W. M. STANLEY 



can multiply in a single cell if the viruses are sufficiently distinct in 

 their requirements. These and similar results have permitted many 

 stimulating inferences regarding the mode of virus reproduction and 

 have suggested innumerable approaches for future experimentation. 



Work on the changing of the chemical structure of a virus by 

 means of known chemical reactions has been both encouraging and dis- 

 couraging. It has, in fact, proved possible actually to change the 

 chemical structure of a virus; but so far no change has been found to 

 be perpetuated in the virus particles produced as a result of infection 

 of a host with the altered structures. Thus, the abolishment of the 

 sulfhydryl groups in tobacco mosaic virus or the introduction into the 

 structure of this virus of several thousand acetyl, phenylureido, carbo- 

 benzoxy, benzene sulfonyl, or malonyl groupings yields diverse altered 

 virus structures. Although these are infectious, the disease which they 

 produce is the ordinary tobacco mosaic disease, and is accompanied 

 by the production of particles, not of the respective altered structures, 

 but of ordinary tobacco mosaic virus. However, encouraging results 

 were obtained in a study of the specific virus activity of these chemical 

 derivatives on different hosts. It was found that a property of the 

 virus, which perhaps can best be described as virulence, can remain 

 constant for one host but be modified with respect to a different host, 

 upon formation of a given chemical derivative of the virus. This 

 result lends encouragement to the belief that, eventually, heritable 

 structural changes in a virus will be achieved in the chemical labora- 

 tory by means of known chemical reactions. Contemplation of the 

 implications that would accompany the actual accomplishment of 

 this feat tends to stagger the imagination. 



Spectacular progress has attended studies on the nature of the 

 differences in chemical structure that are responsible for the existence 

 of virus strains. It was indeed fortunate that Nature provided, and 

 the pathologist recognized and separated, two or more strains of each 

 of several different viruses. Strains of a virus appear to arise during 

 the reproduction of a virus by a process which can be regarded as 

 similar to that of gene mutation. It can be presumed that the strains 

 of a virus have arisen from some parent strain, one by one, during the 

 course of the years. Each strain thus probably bears a definite rela- 

 tionship to the parent strain and to each of the similar strains. Each 

 strain causes a more or less different disease; because of this fact it 



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