PROBLEMS CONCERNING THE TUMOR VIRUSES 361 



closed his discussion of the most recent paper reviewed here (Gross, 1957b) 

 with the words, "It would not be surprising to find that such agents, although 

 possibly of a different pathogenic potential, were related." Dulaney et al. 



(1957) were not so cautious and stated: "It is our belief that the parotid 

 gland tumor-inciting agent is separate and distinct from the leukemic 

 agent." 



The problem became more complicated when Stewart et al. (1957a) 

 reported the occurrence of a wider variety of tumors in mice receiving tissue 

 culture preparations or cell-free extracts from a transplanted AK/n leukemia. 

 In addition, Eddy et al. (1958) found that the tissue culture materials elicited 

 different tumor types in newborn hamsters. The implications of this work 

 with tissue cultures are far reaching and emphasize again the application of 

 tissue culture techniques to the tumor viruses. 



The development of the problem of a single virus or multiple viruses 

 being causative agents of these tumors will be watched with interest, 

 especially by those who are working with the chicken tumor viruses. It wiD 

 be recalled these investigators have already advanced the unitarian theory 

 of tumor viruses and the concept of a stem virus without the benefits derived 

 from the use of inbred animals as experimental animals. The establishment 

 of high-leukemia strains of mice has been of invaluable assistance to those 

 now working in the field, and, perhaps, they should reciprocate by giving 

 more attention to the host. It should be firmly established that only newborn 

 mice are susceptible to the agents because, if substantiated, it introduces a 

 new concept into the problems of the tumor viruses. A recent report by Gross 



(1958) revealed that filtrates of a potent "passage" strain of leukemia induced 

 leukemia in the Bittner substrain of C3H mice which were from 1 to 51 days 

 of age. Cell-free extracts could be administered to newborn and older mice of 

 a variety of inbred strains to establish a firm foundation for future work. 

 Such efforts will not only assist the progress of the immediate problem, but 

 will contribute to cancer research by providing information of the value and 

 the limitations of the newborn as hosts for tumor viruses. It is known that 

 newborn animals are in a stage of immunological nonreactivity and this 

 demands a thorough investigation to ascertain the importance of the hosts in 

 all investigations reviewed here. 



Space does not permit a satisfactory review of the efforts of others who have 

 worked with leukemia agents. Friend (1957a,b) discovered an agent in an 

 Ehrlich ascites tumor which has been transferred serially through noninbred 

 adult Swiss mice by means of cell-free filtrates. Whether the virus induces a 

 disease belonging to the categories described by Dunn (1954), or whether it is 

 a variant of the mouse leukemia virus, comparable to the viruses of erythro- 

 blastosis or myeloblastosis in their relationship to the virus of visceral 

 lymphomatosis of chickens, is the major problem. 



