302 BIOLOGY OF THE LABORATORY MOUSE 



contrilnitf (Iftinitt'l}- to (»ui uiKJcrstaiiding of the i)i(K:rssfs of foniialion and 

 growth of spontaneous neoplasms. 



One principle may safely guide us in this discussion. It is the fact that 

 only those who have had direct and continuing, first-hand knowledge of 

 experimentation in all of the three lields are qualified to evaluate with any 

 degree of probable accuracy the relationship between them. This again 

 does not mean that the student of transplanted tumors alone may not con- 

 tribute greatly to our knowledge of the cancer process. The same, of course, 

 applies to investigators who use only induced tumors or who study only 

 spontaneous tumors. All that is meant is that relations between the three 

 types of experimentation are best understood by those who have engaged in 

 all of them. 



With this preliminary discussion we may consider briefly three prin- 

 ciples established by abundant experiments with transplanted tumors which 

 have an important bearing on the problem of spontaneous tumors. These 

 have been considered in a paper by Little (48) . They are as follows : 



1. Transplantation in known and controlled genetic material provides a 

 more delicate test of biological and physiological differences between certain 

 neoplasms than does any other test at present available. 



2. Transplantation experiments in which somatic mutational changes in 

 the genetic constitution of a tumor have been demonstrated afford a most 

 helpful avenue of investigation on the nature and incidence of somatic 

 mutation as a process of importance in cancer research. 



J. Transplantation experiments on the genetics of spontaneous tumors 

 arising in Fi and other hybrid mice, derived from a cross between two 

 inbred strains, give an unusually good opportunity for linkage studies 

 between tumor genes, derived from the parent races, and genes for other 

 characters of a more easily detectable nature. They also should enable us 

 to determine whether hybridization as a process has any influence on the 

 genetic complexity of tumors formed. 



In each of these cases transplantation is being used as an experimental 

 method as an aid in analysis and not as a process which creates important 

 facts de novo. 



Transplantation and the physiological individuality of tumors. — In 

 1920 Strong and the writer (81) published evidence which showed that 

 two mammary adenocarcinomas of the mouse, although histologically 

 indistinguishable, gave very different percentages of continuing growth when 

 inoculated into hybrid mice of known genetic origin. The rate at which 

 these two tumors were eliminated by a negative strain of mice also showed a 



