BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 149 



the technique of serial passages (von Magnus, 1951b). They found that 

 different strains varied in the ease with which they could be induced to form 

 incomplete virus. When the strains were arranged in order of ease, the only 

 biological activity with which the order corresponded was the time taken for 

 effective entry of virus into the cell. At the moment, the significance of this 

 interesting observation is not known. 



C. Partial Cycle of Virus Development Produced by Incomplete Virus 



In earlier work on incomplete virus it was assumed that the incomplete 

 virus was noninfective, but more recently it has become clear that it can 

 initiate partial cycles of virus development. Burnett al. (1954, 1955) carried out 

 some very interesting studies of the growth of incomplete virus in de-embryo- 

 nated eggs. The technique used was to inoculate different doses of incomplete 

 virus, remove the seed by washing, and treat the membranes with RDE in 

 order to obtain a single cycle of growth. The fluids were then harvested at 7 

 hours and the yield of hemagglutinin titrated; in addition, the amount of 

 hemagglutinin produced between 7 and 22 hours was measured. Over a given 

 range of virus dosage the yield of hemagglutinin at 7 hours was proportional 

 to the amount of virus taken up; this applied to both complete and incomplete 

 virus. This finding shows that in both cases a single cycle of virus production 

 was being observed. However, the yield from incomplete virus was much 

 greater than would have been expected from the infectivity of the inoculum; 

 in one experiment, quoted by Burnet et al. (1954), the yield from an incomplete 

 virus preparation with a I /HA ratio of 10~ 3a of the complete virus used 

 gave a yield of one-sixth of the complete virus at all virus dosages tested. 

 Since the yield per particle was independent of the virus dosage and on the 

 basis of calculations of the multiplicity of infection the results could not be 

 explained by multiplicity reactivation. Burnet et al. conclude that a consider- 

 able proportion of the incomplete virus in the seed is able to undergo a single 

 incomplete cycle of multiplication to produce viral hemagglutinin, but that 

 this hemagglutinin is unable to continue to produce full infection. Paucker 

 and Henle (1955b) have also found that virus heated at 37°C. and rendered 

 noninfective was still able to produce hemagglutinin in a single cycle. They 

 suggested that the live virus particles which remained in the seed after 

 inactivation at 37°C. were able to initiate a single cycle of virus growth, but 

 were prevented from initiating a second cycle by viral interference induced by 

 the inactivated virus in the seed; as mentioned earlier (Section VI, B), this 

 interference would not be established immediately and hence there would be 

 no inhibition until the first cycle of virus growth had been completed. In the 

 experiments of Burnet et al. (1954) the restriction to a single cycle of virus 

 production at high virus doses may also be due to a late induction of inter- 

 ference. 



