TECHNIQUE I7 



be of value. In the opinion of the author, although the technical problems 

 of obtaining electron-transparent sections have been solved, that of embedding 

 the material and fixing or otherv^ise treating it so as to obtain reasonable 

 contrast between the internal structures, without gravely compromising the 

 validity of the appearances to be interpreted, has not been solved. The use 

 o{ strong solutions of heavy metals (nearly all of which are active protein 

 precipitants) as " stains " to increase contrast, is especially open to criticism, 

 if the appearance o( such tiny structures is to be taken at its face value. 



Sections of bacteria prepared in this laboratory and accorded minimal 

 treatment show very poor internal contrast, but the form of such nuclear 

 structures as are visible accords quite closely with what can be seen in stained 

 preparations. This is not the case with osmium-treated material. 



Any mechanically sound microtome can be adapted, by gearing-down, to 

 thin sectioning. Methacrylate resin is widely employed for embedding 

 purposes, but polystyrenes have been used with success in this laboratory, and 

 a very wide range of comparable materials is available. 



L: PHASE-CONTRAST MICROSCOPY 



(15, 23, 60, 61, 62, 63, 64) 



By the use of the phase-contrast microscope it has been possible to confirm 

 upon living bacteria the main outlines of the nuclear cycle observable in 

 fixed, stained material. In such untreated material, however, the clarity of 

 the observations leaves much to be desired, and little or no information is 

 obtainable upon those structures which do not differ from their surroundings 

 in refractive index. 



A revolution in the use of the phase-contrast microscope, comparable 

 with the introduction of specific staining methods in classical microscopy, has 

 resulted from the brilliant work o£ Tomcsik, who, by the use of enzymes, 

 antibodies and other proteins has specifically demonstrated a variety of 

 chemically definable materials and structures in the bacterial cell, and has 

 revealed an entirely unsuspected complexity of structural detail in the capsule. 



The most striking of Tomcsik's methods, which is possibly the greatest 

 single advance in cytochcmical technique in the last half-century, and which 

 has potential apphcations in all biological fields, consists in the preparation of 



