BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 143 



characters of the cell surface although their serological behavior is that of 

 virus, since they are specifically agglutinated by viral antiserum. Valentine 

 and Isaacs (1957) observed in an electron microscopic study that, on digestion 

 with acid and trypsin, filaments largely disappeared, whereas trypsin- 

 resistant nucleoprotein rings remained after similar treatment of virus 

 spheres; Ada and Perry (1958) and Burke et al. (1958) found that filamentous 

 preparations of influenza virus contained much less ribonucleic acid than the 

 corresponding spherical forms. Taken together, these results point to filaments 

 having an infective "warhead" (Lindenmann, 1957) containing nucleic acid, 

 and a long tail containing noninfective viral hemagglutinin. 



Morphological studies of filaments in the course of formation and cut in 

 ultrathin sections show that the interior of the filament is continuous with the 

 cytoplasm of the cell (Morgan etal., 1956b), and that the surface of the filament 

 is continuous with the surface of the cell (Bang and Isaacs, 1957). The 

 conclusion suggested by these findings is that viral spheres forming near the 

 cell surface may drag out long filaments of the cell cytoplasm behind them; 

 on the basis of Cairns and Mason's work this process may occur during the 

 time that the virus is trying to free itself by its enzymatic action from 

 mucoprotein present at the cell surface. At some point, the filaments would 

 become nipped off at their base and so be formed of variable length. If this is 

 so, one must conclude that a large part of the cytoplasm near the surface of 

 the infected cell has acquired the specific hemagglutinating and serological 

 behavior of virus elementary bodies (see Section VII). It seems, too, that 

 highly filamentous strains of virus cause little damage to the cell surface. On 

 the other hand, as the virus becomes better adapted to a tissue, filaments 

 become much less common and it may be that the process of adaptation is 

 accompanied by an increasing ability of the virus to damage the cell surface, 

 thus preventing the formation of filaments. In this connection, Bang (1955) 

 has illustrated the different degrees of cell destruction produced by virulent 

 and avirulent forms of virus. Finally, one cannot but be struck by the almost 

 haphazard way in which influenza virus filaments and spheres appear to be 

 assembled together and released from the cells, a picture which contrasts 

 strikingly with the regular array of adenovirus particles demonstrated by 

 Morgan et al. (1956a) within the cell nucleus. 



VI. Incomplete Virus 



In the previous sections of this chapter different intracellular stages in the 

 normal cycle of viral development have been described. This final section 

 is devoted to the abnormal development of virus which occurs under certain 

 conditions, particularly when cells are heavily infected with virus, and which 

 results in the formation of what von Magnus called incomplete virus. There 



