i 4 SCIENCE PROGRESS 



growth, in mice whose resistance is below the average, or rather 

 the suitability of whose soil is above the average. A summary 

 of some observations we have already published will serve as 

 an illustration. The final results of transplanting 32 spon- 

 taneous tumours of the mamma show that 2,278 of the mice 

 inoculated remained alive three weeks later, and 72 of them 

 ultimately developed tumours — i.e. only 1 inoculation out of 

 31 was successful, or 3*2 per cent. A tumour is propagated 

 by a repetition of this process of transplantation from one 

 series of mice to another. But the subsequent transplantations 

 are only exceptionally accompanied by better success. In 

 illustration of the pertinacity necessary in such experiments, 

 I may state that it was not till we had repeated the primary 

 process above described for twenty-seven different sporadic 

 tumours, and also repeated it for each tumour one, two, three, 

 and four or more times, that we alighted on a growth capable 

 of unlimited propagation. This difficulty in starting propaga- 

 tion illustrates merely one aspect of the initial obstacles which 

 those who embarked on the experimental study of cancer have 

 had to overcome in the case of mice, and are still striving to 

 overcome in the case of other animals. 



When once a tumour has been got to grow well in mice 

 of a particular race, similar difficulties are often encountered in 

 transferring it to mice of strange race. Thus we added the 

 study of Jensen's tumour, which was accustomed to Danish 

 mice, to those discovered by ourselves with the greatest diffi- 

 culty. Michaelis and other investigators have failed altogether 

 to get Jensen's tumour to grow in strange mice. We failed also 

 to propagate a tumour sent from Paris by Borrel. Our experi- 

 ence leads me to surmise that those absolute failures may have 

 been due partly to an unfavourable phase of growth of the cells 

 of the tumours. Our initial success with Jensen's tumour was 

 only 4! per cent, of the inoculations. 1 When the tumours 

 grown in English mice were transplanted again into English 

 mice, the percentage soon equalled that obtained by Jensen in 

 Danish mice, and in single experiments greatly exceeded it, 



1 The tumour had been removed from a mouse, was forwarded by ordinary 

 post hermetically sealed in a glass tube, and transplanted into English mice five 

 days later. Our primary transplantations were therefore made with an added 

 difficulty, but they also illustrated the vitality of the tumour cells after separation 

 from the body. 



