XII. ELECTRON MICROSCOPY 411 



While this problem of distortion due to drying still remains the major 

 limitation of the electron microscopy of biological materials, there are 

 indications in recent work that the difficulty is not insurmountable. 

 For instance, in tissue culture preparations, Porter and others {9) 

 have greatly decreased the difficulties by careful fixation with osmic 

 acid vapor. In the case of bacteria, recent work has indicated that 

 the main reason that a large amount of shrinkage occurs on drjdng is 

 the fact that the organisms were previously removed from their 

 natural surroundings and suspended in distilled water. This, as 

 might be expected, appears to wash away one or more layers of mate- 

 rial that normally surround the cells, changes the concentration of salt 

 in the cells through dialysis, and probably causes some other change 

 due to osmotic effects. When organisms are grown directly on the 

 supporting membrane and fed only by diffusion through that mem- 

 brane it has been found that there is very little indication of artifacts 

 due to drying. In this case the only contact that the preparation 

 has with distilled water is during a brief interval in the transfer of 

 the collodion bearing the cells from the agar plate to the wire screen 

 (7). Since a plastic membrane separates the organisms from the 

 water, there is no disturbance of the organisms. The elTccts of sur- 

 face tension and the tendency of the cells and associated material to 

 go into suspension are completely eliminated. Any changes caused 

 by the contact with water must occur by diffusion through the 

 membrane — a relatively mild effect. 



It appears at the present time that there is no obvious funda- 

 mental reason why the geometrical structure or the morphology of 

 organic materials cannot be preserved through the drying process. 

 It is also quite apparent that considerable research must be carried 

 out to understand the action of drying and to determine the best 

 technique to be used. Certainly' the progress made since the careful 

 study of this problem was initiated (within the last five years) has 

 been very encouraging. 



Effect of Vacuum and Bombardment. The major distortion of 

 the specimen occurs on the initial drying in air. It has been well 

 established {8) that except in special cases there is no further change 

 due to placing the dried specimen in a high vacuum. It has also been 

 established that the bombardment by the electron beam also produces 

 no further change, except in extreme cases. It is known, for instance, 

 that if the specimen is subjected initially to an extremely rapid in- 

 crease in electronic bombardment it is possible for large magnitude 



