REPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 563 



tlie preceding pages, arc fully compatible with it. Tliere is, in the first place, 

 a general finding of an eclipse of infectivity following infection. This may be 

 taken as an indication of a drastic change in the structure of the virus in 

 passing to the multiplying state. There is also ample cytochemical and 

 microscopic evidence for a series of stages, different for different groups of 

 viruses, tlirough which the virus materials must go before becoming organized 

 into mature virus particles. Often, the first virus materials to appear in an 

 infected cell are seen in electron micrographs as an undifferentiated matrix, 

 within which the typical virus particles are then formed by a stagewise 

 process of maturation (Gaylord and Melnick, 1953). The frequent intra- 

 nuclear or perinuclear location of these foci of virus production suggests that 

 some interaction with the host cell DNA may be required to initiate repro- 

 duction, even for viruses of the RNA grouj). It seems j)ossible that production 

 of an RNA-containing virus may require some genetic alteration involving 

 a change in the cellular DNA. 



With viruses of the psittacosis group, microscopic observations have 

 suggested that multiplication entails a binary fission of viral elements, 

 which differ morphologically from the mature virus particles (Sigel et al., 

 1951). Such a finding, if correct, would by no means be incompatible with 

 the hypothesis of a multiplying form of the virus distinct from the infectious 

 mature particle. The elementary act of virus multiplication must always be 

 a reduplication of the genetic elements of the virus. It is not surprising that 

 the reduplication process of vegetative (and possibly noninfectious) virus 

 elements may express itself in morphologically recognizable acts of binary 

 fission. Repeated reduplications of virus elements must miderlie the expon- 

 ential kinetics of virus production observed in some viral infections of 

 individual cells (Dulbecco and Vogt, 1953). 



For some insect viruses, a complex reproductive process has been postu- 

 lated on the basis of morphological studies (Bergold, 1953). Although the 

 basic mechanisms are still doubtful, the existence of separate phases of 

 replication and maturation seems very probable. 



V. Virus Multiplication, Cell Function, and 

 Cell Organization 



A. Restatement of the Dual Hypothesis 



The picture of virus multiplication outlined in the preceding sections has 

 a number of unifying features. In all cases, multiphcation appears to be 

 initiated by a genetic portion of the virus particles, which contains nucleic 

 acid and which is either noninfectious or, at least, less infectious than the 

 mature virus by the available tests. The production of new virus entails both 



