Life and Protoplasm - 9 



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Fig. 1-4. An electronmicrograph showing a single bacterial cell (A) surrounded by 

 many particles of bacteriophage (B). The marker (1 micron) shows the extremely small 

 size of the bacterium (Escherichia co/i) and the still smaller size of the bacteriophage 

 particles (strain T.,). The bacteriophage destroys the bacterium when it penetrates the 

 cell. In the present case, however, the clustering of the bacteriophage particles at the 

 surface of the bacterium has resulted from drying the preparation— which must be done 

 before an electronmicrograph can be taken. These specimens were shadow-cast with gold. 

 (Courtesy of T. F. Anderson, University of Pennsylvania.) 



Isolation of a Virus. In 1935 one of the 



viruses, the tobacco mosaic virus, was iso- 

 lated and identified by W. M. Stanley, then 

 at the Rockefeller Institute in New York 

 City. To separate the virus from the many 

 inactive components present in the total juice 

 from an infected tobacco plant, two stages of 

 centrifuging were employed. First, an ordi- 

 nary low-speed centrifuge was used to remove 

 all larger particles, such as bacteria and other 

 microscopically visible bodies. This left a 

 perfectly clear supernatant fluid that retained 

 its infective potency in full strength. The 

 second centrifuge was of the high-speed type, 

 called an ultracentrifuge. Such a machine 

 can develop centrifugal forces of about half 

 a million times gravity. When a solution is 

 subjected to this force, the larger molecular 

 components tend to be thrown out of solu- 

 tion and to accumulate at the bottom of the 



test tube. In the present case, the sedimented 

 material proved to be the virus in a practi- 

 cally pure condition. In other words, this 

 sample of virus was practically free from con- 

 tamination by any inactive materials. After 

 further simple chemical treatment, the virus 

 was obtained in the form of crystals (Fig. 1-5) 



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Fig. 1-5. Crystals of tobacco mosaic virus (X 675). 

 Each crystal represents an aggregate of many virus 

 particles. (Courtesy of W. M. Stanley, University of 

 California.) 



