300 F. M. BURNET 



(1) Morphology of pock on the chorioallantois (CAM). 



(2) Production of hemagglutinin (HA). 



(3) Heat stability as tested by 40 minutes heating at 55°C. and subsequent 

 titration on the chorioallantois (HR). 



(4) Virulence for mice on intracerebral injection of a standard dose of 10 4-5 

 pock-forming units (MV). 



(5) Character of lesion produced by intradermal inoculation of a similar 

 dose in the rabbit skin (RS). Of the strains available the most suitable pair 

 for recombination work were judged to be a rabbitpox strain from Holland 

 RP and a "white" vaccinia variant 7N. Their differences are as shown in 

 Table V. 



TABLE V 

 Marker Characters Used est Poxvirus Recombination Experiments 



Character 7N "white" strain RP (rabbit pox) 



CAM Large opaque white pocks (W) Thin rimmed pocks with central 



hemorrhagic area (H) 



HA Produced in large amount ( + + + ) No hemagglutinin produced and 



no anti-hemagglutinin in serum 

 after recovery (0) 



HR Labile: showing a fall of 4-5 Stable: 1 log 10 or less (S) 



log 10 on heating (L) 



MV Minimal 4 % of deaths ( — ) 100 % deaths (V) 



RS Pink or red papular lesion without Flatter lesion with necrotic purple 



necrosis (N) centre (PC) 



Various types of double infection have been used. The best yield of recom- 

 binant forms was obtained from membranes inoculated with about equal 

 numbers of both types, on which there was an example of pocks of the two 

 parent types overlapping. This common zone will presumably include many 

 cells which have suffered multiple infection by both types of virus. The area 

 was cut out, emulsified, and titrated on the chorioallantois. More recently 

 positive results have been obtained from double infections produced in HeLa 

 cell tissue cultures. For the experiments so far completed only white pock 

 clones have been studied for evidence of recombination. The recombinants 

 reported can be represented as in Table VI, where represents the character 

 of the white vaccinia parent, -f that of the rabbitpox strain and ± an 

 intermediate result. 



Examination of the table suggests no linkage between any of the characters 

 except perhaps between the two virulence markers, where it is by no means 

 complete. Perhaps the most interesting feature is the fact that when either 

 virulence is present it is in all instances of intermediate character. This is 



