268 



BIOLOGY OF THE LABORATORY MOUSE 



JAX C57 black strain as "higher." The F2 generations give the results 

 shown in Table 16. 



Table 16 



There is very evidently no sign of extra-chromosomal influence in the Fo 

 generations. 



Relation of incidence to age. — One fact seems clear in all the experiments 

 thus far recorded. This is the distinctly later age at which non-epithelial 

 tumors usually appear as compared with epithelial mammary tumors. 



A tabulation of the mean age at death of mammary tumor and non- 

 epithelial tumor mice in the same stocks and their hybrids can be made from 

 data derived from the work of Murray, Cloudman and the writer (Table 17). 



Table 17 



* Female is listed first, male second, in the crosses. 



t Numbers of tumor animals too small to provide a significant value for 

 the mean age of incidence. 



It is very evident for the above data that there is involved a very different 

 set of physiological factors in the incidence of the two types of tumors. 



Relation of incidence to sex. — Scattered data derived from various 

 workers over a period of several years indicates that non-epithelial tumors 



