XII. ELECTRON MICROSCOPY 413 



to describe only the application of the electron microscope to the 

 virus problem, since it is in this problem that the most widespread 

 intensive studies have been made and a definite measure of success 

 has been obtained. Nevertheless this section would be very incom- 

 plete if some of the other resenicli piojocts undertaken were not men- 

 tioned. 



The stud.y of animal and insect tissues has been initiated and in 

 the case of muscle and other protein fibers the work of Schmitt and 

 co-workers (^0) has been particularly noteworth3^ Porter (9) on 

 the other hand, has started the study of the structure of animal cells 

 by means of a tissue culture technique. Nothing has as yet been ac- 

 complished in the study of histological sections because the necessary 

 techniques are still in the early stages of development. 



In bacteriology the electron microscope has been used extensively. 

 However, a large pai't of the (^arly woik was purely exploratory in 

 nature and as a result there was very little, if any, organization of 

 the results. Mudd and Anderson ( / / ) with a number of collaborators 

 have made an initial surv(\v of the morphological characteristics of 

 different types of bacteria, which is the most complete at the present 

 time. CI. Knaysi and co-woi-kers, and more recently ('. l^obinow, 

 have undertaken a more careful study of the cytology of bacteria. 

 New prepaiation techniques developed recently by the author and 

 his co-workers are expected to greatly facilitate the latter work by 

 the elimination of many of the artifacts that troubled the early work- 

 ers. At the present time the future use of the electron microscope 

 in bacteriology shows great promise. 



The Virus Problem 



As soon as the electron microscope had reached the point where 

 it was established that high magnification images could be obtained, 

 it, was applied to the examination of various types of viruses. The 

 main part of the early work was concerned with the verification of 

 the dimensions of the particles as obtained by statistical measure- 

 ments, i.e., by electrophoresis, diffusion, and ultracentrifugation. At 

 this point, its most important contribution was the accurate demon- 

 stration of the shapes of the viruses. While the indirect methods, 

 mcluding in particular flow birefiingence measurements, have been 

 able to differentiate qualitatively between rodlike and spherical par- 

 ticles and provide some estimate as to ratio of width to length, it 



