DYEING AND OTHER PROCESSES OF COLOURING 305 



The reduction of a metal in a salt or other compound to the 

 elementary state, with consequent formation of a black deposit, 

 is a familiar process in microtechnique. One must choose a metal 

 the ions of which take up electrons readily. This ensures easy re- 

 duction and stability after deposition. Silver and gold are suitable. 

 Tissue-constituents that can provide electrons will reduce their 

 salts : — 



Ag-^ -f e~ ► Ag n1^ 



Complex silver compounds are often used, especially argent- 

 ammine [Ag(NH3)2]OH. 



The chief substances in the tissues capable of producing micro- 

 scopically-visible deposits of metal are phenolic compounds, such 

 as those present in the granules of the Kultschitzky cells of the 

 intestinal crypts. 



Metallic reduction is very largely used in tracing the course of 

 the axons and dendrites of nerves, but a complex procedure is 

 here necessary, for reducers are not present in sufficient amount 

 to give microscopically visible deposits. It is uncertain what the 

 reducers are, but there is evidence that sulphydr}'l groups are 

 concerned. ^^^ The reducers, whatever they are, produce sub- 

 microscopic 'nuclei' of silver, similar to those present in an ex- 

 posed but undeveloped photographic plate. In order to make a 

 visible deposit, an enormously greater amount of silver must be 

 deposited round these nuclei. Luckily the nerve-fibres take up a 

 lot of unreduced silver, the ions of which associate themselves 

 with histidine and other amino-acid constituents of the proteins of 

 the fibre. ^^^ The double process of nucleus-formation and storage 

 of unreduced silver is called 'impregnation'. The impregnated 

 tissue remains transparent. When the tissue is transferred to a 

 photographic developer, the stored silver is reduced and deposited 

 round the nuclei. Thus the deposit is particulate. The particles can 

 be resolved by the electron-microscope.^^^ Together they form a 

 black mass that appears continuous under the light-microscope. 

 The term 'impregnation' should not be used unless the process of 

 silvering (or the deposit of some other metal) involves two separate 

 steps; the word is then applicable to first step only, during which 

 no visible deposit is made. 



The fundamental dissimilarity between silvering and dyeing 



does not need to be stressed. 



Impregnation methods have been much used in the study of 

 u 



