INTRODUCTION 79 



the basis for the Feulgen reaction for thymonucleic acid. Other 

 sugars will react with reduced fuchsin at higher temperatures, 

 but this procedure has not yet been studied in connection with 

 cytochemistry. There is a sharp distinction between the hy- 

 drolysis conditions which will liberate free aldehydes from acetal 

 and that required for splitting most glycosides— the acetals are 

 liberated by a brief hydrolysis with 0.1 N acid in the cold, whereas 

 most glycosides need much more vigorous treatment. The action 

 of 0.1 N hydrochloric acid which is sufficient to complete the 

 liberation of aldehyde from acetal has no significant effect in 

 liberating purine from thymonucleic acid. Consequently, the 

 two steps in hydrolysis give a clear-cut separation of these two 

 types of aldehyde. 



It is very important with all these techniques to avoid condi- 

 tions under which oxidation of a tissue component may occur, for 

 then adventitious aldehyde groups may appear in the tissue. 

 Protection against this may to a large degree be secured by using 

 a reducing substance such as formaldehyde as a component of 

 the fixative, by carrying out the steps prior to exposure to re- 

 duced fuchsin under anaerobic conditions as far as possible, and 

 by avoiding the presence of oxidising substances such as mer- 

 cury and dichromate in the fixative. There seems to be no ad- 

 vantage gained by using oxidising agents other than periodate 

 when it is wished to oxidise groups in tissue, Periodate is a fairly 

 specific oxidising agent for glycols and /^-hydroxy amines. Other 

 oxidising agents which have been used, such as mercury and per- 

 manganate, are not sufficiently specific in their action to be very 

 valuable in cytochemistry. 



In the examination of tissue sections for aldehydes we must, 

 of course, conform with the usual requirements of cytochemistry. 

 Thus it is necessary to demonstrate: 



1. That the aldehyde is present in fixed tissue in its physiologically 

 normal position. 



2. That the colour in the cytochemical reaction produced is indeed spe- 

 cific for aldehydes. 



3. That no significant degree of destruction of aldehyde occurs during 

 the procedure. 



4. That the colour developed in the cytochemical reaction remains in the 

 physiologically normal site of the aldehyde, and has not diffused to some 

 other part of the tissue. 



