412 JAMES HILLIER 



changes to occur in the specimen, presumably due to a release of 

 bound water that occurs at high temperature and hence, simply 

 explodes the specimen by steam pressure. A slow increase of elec- 

 tronic bombardment on the same specimen, however, will seldom 

 show any effect until extreme intensities are reached. Even in this 

 case, the change usually is one of slight shrinkage accompanied by a 

 slight increase of the transparency of the specimen to the electron 

 beam. It appears that in this case the bound water is released suf- 

 ficiently slowly so that there is no rupturing of the specimen. 



Generally speaking, extremely high intensity bombardment of 

 biological specimens is to be avoided for a number of other reasons 

 including the rupturing of the supporting membrane and the sub- 

 limation of salts and possibly other materials in the specimen. 



Thickness of the Specimen. One of the factors restricting the 

 range of specimens that can be examined with the electron micro- 

 scope is the thinness required. In the case of examination of tissue 

 .the problem of obtaining cut sections 0.2 /x in thickness is a difficult 

 one. First there is the problem of actually cutting such a section. 

 A reasonably satisfactory solution appears to have been supplied by 

 Pease and Baker (18), as already mentioned. The problem of ade- 

 quate fixation still remains. However, with the means for cutting 

 the sections and the means for observing them at hand, workers in- 

 terested in this problem will be well equipped to discover a solution. 



E. OUTSTANDING ACCOMPLISHMENTS OF THE 

 ELECTRON MICROSCOPE 



In introducing the electron microscope to their fellow scientists, 

 it was natural that the early workers, including the writer, should 

 speak and write enthusiastically of its research possibilities, even 

 though many of the statements were based more on intuition and faith 

 than on available scientific evidence. That less than ten years later 

 the selection of the most important contribution of that instrument 

 has become a difficult task is probably the most outstanding aspect 

 of its accomplishments. Future possibilities estimated on the basis 

 of present work make the predictions of the early workers appear 

 conservative in the extreme. 



In discussing the accomplishments of (lie electron microscope in 

 biology, a number of different researches could be described in con- 

 siderable detail. In the present work, however, it seems desirable 



