THE GENETICS OF SPONTANEOUS TUMOR FORMATION 261 



This is, in all probability, a significant difference and is, therefore, of 

 interest. 



In analyzing the possible reason for the decreased incidence of mammar}^ 

 tumors in yellow mice, the following suggestions were made by the writer: 



'*A study of the physiology of reproduction of yellow and non-yellow 

 mice within the yellow stock suggests that the yellows pass through their 

 reproductive cycle earlier than do the non-yellows. The duration of the 

 cycle in the two forms is essentially equal. This fact would satisfactorily 

 explain the earlier incidence of mammary tumors in yellow mice. 



"The lower incidence of mammary tumors in yellows as compared with 

 non-yellows may be at least in part due to the same phenomenon. This 

 would follow because the opportunity for mammary tissue in yellow mice of 

 cancer age to be continuously affected by ovarian secretion would be less 

 than in non-yellows. This would result in a higher percentage of vellows 

 reaching an age at which stimuli from the ovary ceased before the mammary 

 tissue had reached an age at which tumor formation is most frequent." 



These data show that it is not accurate to lump together different colored 

 mice in calculating the incidence of mammar\' tumors. They also indicate 

 the need for further study of this general topic. 



Summary. — To summarize the situation as regards epithelial breast 

 tumors in mice, one may state: 



1. These are the commonest type of spontaneous neoplasm in unselected 

 material. 



2. Strains have been produced by inbreeding and selection which mav 

 give as high as 93^0 or as low as 0.5*^ incidence of these tumors in breeding 

 females. 



J. There is conclusive evidence that the incidence of these tumors is not 

 due to a single recessive gene. 



4. There is a definite extra-chromosomal influence which is directlv 

 transferable from female parent to her progeny. 



5. This influence is at least in part, if not entirely, transferable through 

 the milk of the mother. 



6. It is probable that genetic factors also play a part in determining the 

 tendency to form tumors. 



7. Hormonal influences also affect the incidence of breast tumors. 



Epithelial Lung Tumors 



Most of the observed lung tumors in mice are epithehal in origin, being 

 of the adenoma, adenocarcinoma or carcinoma simplex types. These 



