62 II, FUNDAMENTAL STRUCTURE OF PROTOPLASM 



as 1.5 per cent (27), whereas equine encephalitis virus contains so much 

 as 50 per cent (28). Usually a considerable quantity of cholesterol is 

 present in the lipids of vaccinia virus particles ; however, Hoagland et 

 al. (29) have shown that cholesterol can be removed from the particles 

 by extraction in the cold with ether without affecting virus activity. 

 According to our experiments, as above mentioned, inactivation occurred 

 if the viruses were treated with ether at a laboratory temperature, 

 whereas Harris succeeded in eliminating lipids at — 65"C. from des- 

 sicated rabies virus with petroleum ether and found that the virus be- 

 came stable by this procedure (30). 



It should naturally be expected that viruses become stable on the 

 lipid removal, since lipids may render the protein molecules easily 

 changeable ; when lipids are removed the protein molecules may be en- 

 abled to combine directly to form a stable structure. The remarkably 

 stable nature of crystalline viruses may be explained in this respect. A 

 virus may, however, become the more estranged from living matter, 

 the more the lipids are eliminated, since life can appear only on the 

 stage of instability. 



3. The Length of Crystalline Virus Particles 



Some plant viruses are isolated by ultracentrifugation in a form of 

 minute rod-shaped particles containing no lipids. Such particles are 

 not artefacts, because they can be demonstrated in infected leaves 

 themselves, and the sap itself shows the anisotropy of flow. From 

 electron micrographs the length of a particle of tobacco-mosaic virus 

 was estimated to be about 0.3/^, and the width about 15 vafx. Its "mo- 

 lecular weight" was calculated to be 40,000,000 (31). 



It is a noteworthy fact that the particles of various strains of 

 tobacco-mosaic virus are proved in electron micrographs to be similar 

 in both size and shape regardless of the presence of the profound 

 differences in their chemical composition. In discussing this fact Stanley 

 stated that despite the differences in composition some general directive 

 force appears to be effective during the construction of these virus 

 particles (32). Is it really necessary to postulate such a mysterious 

 being as a general directive force ? 



The majority of workers considered that there are striking differ- 

 ences between plant and animal viruses, since the plant viruses are iso- 

 lated in rod-shaped particles consisting of a protein containing no lipid, 

 whereas the latter viruses in particles having complex chemical composi- 

 tions containing lipids, never being isolated in rod-shaped particles 

 unlike the former. It is, however, known that some animal viruses can 



