BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 115 



Sanders (1953) found that when mice were inoculated in the tongue with 

 the GD VII strain of encephalomyocarditis (EMC) virus, spread of virus 

 occurred along the nerve to the hypoglossal nucleus. If this nucleus was 

 removed 48 hours after inoculating the tongue, a suspension prepared from 

 frozen and thawed nuclei contained no detectable infective virus, whereas 

 suspensions prepared by incubating intact nuclei in vitro at 37°C. developed 

 3000 LD 50 of virus per milligram of tissue. In these experiments it was not 

 possible to say how much virus was initially present in the intact nuclei 

 before incubation. Recently, Sanders et al. (1958) have studied the growth 

 of EMC virus in agitated cultures of Krebs 2 mouse carcinoma cells under 

 conditions where the cells remain separate, the virus being titrated by a 

 plaque technique. At a multiplicity of infection of 0.1, about 95 % of the 

 virus is adsorbed to the cells within 15 minutes. During the lag period the 

 amount of virus recoverable by breaking up the cells with glass beads was 10 4 

 plaque-forming units per milliliter, whereas the number of infective centers, 

 determined by direct plating of intact cells, was 10 6 per milliliter. Hence, the 

 recovery of virus during the lag period was 1 % of the number of infective 

 centers. 



As far as has been investigated, therefore, small animal viruses behave like 

 bacteriophages in showing a low recovery of infective viruses during the lag 

 period. 



3. Medium-Sized Viruses 



Hoyle (1948) noted that during the lag period there was a low recovery of 

 influenza virus from an infected chick chorioallantoic membrane. Henle 

 (1949) carried out much more detailed experiments on similar lines; he found 

 that after injecting various doses of influenza virus into the chick allantoic 

 cavity about 30 % of the virus infectivity of the inoculum could be recovered 

 in the allantoic fluid during the lag period. Henle interpreted this to mean that 

 70 % of the inoculum was taken up by the cells, but in view of later studies, 

 which showed that the infectivity of influenza virus is reduced during incuba- 

 tion at 37°C. at an appreciable rate (Horsfall, 1954; Paucker and Henle, 1955a), 

 conclusions based on the amount of the more stable virus hemagglutinin 

 taken up by the cells are probably more reliable. Cairns and Edney (1952) 

 reported that over a wide range of virus dosage about 50 % of influenza 

 virus hemagglutinin was taken up by the allantoic cells during a 4| hour 

 incubation period. Hoyle and Frisch-Niggemeyer (1955) arrived at a very 

 similar figure for virus labeled with P 32 by studying the residual radioactivity 

 in the allantoic fluid after allowing a li hour period of absorption. Horsfall 

 (1954), on the other hand, found an exponential decline in the titer of 

 unadsorbed virus with time. When Henle's (1949) figures are calculated on 

 the conservative basis that 50 % of the seed virus is taken up by the cells, it 



