136 ALICK ISAACS 



(Henle et al., 1954). In addition, Schlesinger and Karr (1956) observed that in 

 a similar system, when cells were infected at a high multiplicity, i.e., when 

 presumably all cells were infected initially, periodic increases and decreases 

 occur in the amount of liberated virus, associated with stepwise breakdown 

 and partial restoration of the inhibitor of viral hemagglutination present in 

 the chorioallantoic membrane. These periodic increases and decreases appear 

 to correspond in time to the cycles observed by Cairns (1952). Recently, 

 Cairns (1957) has shown that there is asynchrony in the initiation of infection 

 by influenza virus, with great variation in the time before hemagglutinin 

 appears in the allantoic fluid after infection with very small inocula. This 

 finding makes it even more difficult to disentangle the different cycles which 

 together make up the normal growth curve. 



Tyrrell (1955) found that when influenza virus was grown in tissue cultures 

 of chick embryo lung the average yield of virus was 650 hemagglutinating 

 particles per cell. In this technique one hemagglutinating particle corresponds 

 roughly to one infective dose and 10 virus particles as counted in the electron 

 microscope (Tyrrell and Valentine, 1957), i.e., the yield was about 6500 virus 

 particles per cell. The period of liberation observed was 72 hours. Recently, 

 Ledinko et al. (1957) studied the growth of influenza virus in trypsinized 

 suspensions of chick embryo lung cells, adding RDE after the initiation of 

 infection in order to restrict the growth of virus to a single cycle. After a lag 

 period of 2|-2§ hours there was an exponential rise in the titer of extracellular 

 infective virus for about 2 hours, with a rate of increase of roughly 10-fold per 

 hour. 



D. Newcastle Disease Virus of Fowls (NDV) 



Rubin et al. (1957) carried out one-step growth curve studies of NDV grown 

 in monolayers of chick embryo lung epithelium, the cells being infected at a 

 multiplicity of 2 infective doses per cell. The lag period, as measured by the 

 first appearance of new virus, was 3-4 hours but the average latent period, 

 defined as the time when the yield of virus corresponded to one infective 

 particle per cell, was 5-6 hours. In studying the development of cell- 

 associated viruses, the cells were treated with antiserum before disintegrating 

 them by three cycles of freezing and thawing. The antiserum treatment was 

 necessary to remove virus superficially adsorbed to the cells and to give a 

 clearer picture of the development of intracellular virus. Intracellular virus was 

 found to increase at a nearly exponential rate, with an approximately 30-fold 

 increase between the 4th and 6th hours before the increase tailed off. The 

 curve of extracellular virus increased more steeply and 24 hours after infection 

 there were roughly 1000 infective doses of virus released per infected cell. 

 About 50 % of the final yield of virus was produced at 8-10 hours after the 

 initiation of infection. 



