CLONAL PHENOMENA 



the infection was being transferred. It was a commonplace that 

 the material might contain contaminant or perhaps syner- 

 gistic bacteria, but there were standard methods by which 

 these could be eliminated. 



Once an experimental infection had been produced it 

 was transferred to the next host in some technically con- 

 venient dose, the usual objective being the Pasteurian one 

 of ' adapting ' the virus to the experimental host until it had 

 a high and constant virulence. There is still a tendency to 

 write as if adaptation of a virus were a simple function of the 

 number of passages it had undergone in the new host; not 

 so long ago this attitude was universal. Today it is reasonable 

 to ask that such an approach should be replaced by a recogni- 

 tion that every significant situation involving the functional 

 activities of a virus needs to be looked at from the point of 

 view of population genetics. 



The behaviour of strains of influenza A virus on first isola- 

 tion offers a particularly good example which has been 

 extensively studied in Melbourne. During an extensive 

 influenza epidemic, involving service and civilian personnel, 

 in May 1942, we used the two newly developed techniques 

 of amniotic inoculation of the chick embryo and of haemag- 

 glutination for the isolation of virus from human throat 

 washings (Burnet and Bull, 1943). It was soon found that 

 amniotic fluids containing the primary isolate agglutinated 

 guinea-pig or human cells to much higher dilution than fowl 

 cells. A typical fluid might have what we called an F/G 

 ratio of 1 0/ 1 60, with the fowl cell titre as numerator and the 

 guinea-pig cell titre as denominator. At the same time it was 

 observed that isolation usually failed when allantoic inocula- 

 tion was used. Yet after one or two passages in the amniotic 

 cavity all strains grew freely in the allantoic cavity, giving a 

 fluid which would generally agglutinate fowl erythrocytes to 

 a slightly higher titre than guinea-pig cells. We called this 

 change of influenza A virus when it was transferred from the 

 natural human host to the allantoic cavity of the chick 



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