348 H. B. ANDERVONT 



delayed but, according to the published data, they were neither negative nor 

 erratic. 



Throughout the remainder of this discussion, unless stated otherwise, all 

 mice were less than 24 hours old when used as test animals for the activity of 

 tumor or normal tissue extracts. 



Gross' (1951a) first evidence of a leukemia-inducing agent was obtained 

 when strain C3H mice received supernatant fluids from centrifuged extracts 

 of leukemic tissues from high -leukemia strain AK/n mice. Fourteen mice were 

 inoculated and 7 developed leukemia 8 to 11 months later. Likewise, the 

 administration of cell suspensions of AK/n embryos to 6 strain C3H animals 

 produced leukemia in 4. With this as a start, leukemic tissues from strain 

 AK/n as a source of virus and strain C3H as test mice were used to show: (1) 

 filtrates contained the virus (Gross, 1951b, 1952b, 1953c,d); (2) heated 

 extracts (Gross, 1953a) or filtrates (Gross, 1953c,d) did not elicit leukemia; 

 (3) centrifugation at high speeds (144,000 g) sedimented most of the activity 

 in cell-free extracts (Gross, 1953a,d). Additional experiments confirmed the 

 presence of the virus in AK/n embryos (Gross, 1952b). Gross (1953b) used 

 strain C3H as test animals to find the virus in cell suspensions of testes and 

 ovaries from AK/n mice; of 12 inoculated with testicular extracts, 8 developed 

 leukemia, and of 14 inoculated with ovarian extracts, 6 did so. 



In a subsequent paper Gross (1954a) summarized his results and compared 

 them with those of others who had studied the virus of visceral lymphomatosis 

 of chickens and, on the basis of his detection of the mouse virus in embryos, 

 testes, and ovaries, discussed the egg-borne transmission of both viruses. 

 He wrote: "It can only be stated that leukemia in chickens or mice, is not 

 transmitted from one host to another in the conventional manner hitherto 

 observed in the common communicable diseases." This statement is quoted 

 because it is relevant to the previous discussion of chicken leukemia in this 

 chapter, and because it is surprising that it was written by an investigator 

 interested in tumor viruses, especially by one (Gross, 1952b) who had recently 

 participated in a conference during which Burmester (1952), in his discussion 

 of the chicken virus, had stated: "This agent is spread and produces the 

 disease by direct contact and probably by the aerogenous route." 



Gross (1954b, 1955b, 1956) advanced the problem when he found that the 

 strain AK/n virus evoked leukemia in strain C57BR/cd mice and that 

 centrifuged cell suspensions of leukemic tissues from high-leukemia strain 

 C58 mice were also capable of eliciting leukemia in strain C3H mice. Filtrates 

 of leukemic extracts from C58 mice produced leukemia in strains C3H and 

 C57BR/cd. He also found (Gross, 1956) that cell suspensions of strain C58 

 embryos produced leukemia in strains C3H and C57BR/cd. These results 

 widened the scope of the studies by showing that his earlier findings could be 

 duplicated with tissues from another high-leukemia inbred strain. 



