THE GENETICS OF SPONTANEOUS TUMOR FORMATION 271 



inbred strains, C58, "A surprising number of animals were found at autopsy 

 to have enormous spleens, large livers and swollen lymph nodes. They 

 were dying with leukemia, in most cases of the lymphatic type." 



* Both epithelial and non-epithelial tumors are included. 



In this strain, among over 700 mice observed until death, the incidence of 

 spontaneous leukemia was 90%. Since the strain was presumably homo- 

 geneous genetically, the presence of a group of 10% which failed to develop 



Table 22 



leukemia showed that in these mice extrinsic influences of some sort were 

 deciding whether or not an animal became leukemic. 



When males of this strain were crossed with females of a non-leukemic 

 strain, the incidence of leukemia in the resulting Fi hybrids was 45%. 



MacDowell defines the term leukemia as a neoplastic growth of the white 

 blood cells, differing from cancer in the fact that these cells do not remain 

 localized but move throughout the body. He further points out that 



