Spontaneous Cancer in Mice. 533 
transplantation. In short, cancer in the mouse is essentially the same 
process as In man. 
There are, however, certain characteristic features of its incidence in the 
mouse attracting attention and demanding an explanation. The enormous 
preponderance of tumours of special organs, in particular the frequency of 
growths of the mamma, constitutes one of them. The frequency of multi- 
centric development stands out as a further characteristic feature of 
tumours of the mamma in this animal. The data accumulated point 
towards an increased liability of the whole mammary system of the mouse, 
either as an inherited condition, or as the result of a general diseased 
condition, with diffuse changes in the mamma, leading to development of 
cancer. The same consideration applies to the adenomatous nodules in the 
lungs; they are also remarkably frequent and most often multiple, and 
to a less extent the same considerations apply to the lymphomatous tumours 
occurring in the mouse. 
It has been generally assumed that the conditions of life of the mouse—in 
particular, factors connected with the physiological demands made upon the 
mamma, through the constantly repeated pregnancy and lactation—would 
suffice to account for the frequency of the mammary tumours. This point 
has been investigated experimentally, and the previous history of a number 
of mice spontaneously attacked by cancer, which have been observed since 
birth in the laboratory, yields no evidence of these physiological factors 
playing any part. 
The records of the breeding experiments of the laboratory with regard to 
the previous history of the mice which have developed spontaneous cancer 
of the mamma give the following data:—Of 74 mice which had developed 
mammary tumours, and which were under observation from birth, only 33 are 
recorded to have littered previously to the development of the tumour, while 
no litter is recorded for 41. Analysing this latter figure, there is conclusive 
evidence that 20 mice have been completely isolated since quite young, and 
have never been with a male; and whilst the other 21 have not been so 
strictly isolated, but may at some time or other have been in a cage alone 
with a male, none of them have littered. Of the 33 mice which had littered 
previously to the development of the tumour :— 
1 litter is recorded in 11 cases. 
2 litters are recorded in 7 ,, 
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