64 EXPERIMENTAL RESEARCHES 



As you know, mouse cancers are tumours 

 showing a far-reaching analogy to human 

 cancer, and it would therefore appear most 

 justifiable to regard the experiences gained with 

 them as the solid foundation of an experimental 

 study of tumours. Besides several analogies, 

 however, the mouse tumours show certain 

 special features, chief among which is the mode 

 of formation of metastases. It is most remark- 

 able that the highly virulent carcinomata which 

 have been cultivated in numerous generations, 

 very rarely show macroscopically visible metas- 

 tases, whilst the slowly growing spontaneous tu- 

 mours relatively more often, though also not very 

 commonly, form large secondary nodules in the 

 lungs. Our interest in this peculiar behaviour 

 must even be increased, since Haaland has 

 shown that microscopical metastases are by no 

 means uncommon, but that they almost always 

 remain below the limit of macroscopical visi- 

 bility. I endeavoured to solve this problem by 

 making, so to speak, artificial metastases, viz., 

 by studying in animals affected with an inocu- 

 lated tumour the result of a second inoculation. 

 I found the interesting fact, that the effect of 

 the second inoculation was inversely propor- 

 tional to the energy of growth of the primary 



