BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 139 



of time. Rubin et al. (1955) tried to measure the release time for this virus. 

 They assumed that the rate of increase of extracellular virus is at any instant 

 proportional to the concentration of the intracellular virus present at that 

 instant and to a velocity constant. This can be expressed as dcjdt = K^t), 

 where K x represents the probability per unit time that an intracellular virus 

 particle will be released. (It is, however, theoretically possible that K x might 

 alter during the growth cycle, if for example, the release of virus was accelera- 

 ted as cell damage increased.) Therefore, HK X is the average time for virus to 

 be released once it has become infective, and it can be calculated either mathe- 

 matically or graphically from the experimental data on the rate of increase of 

 intracellular and extracellular virus. For western equine encephalitis virus 

 growing in chick embryo monolayers the average release time was calculated 

 to be just under one minute. 



Lwoff et al. (1955) noted an extremely rapid release of poliomyelitis virus 

 from individual monkey kidney cells, practically all the virus being released 

 within half an hour. On the other hand, Howes and Melnick (1957) concluded 

 that Type I poliomyelitis virus was slowly released from monkey kidney 

 monolayers. This is based on the findings that the increase in extracellular 

 virus began about one hour after the increase in cell-associated virus and that 

 the free virus was always much lower in titer. Even after 11 hours' growth, 

 less than 20 % of the total virus was free. These findings are very different 

 as regards extracellular virus from those of Dulbecco and Vogt (1955), who 

 measured the rate of increase of extracellular (but not cell-associated) virus 

 in the same cells. The only obvious difference between the two sets of experi- 

 ments is that Dulbecco and Vogt used a one-step growth curve technique in 

 order to minimize adsorption of newly released virus to fresh cells, whereas 

 Howes and Melnick assayed samples from the monolayers directly, where 

 presumably newly released virus might be rapidly adsorbed to fresh cells. 

 Possibly, therefore, rapid release of virus is a more accurate picture for 

 poliomyelitis virus, but direct comparison of the two techniques might help 

 to decide this point. 



B. Myxoviruses 



Cairns (1952) studied the release of influenza virus from eggs in- 

 fected with virus at limiting infective dilution, where presumably a single 

 allantoic cell was infected initially. He found that virus liberation occurred 

 over a period of 3 hours. The technique was not suitable for measuring further 

 liberation of virus since additional cycles of infection could not be prevented. 

 However, Henle et al. (1954) inoculated de-embryonated eggs with 10 6 ID 50 

 of influenza virus and measured the differential (hourly) release of virus. 



