352 H. B. ANDERVONT 



The use of such hybrids as experimental animals has two serious dis- 

 advantages. First, they already carry the virus, if Gross is correct in his 

 opinion that embryos derived from high-leukemia strains are infected. Second, 

 hormones, ionizing radiations, chemical carcinogens, and diet may hasten 

 the time of appearance and increase the incidence of a tumor in experimental 

 animals possessing the predisposition to its spontaneous development. Hence, 

 an accelerated occurrence of the tumor in them is inconclusive evidence of the 

 activity of an extrinsic virus. Suitable hybrids for detection of the leukemia 

 virus should be procured from strains showing low incidences of the spon- 

 taneous disease. 



In this brief review of efforts to reveal a virus as the causative agent of 

 mouse leukemia no attempt was made to include all the recent literature. It 

 is hoped the reader is aware that time and more work are needed before the 

 viral etiology of the disease is clarified. Gross has shown that a transmissible 

 agent, presumably a virus, is important, but the attempts of others to 

 confirm his results have been conflicting and, to those not directly involved, 

 confusing. The reason for the present state of affairs could be that the investi- 

 gators, including Gross, have given a large part of their attention to the study 

 of other tumors which arose unexpectedly in the experimental animals. Some 

 of these will now be discussed. 



2. Parotid Gland Tumors 



This tumor was the leading culprit in the diversion of interest, and for good 

 reasons. It arose in mice after they had received extracts of leukemic tissues 

 and was a new type of tumor or, at least, one not recognized previously in the 

 mouse. It has been described adequately by Dunn (Law et al., 1955) and by 

 other pathologists whose descriptions were included in the first few following 

 references to Gross. Considerable space could be given to this tumor but, for 

 the purpose of this chapter, a brief review will suffice. 



Gross (1953c,d) recorded the occurrence of parotid tumors in strain C3H 

 mice after they had received centrifuged filtrates of AK/n leukemic tissues or 

 centrifuged extracts from AK/n embryos; C3H mice inoculated with C3H 

 embryo extracts "remained in good health." When AK/n leukemic extracts 

 were centrifuged at 144,000 g from 1 to 2 hours, the supernatants and resus- 

 pended sediments contained the parotid tumor-inducing agent. Gross (1954b) 

 found that filtered AK/n leukemic extracts did not produce parotid tumors in 

 C57BR/cd mice. 



In a subsequent paper, Gross (1955b) gave more information concerning 

 the parotid tumor response of C3H mice exposed to filtrates of AK/n leukemic 

 tissues and extracts of AK/n embryos. He emphasized the fact that parotid 

 tumors never arose spontaneously in his subline of C3H mice. A later 

 publication (Gross, 1955c) included considerable data on the occurrence of 



