270 



BIOLOGY OF THE LABORATORY MOUSE 



Table 19 



The Incidence of Non-epithelial Tumors in Intense and in Dilute 



Mice in Two Series of Crosses 



another in various physiological activities has been pointed out by Little 



(58). 



The parent strains used were : 



(a) Mus bactrianus, a small, slowly maturing, relatively infertile species. 



(6) JAX C57 black, a strain derived from Mus musculus, large, rapidly 

 maturing and fertile. 



The tumor incidences in these strains and in their Fi hybrids are com- 

 pared in Table 20. 



Table 20 



The increase in the hybrids is striking. There was also a definite increase 

 in multiple tumors among the hybrids (Table 21). 



Leukemias 



By far the most extensive and important work in this field has been done 

 by MacDowell and his associates. It has been admirably summarized and 

 discussed by him (67) in a recent general paper. 



He describes the origin of his material which is based upon carefully 

 controUed inbreeding over an extensive period. As he states, in one of his 



