Resistance of Mice to the Growth of Cancer. 165 



Historical. 



Jensen stated in papers appearing in 1901, 1902, and 1903* that he had 

 observed the complete disappearance of tumours from mice that had been 

 inoculated successfully. He added, however, that the " results of attempts 

 to cure mice of tumours have hitherto been uncertain ; at times they have 

 been positive to an unexpected extent, at other times they have been 

 entirely negative." For this reason he postponed the detailed discussion 

 of his experiments till he had come to a clearer understanding of their 

 significance, and to more reliable results. Kef erring to the fact that 

 40 to 50 per cent, of the total inoculations of his tumour had been 

 unsuccessful, Jensen interpreted this to mean that the mice were protected 

 by some natural means. It seemed to him impossible to explain the fact 

 that a tumour which was growing well at the time of its removal from one 

 mouse should be unable to grow in other mice, except on the assumption 

 that the tumour had been transferred to a more refractory group of mice ; 

 but he stated, " it appears that the nature of the inoculated tissue is of no 

 little importance for the success attained. So far as it is possible to form 

 an estimate, the results appear less certain when small young tumours are 

 used which are actively growing, and the percentage of successful inoculations 

 is higher if the healthy portions be selected from older and . larger tumours." 

 Jensen especially pointed out that mice which had been unsuccessfully 

 inoculated at the first essay were also refractory to subsequent inoculations ; 

 but he was careful to point out that it would be a mistake to assert that 

 the second negative result was solely the consequence of natural refractoriness, 

 since it would be wrong to assume that the material inoculated and absorbed 

 in the first instance had not contributed to the exemption subsequently 

 exhibited. 



Borrelf drew attention to the difficulties attending primary transplantation 

 and recorded suggestive experiments directed to obtaining a polyvalent 

 immune serum against human cancer, on the assumption that distinct sera 

 might be necessary for tumours of different histological types. 



In November, 1903, we were able to add the study of Jensen's tumour 

 to that of tumours discovered by ourselves, and we at once began investiga- 

 tions along the lines indicated by Jensen. In our earlier experiments, the 



* u Nogle Fors^g med Kraeftsvulster," Lecture, December, 1901 ; " Fors^g med 

 Musecancer," Lecture, April, 1902, ' Biolog. Selskabs Forhandlinger, Kobenhavn,' 

 1901 — 02 ; " Experimentelle Untersuchungen Uber Krebs bei Mausen," ' Centr. f. Bakt.,' 

 vol. 34, 1903. 



t " Epith61ioses infectieuses et Epitheliomas," 'Annales de l'Inst. Pasteur, 5 vol. 11, 

 1903 ; ' Koy. Soc. Proc.,' vol. 73, 1904. 



