MASKING 91 



transformed fibroma virus units has to be at a certain critical level at 

 the time that the fibroma virus would normally begin exerting its pro- 

 tective effect within the host for the transformation to become apparent. 

 It is also probable that other factors than the mere ratio of fibroma to 

 heated myxoma virus are of importance in determining the success or 

 failure of transformation because Dr. Margaret Smith, working with the 

 phenomenon in my laboratory, frequently experienced failures even 

 when all of the known factors were controlled to the letter. 



The exact mechanism by which heat-inactivated myxoma virus 

 transforms fibroma virus to myxoma virus is not understood. I men- 

 tioned at the outset that the phenomenon appears superficially to be 

 something of a mixture of "recombination" and "multiplicity reactiva- 

 tion." The recombination phase of the affair concerns mainly the 

 fibroma virus which is the living component of the reaction mixture. 

 To completely parallel the phage phenomenon of recombination, it 

 should acquire new characters from combination with living myxoma 

 virus. Actually the component of myxoma virus with which fibroma 

 virus apparently combines in the transformation is capable of being 

 perpetuated as such once it combines with fibroma virus. It thus seems 

 potentially at least to possess one of the attributes thought to be char- 

 acteristic of living matter, namely, reproduction. The "multiplicity 

 reactivation" phase of the myxoma transformation concerns mainly the 

 thermo-stable component of myxoma virus since alone this component 

 has all of the earmarks of an inactivated material so far as propagation 

 "on its own" is concerned. However, in combination with fibroma virus 

 this apparently inactivated and "dead" component becomes reactivated 

 apparently as a result of the replacement of the component, fibroma 

 virus, which was destroyed during the heat-inactivation process. 



From all of this it seems to me that the Berry-Dedrick phenomenon 

 of myxoma virus transformation has quite a lot in common with some 

 of the exciting tricks that bacteriophages can be made to do. The com- 

 plicating factor, as I mentioned at the outset, which prevents and will 

 continue to prevent any absolute parallelism between observations 

 made with the animal viruses on the one hand and with the bacterial 

 and plant viruses on the other is the presence, in the hosts of the animal 

 viruses, of antiviral immunity. This specific immune response to in- 

 fecting viruses will forever be present in the animal field to interfere 

 with or prevent those beautifully clear-cut and definite results so often 

 achieved by the plant virus and bacteriophage people. However, despite 

 this, there is undoubtedly much that can be learned from the plant and 

 bacterial virus work that can be rather directly applied with profit in 

 the animal virus field. 



