CLONAL PHENOMENA 



In this Study he used the strain BEL.D, which he had 

 found to grow more actively and uniformly in the allantoic 

 cavity than any other of those available. A large series of 

 eggs was infected with an inoculum containing less than one 

 infective dose. Samples were taken from each egg at two- 

 hourly intervals and stored until it was known by a positive 

 haemagglutinin test whether an infection had been induced. 

 All samples from such positive eggs were then titrated. The 

 results showed that there was a wide variation in the lag 

 period before new virus appeared. Once any new virus was 

 liberated the subsequent course of increase was very uniform 

 but the lag in initiation ranged from 2 to 28 hours. Appro- 

 priate experiments indicated to Cairns that the variation 

 could only be attributed to random difficulties in virus and 

 cell making an effective (that is, infection-producing) inter- 

 action. Since some particles took as long as 28 hours to 

 provoke first liberation of virus and since the half-life of 

 infectivity at 37° is of the order of 8 hours, it becomes highly 

 probable that many particles fail to initiate infection simply 

 because they are thermally inactivated before an effective 

 interaction is made. This is probably the simplest — though 

 not necessarily a complete — explanation of the tenfold dis- 

 crepancy found by Donald and Isaacs (1954) between the 

 number of morphological particles counted by electron micro- 

 scopy and the infectivity of the same preparation of influenza 

 virus. Under such circumstances, there will obviously be 

 scope for selective survival of any virus particles which, by 

 mutation or otherwise, have developed inheritable qualities 

 which allow them, on the average, to make earlier effective 

 contact with the susceptible cell. The other example, which 

 I shall have to discuss more briefly than it deserves, is con- 

 cerned with the changes in the virulence of myxomatosis 

 virus after its introduction amongst the wild rabbit popula- 

 tions of Australia. 



This is one of the most interesting ecological experiments 

 that has ever been undertaken and it is fortunate that 



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