1912.] Resistance to the Growth of Implanted Cancer. 207 



experiments with rat and mouse tumours, the success or failure of re-inocula- 

 tion has been found to be determined solely by the nature of the reaction set 

 up by the first implanted tumour. 



All of the above experiments have been chosen with the special view 

 of demonstrating how very differently the parenchymata of various 

 tumours of different types behave in regard to the alteration of the 

 suitability of the soil which they induce, and the detailed description of 

 this class of experiments given above will now be followed by a general 

 discussion upon the interpretation which is to be put upon the results 

 obtained. 



On the one hand, two tumour strains have been shown, 63 and T, which, 

 in the course of their development, do not alter the suitability of mice for 

 re-inoculation ; on the other hand, a tumour strain has been shown which so 

 alters the animals that all are refractory to subsequent inoculation. It is, of 

 course, apparent that such wide differences can only be attributed to inherent 

 properties of the tumour parenchymata, and the contrast in their behaviour 

 may be drawn by stating that the parenchyma of strain 206 induces a 

 resistance which the parenchymata of 63 and T fail to do. The terminology 

 of modern immunity studies would label the former an efficient antigen, 

 whereas the latter would be inefficient. In these extreme cases the 

 differences are so wide, and the reactions so marked, that the medium, 

 i.e. the inoculated mouse, can be regarded as indifferent. When tumours 

 are considered, however, which only induce resistance in a certain percentage 

 of cases, slight differences in the medium turn the scale for or against 

 the inoculated graft in individual cases. 



To take the specific instance of strain 199, why does this strain induce 

 resistance in 60 per cent., and fail to do so in the remaining 40 per cent. ? 

 The parenchyma, which has been distributed over 10 mice, for example, 

 although of exactly the same quality and quantity throughout, fails to 

 induce resistance in four mice. Again, in the extreme cases of strains 63 

 and T, resistance is induced occasionally in a certain number of animals, 

 whilst strain 206 sometimes gives rise to progressively growing tumours in 

 animals exhibiting no reaction of resistance. These variations in the 

 development of resistance in the individuals composing a series must be 

 regarded as the expression of slight differences in the constitution of the 

 animals composing such a series, and, whilst in general the reaction is 

 determined by the tumour parenchyma, a slight individual peculiarity is 

 sufficient at times to determine or prevent the development of resistance. 

 Tumour strains such as 63 and 206 usually mask all individual variations, 

 but strain 199 and many others bring them out with distinctness. 



