304 BIOLOGY OF THE LABORATORY MOUSE 



the nature of a somatic mutation. The question is still undecided, but it is 

 certain that some of the most favorable material in which to study it is to 

 be found in the modification of transplantable neoplasms. Such tumor 

 tissue can be subjected in various amounts to chemical and physical condi- 

 tions which have been shown to be mutation producing agents. Changes 

 in the tumor can afterwards be studied and recorded. Controlled series 

 of normal tissue subjected to the same agents can be maintained. 



Treatment of various clearly defined sites in animals of known tendency to 

 produce spontaneous tumors of different types with agents likely to produce 

 mutation should give interesting information as to whether these agents 

 increase the incidence of spontaneous cancer. If they do so the relation of 

 this increase to the higher mutation rate in germ cells affected by similar 

 agents should be interesting and important. 



Genetic studies of tumors originating in hybrids. — Transplantation 

 studies of tumors of this type should add more knowledge to the genetic 

 analysis of spontaneous tumors by providing evidence for linkage between 

 genes which underly the growth of transplanted tumors and some other 

 known Mendelizing genes. As the number of known genes in mice increases 

 and linkage becomes more generally recorded, the chance of finding genes 

 related to the process of spontaneous tumor formation should similarly 

 increase. If there is no evidence of such linkage when it may fairly be 

 expected, the negative findings will themselves be important in determining 

 the relative roll of chromosomal inheritance and other etiological factors in 

 spontaneous tumor formation. Preliminary evidence of linkage between 

 genes determining the growth of certain spontaneous tumors and those for 

 certain types of coat color has already been obtained. The need of obtain- 

 ing rapidly the largest possible number of genes is evident and the field of 

 tissue transplantation (more particularly that of tumors) should give us 

 valuable new information. 



At all events, the genetic analysis of transplanted and induced tumors 

 has a direct and permanent bearing on similar studies with spontaneous 

 neoplasms. 



Relation to Transplantation of Normal Tissue 



Little has been said in this chapter on the bearing of tumor transplanta- 

 tion to the genetics of normal tissue transplants. 



This omission is not due to the fact that the subject lacks importance or 

 interest. It results from the somewhat extraordinary fact that so little 



