FIXING AND HARDENING AGENTS. 37 



weeks or months ; but this exaggerated fixation is clearly 

 only justifiable in very special cases, if at all. Wash out 

 very thoroughly in water (running, 24 hours, or treat as 

 directed for chromic acid 37). Stain with alum haema- 

 toxylin if you wish to stain in toto (staining in this way 

 with other reagents is possible, but difficult). Stain sections 

 with safranin or other basic coal-tar colour, or with iron 

 haematoxylin. 



For fixing with the strong mixture you need only take a 

 bulk of liquid of some 4 times the volume of the objects (but 

 with the weak mixture the proportion should be increased). 

 Both of them are first-rate fixatives of cellular structures, 

 both as regards their preservation and as regards their 

 optical differentiation. But they must be properly used, and 

 not applied to objects for which they are not fitted. For 

 instance, their power of penetration is extremely bad ; they 

 will not fix properly, even in a loose-celled tissue, through 

 more than a layer of about five cells thick. They are 

 therefore suitable only for very small objects or for very 

 small pieces of tissue, such as suffice for cytological or 

 histological work. The strong liquid especially has not the 

 character of a general reagent. As a matter of fact it was 

 recommended by FLEMMTNG in the first instance merely for a 

 very special purpose, the hunting for karyokinetic figures, 

 and not for general purposes. It is still very much used, but 

 in my opinion unadvisedly. In most cases, Bouin's picro- 

 formol will do all that it is intended to do, without its 

 disadvantages. 



It may be used for prolonged hardening, e. g. of small 



pieces of nervous tissue, and is very good for that purpose. 



Fat is blackened (or browned) by it (see 35). 



Chromatin is mordanted by it for basic anilin dyes, enabling 



them to give peculiarly sharp and powerful stains. 



43. Osmic Acid and Bichromate. ALTMANN (Die Elementarorgan- 

 ismen, Leipzig, 1890), takes for his " bioblasts " a mixture of equal parts 

 of 5 per cent, solution of bichromate of potash and 2 per cent, solution 

 of osmic acid. The bichromate ought not to contain any free chromic 

 acid. 



Lo BIANCO (Mittli. Zool. Stut. Neapel, ix, 1890, p. 443) employs for 

 marine animals a mixture of 100 c.c. of 5 per cent, solution of bichro- 

 mate and 2 c.c. of 1 per cent, osmic acid. 



