THE GENETICS OF SPONTANEOUS TUMOR FORMATION 269 



located in tissues or organs not specific to or unequally developed in the 

 sexes are as frequent in one sex as in the other. In this respect they 

 resemble, as might be expected, more closely the epithelial lung tumors than 

 those of the mammary gland. 



It may well be that further and more detailed observations will reveal 

 tendencies for certain types of non-epithelial tumors to occur more fre- 

 quently in one sex than in the other. Such differences will, however, in all 

 probability be minor and secondary and will occur as a reflection of the 

 influence of a physiological distinction between the sexes, less important 

 than those commonly recognized as secondary sexual characters. 



Relation of incidence to coat color. — The difhculty of exact studies in this 

 field is clear. There are, however, certain indications of relationships 



Table 18 



between coat color of certain types and its accompanying physiology on one 

 hand and the incidence of non-epithelial tumors on the other. 



One of the more interesting of these suggestive relationships is to be found 

 in yellow mice which have long been known to be addicted to adiposity. 



In a cross between yellow and non-yellow strains of mice there were 

 among the hybrids nine cases of lipoma or liposarcoma. These are entirely 

 absent in other crosses involving the same non-yellow strain. The actual 

 figures are given in Table 18. 



It seems likely, therefore, that yellow ancestry introducing physiological 

 tendencies towards excess formation of lipoid tissue provides an increased 

 opportunity for the origin of tumors in that tissue. 



Another less clearly defined but potentially interesting relationship 

 between coat color and non-epithelial tumor formation is to be found in the 

 incidence of this type of tumor in "intense" pigmented mice with the gene D 

 as compared with "dilute'' mice homozygous for its allele d (Table 19). 



Relation of incidence to hybridization. — The possible eftect on tumor 

 incidence of crosses between strains of mice that differ widely from one 



