THE STRUCTURE OF VIRUSES 



133 



dogenous manner, it should be emphasized 

 that it is recognized today that virus 

 activity results only from the introduction 

 of virus from without. There is, however, 

 a strong and growing tendency to consider 

 that viruses or similar factors may, upon 

 provocation of the cell, originate endog- 

 enously and give rise to tumors or can- 

 cers. Whether such viruses are actually 

 derived from normal cell constituents or 

 are formed by the mutation of a " masked ' ' 

 virus which is normally or usually carried 

 within the cell, but which in reality is 

 alien to the cell, is not known. The prob- 

 ability that the synthesis of viruses does 

 not differ fundamentally from the syn- 

 thesis of normal proteins within cells, 

 and the fact that, although viruses may be 

 removed and studied apart, they neverthe- 

 less possess most of the properties of liv- 

 ing organisms, cause the attack on the 

 nature and mode of action of viruses to 

 become of signal importance, for it is in 

 reality an attack not only on the problems 

 of abnormal and normal metabolism, but 

 also on the nature of life. The difficulties, 

 discussed by Bohr, which are involved in 

 making studies of the living state without 

 affecting conditions by the very act of 

 study, are lessened in the case of viruses, 

 since they may be removed from cells, sub- 

 jected to various studies, and then rein- 

 troduced into cells without measurably 

 affecting them. This property of sus- 

 pended animation is unique with viruses 

 and is possessed by no other entity to a 

 similar degree, for the same difference that 

 exists between viruses and ordinary living 

 organisms must exist between viruses and 

 the seeds or spores that are usually cited 

 as examples of suspended animation. This 

 and other preceding statements are neces- 

 sarily limited by one's understanding of 

 the nature of the difference between viruses 

 and living organisms, hence this question 

 will be considered. 



The chemist, after a perusal of the prop- 

 erties of the purified viruses that have 

 been isolated and adequately studied, has 

 no difficulty in coming to the conclusion 

 that they are protein molecules. They 



merit the term molecule because, despite 

 many attempts, it has not been found pos- 

 sible to sub-divide the large units without 

 the loss of virus activity. The chemical 

 and physical properties are admittedly 

 those to be expected of large protein mole- 

 cules, and the chemist, well acquainted 

 with the catalytic synthesis of a peptide 

 bond and the autocatalytic formation in 

 vitro of the pepsin and try spin proteins, 

 views virus activity as but a logical and 

 not unexpected extension of the expression 

 of chemical structure. True, the duplica- 

 tion of conditions necessary for the ex- 

 pression of virus activity in vitro has not 

 yet been accomplished, and but little has 

 been done with the large and complex 

 viruses such as vaccine virus, but the 

 chemist, secure in the knowledge already 

 gained, considers that such problems lend 

 themselves to experimentation and looks 

 forward with confidence to their eventual 

 solution. 



On the other hand, the pathologist and 

 biologist, long interested in the expression 

 of virus activity and its results, have con- 

 sidered viruses to be small living organ- 

 isms. Recently, however, there has been a 

 tendency on the part of some of these 

 workers to accept the view that some or all 

 viruses may be something other than living- 

 organisms. Rivers has suggested that the 

 large viruses may be small living organ- 

 isms, the middle-sized viruses representa- 

 tives of an unknown form of life, and the 

 small viruses non-living agents, but that it 

 is impossible to draw lines dividing the 

 groups and that very probably one group 

 shades off into its neighbors. Rivers ap- 

 pears to believe, therefore, that although 

 the large differ from the small viruses, 

 they nevertheless form a continuum from 

 small non-living to large living viruses. 

 Green and others have suggested that 

 viruses are simplified fragments of living 

 protoplasm which arise from organisms by 

 a process of retrograde evolution under 

 parasitism, involving loss of function and 

 associated substance, and that this process 

 may result in forms varying from a single 

 colloidal molecule to entities almost indis- 



