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one finds the numerous arguments that render it inadmissible to re- 

 gard the said agents as autonomous living entities, and of which I 

 mention only the extremely small dimensions, the specific gravity - 

 which, being about 1.25, lies much closer to that of a protein (1.35) 

 than of small bacterial cells (1.10) - the high resistance against typical 

 and universal cell poisons such as mercuric chloride, and the prolong- 

 ed preservation of these entities under conditions where multiplication 

 is virtually excluded; frequently viruses have been found to retain 

 their properties in a quantitatively undiminished degree even after 

 storage for periods of up to ten years. Furthermore, we must remember 

 the strong indications in favour of the non-specific genesis from normal 

 living cells which is also supported by the fact that some infectious 

 diseases must almost certainly be characterized as originating spon- 

 taneously. And finally the strongest argument of all, viz., Stanley's 

 proof that a virus can be isolated in crystalline form. In this manner 

 we can visualize the impasse of the ultramicrobiologists. 



Even though the way out cannot be sharply outlined, an attempt 

 may nevertheless be made to indicate in what direction it will have 

 to be sought. To me personally it is beyond a doubt that it will be 

 biochemistry that is apt to bring us nearer to a solution. Starting 

 with the viruses which, like the tobacco mosaic virus, can be obtained 

 in chemically pure form, this branch of science will permit us to char- 

 acterize more adequately the chemical nature and potentialities of 

 those extremely small particles that can exert such impressive and often 

 devastating effects. But primarily biochemistry must help us to ac- 

 quire an understanding of the manner in which the penetration of the 

 virus into a metabolizing cell causes the latter to shift its normal syn- 

 thetic functions in the direction of synthesizing a virus as a by-product. 



Concerning the first point of this programme it can be stated that the 

 initial steps have already been taken, principally by Stanley and col- 

 laborators. It is obvious that for this kind of work everything depends 

 on the certainty that the isolated crystals actually represent the virus 

 proper. I have already mentioned Stanley's primary proof for. this 

 contention, viz., the repeated recrystallization, now successfully ac- 

 complished fifteen times, with full retention of activity. Nevertheless, 

 I am convinced that there are many among my audience who are not 

 prepared to accept without more ado an identification on this basis. 

 Permit me therefore to mention some further results obtained by 



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