156 HAMATE IN 



bath, giving a progressive stain. The stain is in different tones of 

 blue or red according to the composition of the staining sohition. 

 Neutral or alkaline solutions give a blue stain ; acid solutions 

 give a red one. In order to get a blue stain in preparations that 

 have come out red through the acidity of the staining bath, it is a 

 common practice to treat them with weak ammonia, in the 

 belief that the blue colour is restored by neutralisation of the 

 acid that is the cause of the redness. According to Mayer, the 

 ammonia acts, not by neutralising the acid, but by precipitating 

 the alumina, which carries down the hacmatein with it (if no 

 alumina were present the colour would be purple, not blue). 

 The same result can generally be obtained by merely washing out 

 with common tap- water, which is usually sufficiently alkaline, 

 and can be obtained with certainty by treatment with bicarbonate 

 of soda or acetate of soda or potash. And this is the preferable 

 course, as ammonia is certainly a dangerous thing to treat delicate 

 tissues with. See Scott's tap-water substitute, § 1429 his. Of 

 course this is a different question from that of neutralising with 

 an alkali tissues that have been treated with an acid to correct 

 over-staining. Here the neiitralisation may be indicated in the 

 interest of the preservatioti of the stain. 



Squire {Methods, p. 22) finds that sections can be blued in a 

 few seconds by treatment with a 1 : 1000 solution of bicarbonate 

 of soda in distilled water. Mayer holds that acetate of potash 

 is the most inoffensive reagent to take ; a strength of 0-5 to 

 1 per cent, may be taken. 



Several of these solutions have a great tendency to over-stain. 

 Over-staining may be corrected by washing out with weak acids 

 {e.g. 0-1 to 0-2 or even 0-5 per cent, of hydrochloric acid, or with 

 oxalic or tartaric acid), but this is not favourable to the per- 

 manence of the stain. Carnoy {La Cellule, xii, 2, 1897, p. 215) 

 recommends iodised water. If acids be used, it is well to neutralise 

 afterwards with ammonia or bicarbonate of soda (0-1 per cent.). 



Bicarbonate of soda may be used for neutralisation with 70 

 per cent, alcohol as the vehicle (von Wistinghausen, Mitth. 

 Zool. Stat. Neapel, x, 1891, p. 41). 



Over-staining may be avoided by staining very slowly in dilute 

 solutions. The purest chromatin stains are obtained by staining 

 for a short time (sublimate sections half an hour, say) in solutions 

 of medium strength, such as haemalum diluted ten to twenty- 

 fold with water. The stain obtained either with very strong 

 solutions, or with the slow stain of the dilute solutions, is at the 

 same time a plasma-stain, which of course may or may not be 

 desired. Mayer says that very dilute solutions will give a pure 

 nuclear stain if they have been diluted with alum-solution, or 

 have been acidified. Chrome-osmium material will not yield a 

 pure chromatin stain unless it is very fresh ; it is consequently 



