Viruses 



395 



portant in bacterial filtration technics, these considerations also 

 determine the filterability of viruses. But in ultra-filtration o£ 

 viruses through collodion filters, called gradocol membranes, the 

 size of the pore is more important than it is with porcelain bacterial 

 filters. 



A fundamental contribution to virology was made in 1935 when 

 Wendell Stanley crystalized the virus of tobacco mosaic disease. 



Fig. 71. Crystals of tobacco mosaic virus. 

 American Journal of Botany, 19S7, 24, 59.) 



{From Stanley W. 



These plant virus crystals resemble those of inanimate inorganic 

 crystals, but when the virus is placed in a suitable medium, it 

 reproduces itself. This is further support for the concept that 

 viruses may represent that important link between what man calls 

 living and dead matter. Purification by crystallization is the result 

 of rather rigorous chemical treatment which cannot be applied to 

 animal viruses. 



While most microbiologists accept the idea that viruses are 

 living entities, since, for one thing, they can reproduce themselves, 

 there are still plenty of characteristics of these ultramicroscopic 

 forms which provide lively topics for debate on the living vs. the 



