BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 151 



and outside this group of viruses there are very few published accounts of 

 attempts to prepare incomplete virus forms. This seems a pity, since the 

 phenomenon is most interesting and information on the conditions under 

 which it is applicable to other viruses would be worth having. 



Minis (1956) described some indirect evidence that Rift Valley fever virus 

 could be induced to show incomplete virus forms. Mims noted that when 

 large inocula of virus were passaged serially in mice low infective titers 

 resulted, whereas high titer virus was produced by small inocula. The low- 

 infective virus was shown to be capable of interfering with the growth of 

 infective virus by prolonging the incubation period and reducing the peak 

 infectivity titer reached. However, these effects could be explained in 

 alternative ways and, in particular, there is a possibility that if the seed 

 population of virus particles is heterogeneous in respect of virulence, large 

 doses of virus may act as an antigenic stimulant and produce an apparently 

 similar effect (cf. Schlesinger, 1949). 



Sanders (1957) has shown that encephalomyocarditis virus can be titrated 

 in three different ways. The plaque -forming titer measures the infectivity 

 of a preparation in a plaque titration with ascites tumor cells of mice. The 

 same cells can be used to measure a second property of the virus, its cell- 

 killing ability; this depends on the fact that aqueous eosin stains dead cells and 

 the virus can be titrated indirectly by this method by calculation from the 

 number of unstained cells (the calculation is based on the Poisson distribution). 

 Finally, the virus can be titrated by its hemagglutinin titer, and there is good 

 evidence that all three titrations measure properties of the virus particles 

 themselves. If virus is incubated for more than 1 hour at 37°C, the plaque- 

 forming titer is reduced more than is the titer by the other methods, and the 

 same is true of virus freshly released from cells after infection with high 

 multiplicities of virus. This phenomenon requires further investigation, but 

 the results of these experiments seem to resemble closely those of von Magnus 

 for influenza virus. Recently, Cooper and Bellett (1957) have produced some 

 evidence that an autointerference phenomenon observed with vesicular 

 stomatitis virus grown in chick embryonic cells in vitro is due to the produc- 

 tion of incomplete forms of virus. The incomplete, or T, forms of virus are 

 produced on serial undiluted passages, but not with diluted passages, and 

 only when cells are rnixedly infected with T and live virus particles; also the T 

 forms are spontaneously liberated from cells, as in the case of influenza virus. 



VII. Conclusions 



Throughout this chapter, the attitude adopted has been that although 

 research on the multiplication of bacteriophages has provided the greatest 

 stimulus to the investigation of similar problems in animal viruses, in all 



