350 H. B. ANDERVONT 



Bittner substrains; 17.8 % of the NCI mice and 8.8 % of the Bittner mice 

 developed leukemia. The authors stated that the same extracts were not 

 injected into both substrains and therefore the results should not be compared, 

 but a leukemia incidence of 17.8 % in treated NCI mice was above the 

 incidence of 4 % reported by Gross (1955a). 



Further studies with the Bittner substrain mice are necessary to establish 

 the incidence of spontaneous leukemia in them. Leukemia is a common 

 disease in mice and occurs in almost all inbred strains, but to a far greater 

 extent in certain strains, such as AKR and C58. Gross, however, has not 

 limited his use of experimental animals to strain C3H, for, as stated previously, 

 he has established strain C57BR/cd as suitable for confirmation of his 

 findings. 



a. Transmission of the Leukemia Virus to Progeny. In one of his early papers, 

 Gross (1951b) reported transmission of the virus from infected parents to 

 their immediate offspring. This aroused considerable interest among onco- 

 logists because transfer of a tumor-inducing agent from generation to genera- 

 tion was accepted as good evidence for the presence of a tumor virus. Gross 

 recorded the appearance of leukemia in 9 of 18 offspring born to C3H parents 

 infected with AK/n leukemic cell suspensions and in 4 of 9 offspring born 

 to C3H parents infected with AK/n embryo cell suspensions. While discussing 

 these first findings he stated: "There is no reason to believe that the leukemic 

 agent would stop at this point its vertical trend of transmission." In a later 

 paper, Gross (1952b) reported the occurrence of leukemia in 17 of 46 first 

 generation offspring of infected strain C31I parents. During a brief discussion 

 of the parent-to-offspring transmission he modified his earlier view of 

 continuous transmission by stating: "The possibility, however, must be 

 considered that the leukemic agent may become adapted to the new strain 

 of mice to such an extent that it may, after having passed through one or 

 two generations, fail to become activated even though it may be transmitted 

 through the embryos from one generation to another, and even though it may 

 be carried by the new hosts." This was an unusual interpretation by an 

 oncologist interested in a tumor virus because it placed him in the position of 

 either estabhshing the absence of an "inactive virus" in his test animals or 

 admitting that they carry an "inactive virus." The widespread occurrence of 

 leukemia in many inbred strains of mice could imply, according to this 

 hypothesis, that all strains carry the virus. 



Gross' (1954b) next reference to transmission of the virus from generation 

 to generation consisted of a brief reference to young born to C57BR/cd 

 parents that had been infected with AK/n leukemic extracts; 22 parents 

 produced 147 untreated offspring, of which 10 developed leukemia. The last 

 publication in which Gross (1955b) discussed natural transmission of the 

 virus referred only to C3H mice infected with the virus from AK/n mice. 



