112 ALICE ISAACS 



of the main processes involved. Present ideas have been largely shaped by 

 work on bacteriophages, and, according to inclination, workers have tried to 

 point to resemblances or differences between animal and bacterial viruses. 

 At the moment, the most significant recent observations on virus multiplica- 

 tion are probably those of Hershey and Chase (1952) on bacteriophages and 

 Gierer and Schramm (1956) and Fraenkel-Conrat (1956) on tobacco mosaic 

 virus; these studies make it clear that infection can be initiated by virus 

 nucleic acid preparations from which the bulk of the virus protein is absent. 

 Recently Colter et al. (1957) and Wecker and Schafer (1957a) have described 

 similar experiments with animal viruses and the results suggest tentatively 

 that infection could be produced by "nucleic acid extracts" of encephalomyo- 

 carditis (Mengo) and eastern equine encephalitis viruses (see also Section 

 VII). Apart from these findings, our ideas on how animal viruses multiply are 

 to a large extent governed by our interpretation of experiments on the 

 eclipse phase and on virus recombination. Virus recombination is the theme 

 of Chapter II; in this chapter the eclipse phase will be considered first, because 

 of its importance to our concepts of virus multiplication. 



Burnet (1955) defines viruses as microorganisms less than 0.4/z in diameter 

 which can multiply only within living cells of a susceptible host, and which 

 undergo conversion into a noninfective form as a necessary step in their multi- 

 plication. It is in this sense that the term "eclipse phase" will be used in this 

 chapter. Burnet pointed out that in the great majority of animal virus types 

 it was not possible, on the basis of evidence then available, to conclude that 

 an eclipse phase was present or absent. In the following section the experi- 

 mental evidence bearing on the existence of an eclipse phase for a number of 

 different animal viruses is reviewed. 



II. The Eclipse Phase 



One of the striking findings about bacteriophage multiplication is that 

 within a short time of the initiation of infection, no infective virus can be 

 detected in disintegrated bacteria and this eclipse phase persists until 

 halfway through the lag period.* Many similar investigations have been 

 carried out with animal viruses, but while in all cases the amount of virus 

 which could be recovered in the lag period represented only a fraction of the 

 amount taken up by the cells, in only one case was the situation strictly 

 analogous to that found with bacteriophages, i.e., western equine encephalitis 

 virus (Rubin et al., 1955) in which no virus was recovered from cells during 

 the lag period. In all other cases some infective virus could be recovered 

 throughout the lag period, ranging from an extremely small fraction to quite 



* For the present purpose the term "lag period" is used to indicate the time between 

 the initiation of infection and the appearance of newly formed virus. 



