122 ALICK ISAACS 



in vitro. After homogenizing, the membranes were treated with RDE and 

 3.6-9.4 % of the virus which had been absorbed was recovered. Hoyle and 

 Frisch-Niggemeyer (1955) studied the radioactivity and hemagglutinin 

 content of chorioallantoic membranes which had absorbed 480 agglutinating 

 doses of P 32 labeled influenza virus during a period of 1-2 hours' incubation. 

 In extracts of the membrane, prepared by freezing and thawing and treatment 

 with BDE, there was no demonstrable hemagglutinin, although a titer of 140 

 would have been expected on the basis of the radioactivity. The fact that the 

 radioactivity was not due to hemagglutinin masked by combination with 

 membrane inhibitor was shown by centrifuging the extract at 100,000^ for 

 3 hours, when the greater part of the radioactivity remained in the super- 

 natant. Hoyle and Frisch-Niggemeyer also observed that a high proportion 

 of the phosphorus label could be found in the "residual membrane," i.e., the 

 sedimentable brei from the original membrane extract. This fraction was 

 not apparently tested for its viral content, but Henle (1949) had shown 

 earlier that when extracts of infected chorioallantoic membrane were pre- 

 pared during the lag period and centrifuged lightly (2000 r.p.m. for 20 

 minutes) the bulk of the virus infectivity remained in the supernatant. 

 Isaacs and Lindenmann (1957) found that when heated influenza virus was 

 absorbed by chorioallantoic membranes in vitro and then the membranes 

 were washed, the interfering activity of the virus could not be detected in 

 extracts of the membrane before incubation. If such membranes were 

 incubated for 3 to 6 hours at 37°C. before extraction, interfering activity was 

 found in the membranes and was later secreted into the surrounding fluid, 

 but this interfering activity was not due to the heated virus but to a product of 

 the cell-virus interaction, which they called interferon. 



These examples show that during the lag period of influenza virus growth 

 there is a low recovery of hemagglutinin and of virus enzymatic activity 

 which closely parallels the low recovery of infective virus. The finding with 

 heated influenza virus that hemagglutinin, viral antigenic activity, and 

 interfering activity were lost suggests a fundamental alteration in the viral 

 particle on entering the cell; this point is discussed later. It is unfortunate 

 that only influenza viruses have been studied for their antigenic behavior dur- 

 ing the lag period, but the problem is technically more difficult with other 

 viruses. So far as they go, these results add emphasis to the low recoveries of 

 infective virus during the lag period. 



C. The Significance of Virus Recoverable during the Lag Period 



In most studies described above small amounts of virus could be extracted 

 from infected cells during the lag period, and we can now consider the 

 significance of this recoverable virus. The most important consideration in 

 relation to the question of an eclipse phase is whether the recoverable virus 



