152 ALICK ISAACS 



consideration of work on animal viruses the results should be judged on their 

 own merits. Clearly, there are great differences between the multiplication of 

 animal and bacterial viruses, particularly in the way they penetrate and are 

 released from their host cells. Nevertheless, such differences are quite 

 overshadowed by the many strong resemblances found in their methods of 

 multiplication. More particularly, in plant, bacterial, and animal viruses 

 more and more evidence is accumulating to show that the key role in multi- 

 plication is played by the viral nucleic acid. Some recent work from 

 Tubingen on animal viruses adds emphasis to this conclusion. Wecker and 

 Schafer (1957b) examined chick embryonic cells during the lag period, i.e., 

 30-180 minutes after infection with P 32 -labeled fowl plague virus. They found 

 that in extracts of these cells a high proportion of the radioactivity was not 

 sedimented under conditions when the virus elementary bodies would have 

 been sedimented. From such extracts a proportion of the radioactivity could 

 be precipitated by antiserum to the soluble antigen and part of the remainder 

 was shown to consist of viral nucleic acid by the fact that it was hydrolyzed 

 by ribonuclease. It seems, therefore, that after the virus particles enter the 

 cells many of them are broken down to give soluble antigen (a ribonucleo- 

 protein) and free nucleic acid, with phospholipid in addition. The further 

 course of infection in a similar system was studied by Breitenfeld and Schafer 

 (1957) by means of antibody, labeled with fluorescin isocyanate, to both 

 soluble antigen and hemagglutinin. This technique of localizing antigens 

 within cells was preferred to that of homogenization and differential centri- 

 fugation, since it was found that by the latter method viral constituents were 

 nonspecifically adsorbed to the various cell components. They found that 

 soluble antigen was first detected at 3 hours after infection and was localized 

 to the cell nuclei (cf. Liu, 1955). From 5 hours the fluorescence diffused slowly 

 out of the nucleus in the absence of obvious nuclear damage, and by 14 hours 

 the whole cell showed fluorescence. By contrast, the hemagglutinin was first 

 detected at the fourth hour throughout the cytoplasm, but the nucleus was 

 not stained. The hemagglutinin gradually moved toward the cell margin and 

 at the fourteenth hour the cells showed deeply fluorescing borders. This is the 

 first definite evidence that different viral constituents are synthesized in 

 different cellular sites. Thirdly, Wecker and Schafer (1957c) have now pro- 

 duced additional evidence that infection by eastern equine encephalitis virus 

 can be initiated by viral ribonucleic acid. They extracted infected mouse 

 brains with phenol and showed that these extracts were infective. Further- 

 more, in contrast to a preparation of virus with the same infective titer, the 

 infectivity of the phenolic extracts was sensitive to very small amounts of 

 ribonuclease (but not deoxyribonuclease); it was not sedimented significantly 

 with high centrifugal forces; and it was not inactivated on precipitation with 

 ethanol. There is, therefore, strong presumptive evidence that the infectivity 



