CHEMICAL STUDIES ON VIRUSES — STANLEY 367 



may exist today in nature without our knowledge. This possibility, 

 plus our knowledge that viruses can mutate, provide adequate reason 

 to pause for thought. 



The next virus, shown ai^No. 5 on plate 3, represents particles of the 

 To coli bacteriophage, an agent that has the ability to infect and bring 

 about the lysis or solution of certain bacterial cells. As can be seen, 

 the particles of this virus are sperm-shaped and, like vaccinia virus, 

 appear to be distinguished by a morphological differentiation which 

 can hardly be regarded as characteristic of molecules. 



The virus shown as No. 6 on plate 3 is of considerable interest 

 since it is responsible for tumors or papillomas in rabbits which, in 

 certain species, have been found invariably to progress and become 

 cancers. This is a small, spherical virus about 44 m/t in diameter. 

 Because the transition of the benign virus-induced tumor to the cancer 

 appears to be accompanied by the disappearance of the virus as an 

 infectious agent, this disease offers a most interesting experimental 

 approach to the cancer problem. 



The virus shown as No. 7 on plate 3 is that of southern bean mosaic, 

 and it is a crystallizable nucleoprotein having molecules 31 m/x in 

 diameter. 



The final virus (No. 8) shown on plate 3 is tomato bushy stunt, the 

 individual molecules of which are 30 m/t in diameter and the crystals 

 of which were shown on plate 1, figure 2. 



Now, as can be seen from the virus structure shown on plates 2 

 and 3, the viruses close the gap which formerly existed between the 

 organisms of the biologist and the molecules of the chemist. Although 

 the particles of any given virus appear the same, the particles of 

 different viruses vary greatly in size, in shape, and in composition. 

 Some are crystallizable nucleoproteins, and others consist of particles 

 containing protein, nucleic acid, lipid, and carbohydrate. Some par- 

 ticles have properties quite consistent with those of the molecules of 

 the chemist, whereas others have a degree of morphological organi- 

 zation apparently including in some cases a membrane, and this is 

 hardly consistent with the molecular world. These may, therefore, 

 represent borderline organisms. But all the viruses, regardless of 

 vast differences in structure, are tied together by virtue of the common 

 thread of virus activity. The fact that no virus has as yet been grown 

 in the absence of living cells must mean that they are not independent 

 metabolically but are dependent upon the metabolism of the host cell. 

 This in turn must mean that they possess the unique ability to enter 

 into the metabolic chain of events within cells and, hence, to guide 

 what a cell does. 



This unique ability of viruses to direct cellular metabolism repre- 

 sents the very heart of the virus problem and undoubtedly also carries 



