12 INTRODUCTION 



ation, and precipitates silver in a mixing zone at the surface 

 of the mitochondria. 



Techniques for Cytochemical Demonstration of Enzymes 

 These techniques usually involve carrying out an enzyme 

 reaction with the intrinsic enzymes of a tissue section or smear, 

 etc., under such conditions that one of the products of the re- 

 action is precipitated. Before results of this type can be evalu- 

 ated, it is necessary to have answers to a number of questions. 

 These are: (a) How much of the enzyme is destroyed before 

 and during the cytochemical procedure? If destruction occurs, 

 does it occur to an equal degree at all sites or does it occur 

 selectively at certain sites? (b) Is the insoluble reaction prod- 

 uct precipitated at the actual site of enzyme reaction, or is it 

 precipitated at sites which have a very high affinity for the 

 reaction product? (c) Is the enzyme in the specimen used for 

 cytochemical study in its physiological position in the material, 

 or has it been redistributed by diffusion processes? Studies 

 which do not provide answers to these questions must be dis- 

 carded. It is, no doubt, true that some of the methods now used 

 to study the distribution of enzymes do, in fact, demonstrate the 

 correct site of the enzyme ; it is equally true that there are other 

 instances in which the information provided is greatly mis- 

 leading. 



The Feulgen Technique for Nucleic Acid 



In this technique tissues are heated at 60° with normal hydro- 

 chloric acid for 5 minutes or more, the precise time varying with 

 the fixative which is used and the nature of the specimen. This 

 procedure splits off purine from the deoxy sugar nucleic acid, 

 so that, when the material is subsequently treated with reduced 

 fuchsin, a violet Schiff's base is formed. The procedure of hy- 

 drolysis, however, tends also to make nucleic acids diffusible, 

 probably by reducing the molecular weight, as was suggested by 

 Stedman and Stedman. That this was likely to be the case has 

 been implicit in results known for many years. For example, 

 Bauer showed that, when a fixative like formaldehyde is used, 

 or acetic alcohol, the intensity of the reaction reaches a peak 

 after, say, 5 or 10 minutes of hydrolysis. Further hydrolysis 



