ANIMAL VIRUSES: A COMPARATIVE SURVEY 11 



helpful when technical methods become available for a comparative func- 

 tional study in insects of the viruses which are potential insect-pathogens 

 (see Chapter XV) and the arborviruses. This is perhaps the most promising 

 untilled field still available to the virologist. 



The myxoviruses seem to be characteristic parasites of mucous membranes 

 and their possession of a highly active neuraminidase is suggestive of a distant 

 bacterial origin. The absence of such enzymes in vertebrate tissues makes it 

 unlikely that it could have been evolved in any other fashion. 



V. Is There Any Functional Unity among the Animal Viruses? 



This survey has, in a sense, already answered this question by demon- 

 strating that there are at least six successful patterns of animal virus struc- 

 ture; the body of the work will make it clear that for none of the three proto- 

 types chosen from these have we more than a crude intellectual model of the 

 process of virus reproduction within the cell. There is, however, one extremely 

 important feature common to all the viruses — that they are parasites of 

 mammalian or avian cells. Further, two types of cell, the chorionic epithelium 

 of the chick embryo and the HeLa line of human cells in tissue culture, will, 

 between them, allow examples of all groups to multiply freely. All the viruses 

 seem, therefore, to have been adapted by evolution to make use of the 

 standard structural and metabolic outfit of the vertebrate cell for their 

 effective reproduction. Differences in the susceptibility of different cells are 

 probably to be related mainly to specialized differentiations appearing in the 

 course of development. It is highly significant that embryonic cells and 

 dedifferentiated (malignant or pure line tissue culture) cells are more widely 

 susceptible to allow free multiplication of, and cytological damage by, 

 viruses. 



This is the mam justification for a preliminary and very provisional 

 attempt to provide an orientating discussion of the relationships of the main 

 virus types to the mechanism of the standard, undifferentiated, vertebrate 

 cell. As a starting point it is probably best to adopt Hotchkiss's (1957) sug- 

 gestion that, although the invading virus necessarily makes use of the whole 

 metabolic machinery of the cell, it actively interferes with and deviates the 

 metabolic sequence only at one restricted region of the sequence. The region 

 of interference may differ from one virus to another and, in all probability, 

 is closely correlated with the differences between the major groups of viruses. 

 In a search for the nature of the interference that leads a cell to synthesize 

 virus material instead of its own structure, we are virtually obliged to use as 

 a guiding thread the nature of the viral nucleic acids. The content of the virus 

 particles in nucleic acid is tabulated in Table I for the six groups we have 



