoculations which are successful yield tumours quickly (within, 

 say, ten days). Mice then negative remain so. The fact shows 

 that rapidity of proliferation is closely bound up with an extreme 

 susceptibility to nutritive requirements. Since growth presents 



