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replicas are made with carbon or other types of films. Whether the 

 particles are directly studied, or shadowed, or reproduced, they must 

 first be dried. (This necessitates that only nonliving specimens can be 

 studied.) The drying may introduce many artifacts. To reduce these, 

 several alternative schemes have been followed, including quick freezing 

 before drying; fixing in osmic acid; and replacing the water with 

 liquid C0 2 , and then going around the critical point. Although some 

 of these schemes have added to the detail observed, none of them have 

 dramatically altered the final images obtained with electron micro- 

 scopes. Because several different methods of specimen preparation all 

 lead to the same final images, this indicates that the electron-microscope 

 images do represent the original objects. 



To prepare tissues for electron microscope studies, it is necessary to 

 imbed the tissues in a suitable plastic and then section, before placing 

 them on the wire screens. The sections must be no more than 0. 1 micron 

 thick. These sections are so thin that it is possible to section not only 

 tissues, but also bacteria and red blood cells. The disadvantage of these 

 very thin sections is the large number of serial sections which must be 

 viewed in order to obtain a complete picture of a single cell, and the 

 still larger number to obtain a perspective view of tissue structure. In 

 spite of this limitation, electron microscopy is one of the most important 

 physical tools used in research in cytology. Figures 1 1 and 1 2 show 

 electron micrographs of biological specimens. 



REFERENCES 



1. Oster, Gerald, and A. W. Pollister, eds., Physical Techniques in Biological 

 Research. Vol. 1. Optical Techniques (New York: Academic Press, Inc., 

 1955). 



2. Mellors, R. C, Analytical Cytology: Methods for Studying Cellular Form and 

 Function 2nd ed. (New York: Blakiston Company, 1959). 



3. Palade, G. E., "Electron Microscopy of Mitochondria and Other Cyto- 

 plasmic Structures," Gaebler, O. H., ed., Enzymes: Units of Biological 

 Structure and Function (New York: Academic Press, Inc., 1956) pp. 185-215. 



4. Rayton, W. B., "Microscopes," Glasser, Otto, ed., Medical Physics (Chicago, 

 Illinois: Year Book Publishers, Inc., 1944) Vol. 1, pp. 733-750. 



