512 GOLGI METHODS 



and (2), xxiv, 1891 ; Arch. Hal. Biol., op. cit., § 1026 ; Rif. Med., 

 1891 ; Opera Omnia, I, p. 143, and II, pp. 505 and 607). For 

 hardening, use either a solution of potassium bichromate pro- 

 gressively raised from 1 to 3 per cent., or Miiller's fluid. It is 

 best to take small pieces of tissue, large quantities of hardening 

 fluid, and change the latter frequently. But the reaction can 

 be obtained with much larger pieces, even entire hemispheres. 

 In this case the brain should at first be treated with " repeated " 

 injections of the fixing agent, or this should be injected from 

 the carotid or the aorta. Pieces, particularly if small, begin 

 to be ready for the subsequent treatment eight to ten days 

 afterwards, but it is advisable to wait until the twentieth or 

 thirtieth day of immersion, this being not injurious if prolonged 

 for several months ; it is, on the contrary, to be recommended 

 if the pieces are uncommonly large. 



When it is thought that the tissues have been hardened enough, 

 they are passed directly from the bichromate into 0-5 to 1 per 

 cent, mercury chloride. One generally prefers weak solutions 

 (0-5 per cent.) if pieces have been left in the fixing fluid for a 

 relatively short period, having recourse to the stronger ones 

 (1 per cent.) for materials which have been hardened for many 

 weeks or months. The sublimate solution must be changed at 

 first every day, and later as often as it comes yellowish. At the 

 end of the reaction pieces will be found decolourised and almost 

 wdth the aspect of fresh tissue. To obtain a good reaction, about 

 ten days of immersion in the mercury chloride are necessary if 

 pieces are small, longer periods, and even months, being required 

 for large pieces and entire hemispheres. Particularly fine results 

 were obtained by Golgi from brains which had been kept in 

 1 per cent, sublimate for as long as two years. 



The reaction may be said to have begun by the time tissues 

 are nearly decolourised. From that time onwards sections may be 

 made and mounted if successful. 



Imbedding 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 imbedding them in celloidin. 

 Sections, however made, must be repeatedly washed with dis- 

 tilled 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 

 transforming 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 



