THE SLUPN OF CANCER 13 
the two forms of growth are interchangeable experimentally. 
For different carcinomata Murray, Cramer, and myself have 
shown that this interchange of the mode of growth is dependent 
on the anatomical surroundings of the tumour, and not on an 
alteration of the properties inherent in the tumour cells. It 
therefore suffices to study mere growth, and in doing so one 
may disregard the features of most use in the current classifica- 
tion of tumours. 
The transplantation of a sporadic tumour is effected according 
to our method as follows: The tumour is removed from the 
animal so as to avoid bacterial contamination, and weighed. 
It is divided into portions which are placed in separate 
receptacles. The portions are then inoculated by means of 
hypodermic needles, minute fragments (c7vca o'0I—0'03 grm.) 
being broken off and inserted subcutaneously. The tumour is 
thus distributed over as large a number of mice as possible— 
it:may be fifty or four hundred. The experimenter must then 
possess his soul in patience. Rarely is he rewarded by the 
appearance of true tumours, at the site of inoculation, within 
a fortnight. He may have to wait from three to six months 
before the inoculations can be pronounced to have fallen out 
either positively or negatively. 
The nature of the result is determined mainly by two factors: 
variations in the suitability of the soil the mice afford for the 
growth of the grafts introduced as above; and variations in the 
character of the tumour cells, not only of different tumours, but 
of one and the same tumour. The suitability of the soil may be 
taken to be fairly constant when large numbers of young mice 
of the same age (six to eight weeks) and of the same stock are 
employed. Adult and old mice are not only much less suitable 
than young animals, but they also exhibit greater individual 
idiosyncrasies. Normal mice may be said to offer a certain 
unsuitability, or to be resistant, to inoculation, when tumour 
cells are transferred to them after removal from their natural 
environment in the animal in which they developed. This 
resistance acts as a sieve, sifting out the tumour cells which are 
unable to nourish themselves and proliferate in this new and, 
it may be, very strange environment. Thus the positive results 
of the primary transplantation seldom form a high proportion of 
the animals inoculated. They are probably obtained by the 
segregation of groups of cells of high assimilative energy and 
