1906.] Resistance of Mice to the Growth of Cancer. 185 



applicable to the continued propagation of a single tumour. It is impossible 

 to imagine that the differences brought out by our experiments on the 

 fluctuations in energy of growth are due entirely to variable susceptibility 

 of the animals. The simultaneous appearance of numerous spontaneous 

 absorptions, and associated therewith the rapid diminution in the percentage 

 of success in parallel experiments,* can only be referred to fluctuations 

 in the energy of growth of the parenchyma cells. Any other explanation, 

 and especially an explanation by assuming increased resistance of the 

 animals, introduces an arbitrary assumption which further complicates the 

 conceptions. 



The resistance of the animals can be altered, and this change has been 

 described as an active immunity. We shall continue to speak of resistance or 

 refractoriness. The absorption of tumours which have grown for a time 

 undoubtedly calls forth this alteration in a high degree. The absorption 

 of inoculated material without obvious tumour formation has similar con- 

 sequences. Increased resistance results when blood injected subcutaneously 

 is absorbed. The refractory condition induced in this way is not so perfect as 

 that following the absorption of tumour material. It is, however, perfectly 

 definite, and removes any cogency which might attach to the argument that 

 the induction of a refractory condition by absorption of growing tumour or 

 living tumour cells indicates the existence of a hypothetical cancer virus. 

 The evidence we have obtained that the blood of other animals is unable to 

 induce such an alteration in mice, points to the same conclusion. The 

 absorption of the tumours of animals of alien species when inoculated into 

 mice is likewise devoid of effect. 



All these actions are curiously limited to the production of an 

 insusceptibility to subsequent transplantation. We have not been able to 

 induce such an alteration in animals with growing inoculated tumours that 

 they should become unsuitable for growth to continue once it has started. It 

 would appear that the negative phase in growth of the tumour cells plays the 

 principal role when spontaneous absorption occurs, the fall in the energy of 

 growth and the resistance of the mouse together contributing to the final 

 absorption. Once the cellular graft has been vascularised it is in a much 

 stronger position with reference to conditions which may be unfavourable to 

 it, than is a graft in the days immediately following transplantation. This is 

 much more vulnerable. 



The biological reactions described in this paper are effective by means of 

 the body fluids. They are analogous to those that are now well known as 



* ' Roy. Soc Proc.,' B, vol. 78, 1906. Graphic record on p. 206, and protocol, p. 209. 



