14 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—z.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 44 per cent. of the inoculations.. 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. 
