Virus Action and Replication 129 



levels (44, 45, 46) during the latent period. Another gradation of 

 the disruptive effect of a mukiplying virus on its host cell is found 

 in the case of the myxoviruses, influenza and Newcastle disease 

 virus. It has been shown clearly that, although cell death may be 

 the eventual outcome of encounters between these viruses and 

 HeLa cells (47), infected cells can definitely divide even after 

 production of viral protein has started (48). 



One of the most interesting and perhaps most important kinds 

 of relationship in animal virology is the tumor virus-cell inter- 

 action. Of the tumor viruses which have been adapted to study 

 in cell culture, two, polyoma virus and Rous sarcoma virus, have 

 been most useful in following the outcome of individual cells after 

 infection (49, 50, 51). The influence of infection with these viruses 

 on the overall synthetic capacities of cells is not yet known. The 

 DNA-containing" polyoma virus has been shown to have, initially, 

 a lytic eff"ect on mouse and hamster cells which, except for being 

 slower, is similar in general pattern to the action of a virus like 

 polio (52). 



Eventually, cultures which have been infected with polyoma 

 undergo a microscopic change and simultaneous biologic transition 

 to cultures which no longer produce virus (or do so at a very 

 low rate) but which have acquired the capacity to cause tumors 

 in animals. In an attempt to elucidate the role of the polyoma 

 virus in this transition, Vogt and Dulbecco have analyzed single 

 cells and have found that transformed cells which continue to 

 divide indefinitely do not produce virus and have an increased 

 resistance to infection by polyoma (53). They were unable to 

 obtain any evidence that the viral genome was present in the 

 cell (54). Thus, the vital question of whether the virus has inte- 

 grated with the cell to cause the change to a neoplastic unit or 

 whether the virus, by creating a selective pressure or by invoking 

 a developmental change, has aided in the establishing of a line 

 of cells with neoplastic capacity and increased virus resistance, 

 remains unanswered. 



The other tumor virus which has been extensively studied in 

 vitro, using quantitative techniques, is the Rous sarcoma virus. 

 Rubin and Temin have analyzed the tumor cell transition in 

 chicken fibroblasts brought about by this RNA-containing virus 



