184 Dr. Bashford, Mr. Murray, and Dr. Cramer. [Dec. 10, 



action of two independent factors : (1) the variations from mouse to mouse 

 of the susceptibility to (or suitability for) the growth of cancerous grafts ; and 

 (2) the variations in the energy of growth of the tumour cells. 



The first variable fluctuates between fairly wide limits in individual 

 animals, but when large numbers of young mice are compared together, their 

 resistance acts like a sieve, with unequal meshes, it is true, but, on the whole, 

 of the same average fineness. 



Spontaneous tumours vary considerably in the ease with which they can be 

 propagated, i.e., with which they can pass the sieve. These differences have 

 been regarded as depending on differences in "virulence." There is some 

 evidence to show that the sporadic tumours fluctuate in energy of growth 

 just as do the propagated tumours,* and the direct conclusion as to 

 " virulence " from one primary transplantation may be upset when the 

 same spontaneous tumour gives an opposite result, i.e., is again trans- 

 planted after recurrence. The term " virulence " is therefore unfortunate, 

 and must be used with so much reservation that it would be better discarded 

 altogether.f 



The influence of the individuality, i.e., the sum total of changes 

 due to the past life of the organism, will be to make any mouse different 

 from all others, and these differences will increase the longer the animal 

 lives. The difficulty of obtaining success in the primary transplantation 

 of spontaneous tumours would be accounted for by supposing that the new 

 animals provide an environment to the cancer cells so strange, that they 

 cannot survive the interruption to their nutrition. Their failure to grow 

 does not necessarily imply that they would fail to proliferate in their new 

 hosts if the conditions to which they had been accustomed could be 

 immediately supplied in the experiment. Cells which have lived and 

 become accustomed to the body fluids of one mouse for, say, two years, 

 may easily die or fail to adapt themselves when transferred to the bodies 

 of new animals. The frequency, in our experience, of large metastases in 

 animals spontaneously affected, is in marked contrast to the difficulties in 

 obtaining growth in normal animals, and harmonises well with this view. 

 What has been said of the differences in transplantability between 

 spontaneous tumours appears to hold also with reference to separate strains 

 of propagated tumours. If the resistance of the mice be made the criterion 

 for the energy of growth of sporadic tumours, the same standard is equally 



* Vide ' Eoy. Soc. Proc.,' B, vol. 78, 1906, p. 220. Graphic record of transplantation of 

 a sporadic tumour (XIX) which recurred three times after partial excision. 



t Vide ' Sci. Keport II,' Part 2, p. 40 ; Ehrlich, ' Berl. Klin. Woch.,' No. 28, 1905, where 

 he also proposes " energy of growth " as an alternative. 



