286 F. M. BURNET 



Since the only markers used in Hirst and Gotlieb's (1953) work were Aa and 

 Gg, no evidence on this point is provided by their experiments. 



D. Phenotypic Mixture and Heterozygosis 



Early in the work on recombination between MEL and WSE strains in the 

 allantoic cavity, it was observed that the primary yield of fluid often showed 

 anomalous results when tested with antisera against the two parent strains. 

 Some fluids contained what appeared to be a considerable proportion of 

 hemagglutinin neutralized by both antisera. This was mentioned by Lind and 

 Burnet (1953) and reported by Fraser (1953). 



a The X 3 strain was significantly neutralized in ovo by both antisera. 



A clearer picture of this phenomenon was provided from the experiments 

 of Gotlieb and Hirst (1954), who were concerned with double infections by 

 the strains MEL (M) and WSN (W). The latter strain resembles NWS of 

 Stuart-Harris (1939) in general properties but was a separate derivative of 

 WS obtained originally by Francis and Moore (1940). Gotlieb and Hirst (1954), 

 using cross-absorbed immune sera, obtained from double infections a 

 proportion of fluids whose HA was neutralized to considerable titer by both. 

 These are called X 1 fluids. When such a fluid is titrated, both M and W type 

 fluids are obtained among the progeny, as well as a proportion of X x fluids 

 from embryos inoculated at low dilutions. 



With continued titration passage of X type fluids, some were obtained 

 which gave a much higher proportion of doubly neutralizable fluids, including 

 some from dilutions near the limit of infectivity. These were referred to as X 2 

 fluids. They did not represent a stable form since they always gave a propor- 

 tion of pure M and W fluids at limit dilutions. 



Finally, an atypical fluid was obtained which, although predominantly W 

 in type, was also significantly neutralized by anti-M serum. This bred true and 

 was called X 3 . The characters of these fluids are illustrated in Table I. 



