346 H. B. ANDERVONT 



cells and the only known lesion indicative of its presence prior to the 

 appearance of neoplasia is the early occurrence of hyperplastic nodules in the 

 glands. The observations by Jones (1956) that hyperplastic nodules are also 

 numerous in the glands of old mice free of the virus calls for further explora- 

 tion of the possibility that the virus acts as an accelerator in the production 

 of tumors. If it can be established that the role of the virus in the production 

 of the tumor is to provide these nodules in which the tumor develops, then 

 the virus has done much to further knowledge of the tumor viruses, for this 

 would imply that any virus capable of producing benign hyperplasia could be 

 implicated in the origin of cancer. 



A brief survey of the literature produced only one reference to the applica- 

 tion of tissue culture techniques with the virus. Pikovski (1953) reported the 

 survival of the virus for 5 months in tissue cultures of chicken fibroblasts. 

 Tissue cultures of mouse embryos may provide an excellent substrate for 

 cultures of the mammary tumor virus. 



The major contribution of this virus to the tumor viruses has been the 

 establishment of the importance of the host in any host-virus relationship. 

 Regardless of whether the virus is found to be the direct cause of the tumor or 

 whether it acts indirectly by producing a benign lesion which, under suitable 

 environmental conditions, proceeds to malignancy, the genetic constitution 

 of the host is of much importance in the sequence of events which terminates 

 in cancer. 



F. Leukemia of Mice 



Since 1951, an effort to establish a causal relationship between a virus and 

 leukemia of mice has been the most interesting, exciting, and confusing 

 feature of the tumor viruses. It has been the most interesting and exciting 

 because oncologists familiar with the characteristics of mouse leukemia and 

 its widespread occurrence in all inbred strains have seriously considered the 

 possibility that a virus could be involved. The establishment of strains 

 showing high or low incidences of spontaneous leukemia, together with 

 experience gained from studies of mouse mammary cancer, made possible a 

 rational approach to the problem. 



As the work progressed it became clear that the task was not easy. It 

 became very complex when the test animals developed a variety of tumors 

 which were not characteristic of leukemia. This difficulty had not been 

 encountered with other virus-induced tumors which were reproducible by the 

 administration of filtrates to susceptible tissues of suitable hosts. Further- 

 more, those who entered the field published contradictory and, at times, 

 confusing results. A review of the literature gives the impression that the 

 co-operation and free exchange of information maintained by early investi- 

 gators who worked with the mammary tumor virus has, thus far, been 

 lacking in studies of leukemia. 



