SIZE, SHAPE, AND HYDRATION OF VIRUSES 17 



fact that the virus has to be studied in a high vacuum where it 

 loses a great many of its structural features as it loses its un- 

 bound water of hydration. Thus it is quite possible that many 

 characteristic features of viruses as seen in electron micrographs 

 are actually produced in the drying jirocess and are not real. 



Also, it has been pointed out by Anderson (1950) that in 

 drying a preparation the virus is exposed to strong surface 

 forces. He has overcome this by the ingenious method of sus- 

 pending the object to be viewed in liquid CO2 or NoO (which 

 unfortunately rec(uires very high pressure) and then raising the 

 tenq^erature past the critical point, thus avoiding exposing the 

 virus to passage through a meniscus. The N2O is then allowed to 

 escape, and the Formvar membrane is recovered from the bottle 

 and used for microscopic observation. Before this can be done, 

 water must be replaced by a series of other solvents. Viewed in 

 this way, preparations do seem different, notably they are less 

 flattened down. 



Another disadvantage of high-magnification microscopy is 

 the high concentration needed to guarantee that even one par- 

 ticle will appear in the field. About 10^^ particles /cm^ are neces- 

 sary, so that one useful adjunct of virus microscopy is a high- 

 speed centrifuge for concentration. This process, in turn, may 

 alter the characteristics of the virus. 



Many viruses, notably vaccinia and some phages, show 

 definite signs of internal structure. These represent a first step in 

 establishing the internal structure of viruses, a subject which 

 is just starting. Correlation between these structural units and 

 virus functit)n is most desirable but has not yet been successful. 



Viruses of very characteristic shape, like tobacco mosaic 

 virus, can readily be counted with the electron microscope. 

 This secondary use is very valuable in establishing growth 

 curves for plant viruses. 



Electron microscopy, in the hands of exjjcrts, can give con- 

 siderable information about details of structure. For example, 

 Williams (195'2) has subjected tobacco mosaic virus to sonic 

 disturbance, thus breaking the virus rods into shorter sections 

 which can occasionally stand on end. When these are shadowed 



