956 



THE ANIMAL VIRUSES 



size of the larger particles. The size of the smaller viruses may be overestimated 

 by ultra-violet microscopy, because of failure of perfect resolution (see Barnard 

 1937). Electron micrographs vary somewhat according to the mode of prepara- 

 tion of the film. Conclusions drawn from the rate of sedimentation in a gravita- 

 tional field are affected by the density of the virus particles, and density appears 

 to be variable and is difficult to determine with accuracy (see Smadel et al. 1938). 

 Nevertheless, the degree of concordance is sufficient to justify us in assigning 

 with some confidence mean particle diameters to the more important viruses. 

 In Table 59 the size of the commoner viruses and some of the bacteriophages is 



compared with Staphylococcus on 

 the one hand and the larger pro- 

 tein molecules on the other. 



The method of reproduction of 

 tlie filtrable viruses is still in doubt. 

 The evidence so far obtained seems 

 to favour binary fission. Eisenberg- 

 Merling (1943) has described a com- 

 ])lex life cycle for the vaccinia 

 \'irus, but his observations and the 

 conclusions he draws from them 

 must await confirmation. 



Microchemical analysis of vac- 

 cinial elementary bodies has re- 

 \ealed the presence of ash, carbo- 

 liydrate, fat, and nitrogen, a part 

 of which is undoubtedly in the 

 form of protein (Hughes et al. 1935). 

 Further observations by Hoagland, 

 Smadel and Rivers (1940) have led 

 to the identification of neutral fat, 

 phospholipin, reducing sugar after 

 hydrolysis, and thymonucleic acid. 

 McFarlane and his colleagues 

 (1939) believe that there is a shell 

 lipin arranged around the particle in two or three loose layers, but no real 

 membrane. These results suggest that the composition of the larger virus particles 

 is essentially similar to that of ordinary bacteria. The nature of the smaller 

 viruses is in greater doubt, Stanley's -work (see p. 966) indicating that they may 

 be no more, in fact, than macromolecules of protein. (For a general review of 

 the properties of vaccinial elementary bodies, see Smadel and Hoagland 1942.) 

 Habitat. — With the exceptions noted below, all the filtrable viruses at present 

 known are associated with living cells, whether in the animal or the vegetable 

 kingdom. This does not mean that they are never found apart from disease pro- 

 cesses, for their presence has been demonstrated in healthy carriers ; but it does 

 mean that they are essentially parasitic. The existence, however, of saprophytic 

 viruses is suggested by the work of Barnard (1935). Hitherto the only satisfactory 

 criterion of the presence of a virus has consisted in the production of characteristic 

 lesions in a susceptible animal by a suitably prepared filtrate — a technique, it may 

 be remarked, that automatically excludes the discovery of a saprophytic virus 



Fig. 2.3.3. — Electron Micrograph of Vaccinia 

 Virus Elementary Bodies (x 28,000). 



[From Green et al., 19-12.] 



