174 



BIOLOGY OF THE LABORATORY MOUSE 



such as C57 leaden, black hairless and Mus htutrianus, our impression would 

 have been that mammary tumors are rare. The reverse would have been 

 true had we been using only such stocks as the A albino, the C3H, the dba 

 and similar lines with a high incidence of spontaneous breast cancer. To 

 give more concrete examples, the A albino line is high in the incidence of 

 both lung and mammary cancer, while lung cancer is uncommon in most of 

 our other stocks. On the other hand, the ce (extreme dilution) stock has an 

 abnormally high number of tumors of the ovary, while most of the melanomas 



Fig. 89. — A diagrammatic drawing of the maximum extent of the mammary system 

 of the mouse shown in the ventral and lateral aspects. The large black dots represent 

 the nipples and the stippled areas the mammary glands. 



have been observed in the dba (dilute brown) stock, and tumors of all types 

 are rare in Mus hactrianus. Certain tumors will probably always be con- 

 sidered uncommon, but as an increasing number of inbred stocks are devel- 

 oped and studied, we shall be able to find certain lines of mice which will be 

 of exceptional value in advancing our present knowledge of the more rare 

 forms of spontaneous neoplasms. 



Tumors or the Mammary Region 



Since the majority of investigators, especially the pioneers, working with 

 spontaneous tumors in mice confined their studies mainly to the most avail- 

 able forms of neoplasms, they investigated chiefly those outside the body 

 cavities and thus much of the work has been done with tumors of the mam- 



