MASKING 89 



rabbits against myxoma virus conferred by infection with fibroma virus 

 was interpreted as indicating that the fibroma virus was antigenically 

 only a partial replica of myxoma virus. The antigenic components 

 comprising fibroma virus and common also to the myxoma virus were 

 sufficient to establish in fibroma-infected rabbits a state of resistance to 

 myxoma, but, because they represented only partially the antigenic 

 composition of myxoma virus, this resistance was not the complete im- 

 munity conferred reciprocally by two identical viruses. In short, the 

 cross-immunological data suggested strongly that myxoma virus was 

 composed of fibroma virus plus some other antigenic component and 

 that this other hypothetical component was responsible for the lethal 

 virulence of myxoma virus in rabbits. 



The possibility that myxoma virus might be a complex containing 

 fibroma virus as one of its components aroused interest in two labora- 

 tories, but the workers in these laboratories approached the problem 

 from exactly opposite directions. In one instance attempts were made 

 to convert myxoma to fibroma virus by removing the hypothetical com- 

 ponent of the myxoma agent, while in the other the conversion of 

 fibroma to myxoma virus was attempted by adding the hypothetical 

 component of myxoma virus to fibroma virus. 



In the case of the first experimental approach, myxoma virus was 

 passed through fibroma-recovered rabbits for eight serial passages test- 

 ing virus at each passage for its possible reversion to the fibroma type 

 (13). No alteration in the original fully lethal character of the myxoma 

 virus was shown. In another group of experiments designed to achieve 

 the same end, myxoma virus was passed in series through cottontail 

 rabbits for ten passages covering a total elapsed time of 140 days during 

 which the myxoma virus was not out of cottontail tissue (14). Myxoma 

 virus, as I neglected to mention earlier, induces in cottontails a nonfatal 

 disease in which the only manifestation of infection is a fibromatous 

 tumor, identical in appearance to naturally occurring cottontail 

 fibromas, developing at the site of inoculation. It was reasoned that, 

 since myxoma virus in the cottontail induced lesions and a clinical 

 picture similar to that caused in this species by fibroma virus, some 

 change in myxoma virus might be expected to occur as a result of its 

 prolonged sojourn in cottontails. However, such did not prove to be the 

 case and the virus recovered from the cottontail lesions even at the end 

 of ten serial passages and a total residence of 140 days was still fully 

 lethally pathogenic myxoma virus as demonstrated by its activity in 

 domestic rabbits. It was apparent that, at least so far as the techniques 

 applied in these experiments were concerned, no removal of a hypo- 



