Organic Substances 91 



trations are of little importance. Small pieces of tissue are 

 fixed directly in this fluid. In case of intravascular injection it 

 is advisable to perfuse the tissue with an isotonic (about 5 

 per cent) solution of glucose first, because otherwise silver 

 chloride and silver protein precipitates may clog the vessels. 

 The reagent is allowed to act for not much longer than neces- 

 sary for complete penetration of the tissue (5-20 minutes, 

 depending on its size ) ; the excess of silver is removed by re- 

 peated changes of 1 per cent Na thiosulfate, and the latter 

 by distilled water. The tissue may be frozen-cut or dehy- 

 drated and embedded. A black granular precipitate indicates 

 the presence of ascorbic acid. Unless exposure to the silver 

 solution is unduly prolonged (in which case urates may re- 

 act), false positive reactions need not be feared. 



Gold chloride^^^ can be used instead of silver nitrate; the 

 results are claimed to be identical. 



To the writer's knowledge, the demonstration of ascorbic 

 acid has not been tried in sections of frozen-dried material. 

 It is not even certain that the reduction of silver nitrate is 

 prompt enough for a sharp localization. However, in attempts 

 at using frozen-dried tissues it should be borne in mind tliat 

 ascorbic acid is soluble in alcohol; therefore, the reagent 

 should be applied to deparaJBfinized, air-dried sections. 



B. LIPIDS 



"Lipids" (synonyms: "lipoids," "fatty substances") is a 

 term which will be used to denote a large group of miscel- 

 laneous chemical substances classified together, for histo- 

 chemical purposes only, by their solubility properties. These 

 properties are insolubihty in water and solubiUty in several 

 or all of the so-called "fat solvents" (alcohol, ether, chloro- 

 form, benzene, pyridine, acetone, etc.). 



The histochemistry of hpids has been reviewed very thor- 

 oughly by Cain,^ and the reader interested in the topic is 

 urged to consult this excellent paper for detailed inf onnation. 



An immense variety of lipid substances occurs in the ani- 



1. Cain, A. J.: Biol. Rev. Cambridge Phil. Soc, 25:73, 1950. 



