CHAPTER XXXIV. 471 



Embedding is not necessary, but in many cases desirable. It can 

 be easily carried out by washing pieces in many changes of alcohol 

 of ascending strengths and embedding them in celloidin. Sections, 

 however made, must be repeatedly washed with distilled water, 

 otherwise they will soon be spoilt by the formation of opaque 

 granules and needle-like crystals which very much hinder proper 

 observation. After dehydrating, sections can be passed through 

 creosote and turpentine and mounted, preferably without a cover- 

 glass, in dammar or balsam. 



It is, however, preferable to treat sections by the following 

 fixing-and-toning process which was suggested by Golgi for trans- 

 forming the whitish mercury impregnation (to which the reaction 

 is due) into a full-black stain, much more suitable for observation 

 under high power. Moreover, the process helps in preventing the 

 formation of opaque precipitates, and allows of mounting in the 

 usual way without any danger of spoiling the specimens. 



One proceeds thus : Sections of pieces embedded in celloidin are 

 thoroughly washed in many changes of water, and then transferred 

 for a few minutes into a photographic fixing and toning bath to be 

 prepared at the moment of using, as follows :- 



Solution A. 



Distilled water ..... 1,000 c.c. 



Sodium hyposulphite .... 155 gr. 



Potassium alum . . . . . 20 ,, 



Ammonium thiocyanate . . . 10 



fSodium chloride . . . . . 40 ,, 

 Allow to stand for eight days and then filter. 



Solution B. 



Gold chloride . . . . . 1 gr. 



Distilled water . . . . . 100 c.c. 



For use take 50 c.c. of sol. A, 7 c.c. of sol. B, and 

 40 c.c. of old combined bath. 



From the fixing and toning bath sections are transferred into 

 distilled water and again thoroughly washed ; they are then slightly 

 counterstained with an acid solution of carmine diluted with some 

 alcohol, dehydrated, cleared, and mounted in the usual way. 



The elements stained by the method are : (1) Nerve cells with 

 all their processes and ramifications. (2) Nuclei, which is not the 

 case with the silver process. (3) Neuroglia cells. But the reaction 

 in this case is far less precise and complete than that obtained by 

 the silver method. (4) Blood vessels, and particularly their muscle 

 fibre-cells. 



