SELECTED PAPERS 



ready long before Stanley's discovery, had denied these viruses the 

 quality of autonomous parasitic living organisms. 



But what are the consequences of this recognition? If one realizes that 

 viruses can be characterized only by virtue of the destructive effects 

 they exert on particular cells, and remembers that this effect is prima- 

 rily characterized by a multiplication of the destructive agent itself, the 

 denial of autonomous existence to this agent almost inevitably implies 

 that it must be derived from the host cell itself. In consequence many 

 investigators are inclined to regard the virus - and this applies partic- 

 ularly to the bacteriophage as well - as something produced by the 

 infested cell which has been designated as enzyme, metabolic disturb- 

 er, lethal factor, hereditary factor, or, simply, gene. 



It will immediately be sensed that even such a concept entails many 

 difficulties. The unlimited propagation at the expense of the living 

 cell, the consequence that these cells must always carry the agent, or, 

 at least, the potentiality to produce the agent of the specific disease by 

 which they are ruined, these make it hard to reconcile this concept 

 with the general experiences concerning the diseases involved. But it 

 will be evident that the basic principle common to all these ideas 

 would triumph if one were to succeed in unleashing the destructive 

 agent by a non-specific intervention in the life of the cell, and, further- 

 more, in demonstrating the unlimited transmissibility of this agent. 



Some very remarkable results have been obtained along these lines. 

 It is the great merit of the eminent virologist, Doerr, to have shown 

 long ago that it is possible by utterly different non-specific treatments 

 to induce at will in perfectly healthy humans a typical herpes rash, 

 and, as a corollary, to have proved that in the thus caused skin erup- 

 tions there is found a submicroscopic, filtrable agent with which the 

 disease can be perpetually transmitted to man and experimental an- 

 imals. A fully comparable situation has now been demonstrated to 

 exist beyond a doubt in the case of various malignant tumors. These 

 can be elicited in healthy animals by injecting them with tar or arse- 

 nite, and they can thereafter be transmitted at will by tissue filtrates. 

 Moreover, the literature contains several accounts of cases in which the 

 experimenter has succeeded in evoking bacteriophage-like phenomena 

 in entirely normal bacterial cultures by means of certain non-specific 

 procedures. In this connexion the recent observations of Kendall are 

 of importance ; he could induce phage formation in three different 



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