COAGULANT PRIMARY FIXATIVES 33 



the tissue has been fixed by a substance that does not itself 

 unmask. ^^ 



Reactions with nucleic acids. Precipitates but does not fix them. 



Reactions with lipids. These are not chemically changed by 

 ethanol, which tends rather to dissolve them than to fix them. 

 This tendency, however, is weaker than is commonly supposed. 

 Tripalmitin and tristearin are insoluble in cold ethanol, and so are 

 several other common lipids of the tissues. Triolein, lecithin, and 

 oleic acid are soluble; other fatty acids and cholesterol slightly so. 



Reactions with carbohydrates. Precipitates glycogen without 

 fixing it. 



Rate of penetration. Moderate. 



Shrinkage or swelling. Shrinks excessively. 



Hardening. Hardens excessively. 



Method of washing out. Since ethanol is miscible in all pro- 

 portions with the antemedia (p. 73) used in embedding (and also 

 with water), and since it has no tendency to form an extrinsic 

 artifact, no special washing out is necessary. 



Effect on the appearance of cells in microscopical preparations. 

 It produces a coarse coagulum in cytoplasm and nucleus, and 

 destroys mitochondria. Lipid droplets tend to fuse and may 

 dissolve. 



In paraffin sections, cell aggregates are shrunken apart from 

 one another; cytoplasm is often piled up against the cell- 

 membrane on the side opposite to that from which the fixative 

 penetrated. Lipid droplets are not present; chromosomes are not 

 distinctly seen. 



Most fixatives render tissues more basic or more acidic than 

 they were in the unfixed state, because they react with and block 

 the acidic or basic side-groups of certain amino-acids. As a result, 

 the affinity of the protein for basic and acid dyes is changed 

 (p. 90). Ethanol is exceptional in this respect. Since it does not 

 attack any of the side-groups, it leaves proteins neither more basic 

 nor more acidic than they were before fixation, and dyes may 

 therefore be used to find whether basic or acidic side-groups pre- 

 dominated in the living tissues. Nucleoprotein remains strongly 

 colourable by basic dyes, but it is not stabilized in position in the 



D 



