l6 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



J: THE STAINING OF FLAG ELL A 



The classical methods for the demonstration of flagella may be obtained 

 from any elementary text-book upon practical bacteriology. Flagella are 

 too small to be resolved by visible light although their presence can be 

 determined by dark-groimd illumination or phase-contrast microscopy. 

 Their staining depends upon the aggregation of soUd material upon their 

 surface, to increase their apparent size. These methods are o£ little or no 

 cytological value, and are not entirely to be relied upon, even for information 

 upon the presence or absence of flagella, or upon their arrangement, as they 

 have, in the past, given contradictory evidence upon these points. 



K: ELECTRON MICROSCOPY 



(4, 21, 30, 33, 34, 35, 49, 54, 55) 



The electron microscope suffers from two defects in its application to 

 biological materials ; the specimen to be examined must be completely 

 desiccated before introduction into the vacuum chamber, and the penetration 

 of the electron beam is so low that only the thinnest specimens can be properly 

 defined. For these reasons, until recently the most valuable contribution of 

 the electron microscope to bacterial cytology was in the study of flagella. 

 In the last few years, however, valuable studies have been made of cell envelopes 

 in disrupted bacteria, and even more promising has been the development of 

 techniques which enable ultra-thin sections o( bacteria to be cut and examined 

 by the electron microscope. The electron beam has also been put to a slightly 

 different purpose in the study of diffraction patterns produced by the cell walls 

 of bacteria and other micro-organisms. 



The scope of this section does not pernnt a detailed description of the 

 techniques of electron microscopy, upon which several complete books have 

 been written, but a number of examples are included among the illustrations 

 of the types of information which can be obtained from this source. 



At the same time, some comments upon the sectioning of bacteria may 



