266 



BIOLOGY OF THE LABORATORY MOUSE 



''Melanomas, which are rare, are usually conlined to the base of the tail 

 and are deeply pigmented. 



"Osteogenic sarcomas, which have ordinarily appeared in the long bones 

 or jaw, are superficially very much like fibrosarcomas in those regions. 



"Pathological diagnosis has been obtained for all tumors included in this 

 report." 



The inbred stock on which there has been recorded the most extensive 

 observations is the J AX C57 black stock of the Jackson Memorial Labora- 

 tory (Tables 13, 14 and 15). 



Table 13 



Tumor Incidence in the C57 Black Stock among Animals in Which There 



Are Data for the Full Life Cycle and for Non-epithelial Tumor 



Incidence 



It is doubtful whether any of these groups differs significantly from the 

 others except in the mean age at death of the virgin and of the breeding 

 female non-tumor mice. In the case of the breeding females there is a 

 distinct suggestion of the existence of a greater mortality risk. The virgin 

 females have definitely a greater life span than the other groups. This may 

 account at least in part for the high incidence of tumors since the opportunity 

 to have such tumors which occur at an average age of 711 days was well 

 afforded by the fact that mean age of survival of non-tumor mice was 100 

 days beyond that figure. 



With this brief introductory statement we can next consider the relation 

 of non-epithelial tumor incidence to various factors such as extra-chromo- 

 somal influences, age, sex, etc. 



Absence of extra-chromosomal influence. — In the various experiments 

 with non-epithelial tumors there are no cases of reciprocal crosses between 



