LIFE S FRINGES 



minimum requirement for something that could lay claim to being 

 designated as a living cell. For it must be realized that a further de- 

 crease in diameter causes a rapid decrease in the number of protein 

 molecules that can be accommodated in the particle. 



Owing to numerous excellent investigations the size of various pro- 

 teins and of many different viruses has lately been determined with 

 a fair degree of precision. With reference to proteins we owe this pri- 

 marily to the brilliant work of Svedberg and collaborators with the 

 ultracentrifuge they designed, and in which forces of up to a million 

 times gravity cause a sedimentation of dissolved protein molecules. 

 The sizes of virus particles have been determined by Bechhold, and 

 particularly by Elford and co-workers, who succeeded in imparting 

 a hitherto unachievable degree of reliability to filtration procedures 

 by preparing series of collodion filters with decreasing and very uni- 

 form pore sizes, the so-called gradocol filters. With a relatively simple 

 centrifugation apparatus Elford, during the past year, has moreover 

 managed to obtain results that are in very satisfactory agreement with 

 those of the earlier ultrafiltration method. 



Some of the most important data have been summarized in the 

 figure.* 



It shows first of all a striking continuity; from the diameter of the 

 smallest microscopically visible organisms to that of some of the most 

 common proteins we encounter all possible transition stages. Among 

 the viruses we find some whose size does not differ much from that of 

 microscopically observable bacteria, and, at the other extreme, those 

 whose diameter amounts to only 10-12 mio., as, for example, the foot- 

 and-mouth disease virus, or that of poliomyelitis. The fact is worthy of 

 attention that a particular protein, viz., hemocyanin, the respiratory 

 pigment of the snail, Helix, has a diameter twice that of these viruses, 

 so that its volume is greater by a factor of 8. It is remarkable that the 

 bacteriophages so far investigated also differ considerably in size. 



The principal point is, however, that size cannot be used as a basis 

 for sharply differentiating between ultramicrobes and other viruses; 

 Errera's line of demarcation at 100 m\x separates viruses that in many 

 respects show the most pronounced similarity. Conversely, it is not 

 surprising that the very small dimensions of many viruses have mark- 

 edly contributed to the attitude of numerous investigators who, al- 

 * See note page 331. 



337 



