XII. ELECTRON MICROSCOPY 415 



consisting of a relatively dense head between 50 and 100 mju in diam- 

 eter, though it is not always spherical, and a tail, which is usually 

 60 to 100 m/x long. The fact that most bacteriophages had such a 

 structure could never have been proved definitely with the existing 

 indirect methods of investigation. With the electron microscope a 

 single direct examination of the particles in a purified suspension was 

 all that was necessary. On the other hand, the fact that the bac- 

 teriophage particles have such a characteristic morphology has made 

 them ideal subjects for electron microscope examination since they 

 can be identified wherever they occur. There seems to be no other 

 type of particle or material in their size range that has a similar 

 structure. 



The first examination of bacteriophage was made by H. Ruska 

 in 1940. However, he had rather poor preparations in which there 

 was a large amount of salt. The first clear observation of the bac- 

 teriophage was made by Luria and Anderson in 1942 {15). The 

 latter workers were also able to show the adsorption of the bacterio- 

 phage particles on the whole cell and to show the ultimate lysis of the 

 cell with the production of new bacteriophage particles {16) 



Since that time progress has been rather rapid. Bacteriophage 

 particles for staphylococci and actinomyces as well as a number of 

 different strains for Escherichia coli have been examined with the 

 electron microscope. A concentrated effort is being made to observe 

 the mode of growth of the bacteriophage particles in the bacterial cell. 

 The operation of the instrument is now better understood, with the 

 result that it is possible to adapt it to obtain much better images of 

 internal structures of the bacteria. In addition to this, the techniques 

 for preparing bacterial specimens have been improved, so that it is 

 now possible to examine the organisms at any definite time after 

 infection by the bacteriophage, and under such conditions that there 

 has been no disturbance of the specimen and that no material of 

 interest in the specimen is lost or displaced. (See Figures 6-8, made 

 in collaboration with Dr. Stuart Mudd, University of Pennsylvania.) 



At the time of writing, there appear to be no fundamental tech- 

 nical limitations making it impossible to observe the complete life 

 history of the bacteriophage particle in the bacterial cell, and, unless 

 some difficulty not anticipated should appear, there seems to be no 

 reason for not expecting this important problem to be solved in the 

 near future. 



