32 CYTOLOGICAL TECHNIQUE 



The properties of the selected fixatives are summarized on the 

 pages Hsted here : 



coagulant 



ethanol . . . . . . . p. 32 



mercuric chloride . . . . . p. 34 



chromium trioxide . . . . . p. 37 



non-coagulant 



formaldehyde . . . . . . p. 40 



osmium tetroxide . . . . . p. 45 



potassium dichromate . . . . p. 49 



acetic acid . . . . . . p. 52 



When added at the proper concentration to a solution of 

 albumin, coagulant fixatives separate the protein from the water 

 as a curd or coagulum (p. 20). This reaction serves to define them. 

 They transform the proteins of the cytoplasm (and often the 

 nuclear sap as well) into a sponge-work. This does not necessarily 

 destroy structure at the microscopical level, since the meshes of 

 the spongework may be very fine ; but non-coagulant fixatives are 

 nearly always used in electron-microscopy. The production of a 

 spongework is helpful to the penetration of embedding media, 

 especially paraffin. The coagulant fixatives are much less diverse 

 in their action on cells than the non-coagulants are. 



ETHANOL (ethyl alcohol) 



Standard concentration. Undiluted (absolute). 



Formula. C2H5OH. 



Description. A light, colourless fluid, miscible with water in all 

 proportions. 



Ionization. Not ionized. 



Oxidation-potential. Low (95% ethanol, 0-45 volt). 



Reactions with proteins. A non-additive or denaturing coagu- 

 lant of many proteins. It does not coagulate zein, gliadin, or 

 nucleoprotein. Ethanol is a powerful 'unmasking' agent for lipids ; 

 that is to say, it sets them free from combination with protein. It 

 acts in this way when used as a fixative^ ^ and also when used after 



