H^MATEIN (HJBMATOXTLIN) STAINS. 181 



distilled water, free from even a trace of ammonia) , 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. 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 

 neutralisation may be indicated in the interest of the pre- 

 servation, 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 bicar- 

 bonate of soda in distilled water. MAYER holds that acetate 

 of potash is the. most inoffensive reagent to take ; a strength 

 of O'o to 1 per cent, may be taken. 



Several of these solutions have a great tendency to over- 

 stain. Over-stains may be corrected by washing out with 

 weak acids (e. g. O'l to 0*2 or even 0'5 per cent, of hydro- 

 chloric acid, or with oxalic or tartaric acid), but this is not 

 favourable to the permanence of the stain. CARKOY (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 (O'l per cent.). 



Bicarbonate of soda may be used for neutralisation with 

 70 per cent, alcohol as the vehicle (VON WISTINGHAUSEN, Mitth. 

 Zool. Stat. Neatel, x, 1891, p. 41 ; Zeit. f. iciss. Mik., x, 4, 

 1893, p. 480). 



Over-staining may be avoided by staining very carefully 

 and slowly in extremely dilute solutions. It should be noted 

 that the pm*est chromatin stains are obtained by staining for 

 a short time (sublimate sections half an hour, say) in solu- 

 tions 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, in the Gruridzuge, p. 151, says 

 that very dilute solutions will give a pure nuclear stain if 

 they have been diluted with alum- solution, or have been 

 acidified). Chromosmium material will not yield a pure 

 chromatin stain unless it is very fresh ; it is consequently 

 next to impossible to obtain the reaction with paraffin sections 

 of such material ; they constantly give a plasma-stain in addi- 



