BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 147 



virus. Since a low nucleic acid content appears to be the characteristic 

 deficiency of incomplete virus, it seems that multiple infection may strain 

 the cells' ability to synthesize viral nucleic acid, a hypothesis which might be 

 tested experimentally. 



The second factor which von Magnus (1951b) stressed was that the degree 

 of incompleteness increased with serial passage. In a most detailed series of 

 growth curves he studied the effect of serial passages of PR8 virus in the 

 undiluted form. In the standard virus used to initiate the passages the ratio 

 of infectivity : hemagglutinin titer (I/HA ratio) was about 10 6 . First passage 

 virus had an I /HA ratio of about 10 5 ; second passage, about 10 3 ; third 

 passage, between 10 1 and 10 2 ; and fourth passage, between 1 and 10. Since no 

 attempt was made to wash out the inoculum, the infectivity of the residual 

 inoculum may have accounted for a proportion of the infectivity found, so that 

 the I /HA ratios might have been even lower had the inoculum been removed 

 before the new yield of virus was liberated. This may explain the finding that, in 

 the de-embryonated egg, where the inoculum is removed, there is more incom- 

 plete virus produced in a single passage (Bernkopf, 1950), and the same is true 

 in the intact egg (Cairns and Edney, 1952). Another finding which von Magnus 

 (1951b) noted was that while the I /HA ratio declined on passage, the yield 

 of hemagglutinin was not greatly affected until the fourth passage, when it 

 was one-tenth (or less) of that produced in standard passages. As a result, 

 when a fifth passage was made with the same volume of infected fluid, the 

 number of particles present was considerably less than in earlier passages and, 

 as would be expected on the hypothesis that multiplicity of infection was a 

 critical factor, the I /HA ratio of fifth passage material again rose sharply. 

 The low yield of hemagglutinin in fourth passage material is presumably a 

 manifestation of viral interference, but since interference requires many hours 

 to become established (Fazekas de St. Groth et al., 1952), it is only apparent 

 in these experiments when the amount of infective virus in the inoculum is 

 very small. Experimentally, von Magnus (1954) has demonstrated that a 

 large inoculum of incomplete virus suppressed hemagglutinin production by 

 a large dose of challenge virus, provided a few hours' interval was allowed 

 before the challenge virus was inoculated; an 18-hour interval gave the highest 

 degree of interference. The serial passage experiments thus disclose a complex 

 situation. As a result of multiple infection of cells with standard virus 

 particles, the progeny has a low I /HA ratio and a low nucleic acid content, 

 and on serial passages this process is continued. Experimental addition of 

 about 10 6 ID 50 of standard virus along with incomplete virus did not signifi- 

 cantly influence the yield from the incomplete virus alone; large doses of 

 standard virus led to a slightly greater yield of virus, but with a lower I /HA 

 ratio than when the same dose of standard virus alone was given (von 

 Magnus, 1951c). Hence, when the incomplete virus particles infect a cell at the 



