OYTOLOGICAL FIXING AGENTS. 281 



combined with clearing in glycerin, and staining, may be 

 useful for bringing out reticula and nucleoli. Chloride of 

 gold preserves the forms well, but generally leaves the nuclear 

 structures unstained. Nitrate of silver is hopelessly uncon- 

 trollable in its action. Alcohol has much the effect of chromic 

 acid, but often causes a much greater shrinking of the nuclei. 

 Bichromate of potash and chromate of ammonia bring out very 

 sharply the appearance of a reticulum, but these appearances 

 cannot be accepted as true (1. c., p. 334 et seq.). 



" Those wlio seek to study cell-division by means of bichromate 

 of potash or other chromic salts are hopelessly in the wrong 

 road" And this because of the injurious action of the 

 bichromate, not on the body of the cell, which it preserves 

 well, but on the chromatin structures. Chromic salts are 

 excellent reagents for general histological work, but not for 

 nuclear structures. They dissolve nucleoli, destroy nuclear 

 "networks/' and swell up and distort karyokinetic figures to 

 such a degree that the appearances obtained from them are 

 merely unnatural caricatures of the true structure. 



Altmann's nitric acid method is excellent for the purpose 

 of hunting for cell-divisions in tissues ; but the minute struc- 

 ture of the figures is not so well preserved as it is by means 

 of chromic or picric acid. The same must be said of Kleinen- 

 berg picrosulphuric acid method. 



The best fixing agent in general is the chromo-aceto-osmic 

 acid mixture ( 35, 36). Attempts to omit the chromic acid 

 did not give good results. The omission of acetic acid (as in 

 Max Flesch's formula, 34) causes the figures to be far less 

 sharply brought out. The presence of acetic or formic acid 

 in all osmium solutions is favorable to the precision of subse- 

 quent staining with haematoxylin, picro-carmine, or gentian- 

 violet. But mixtures of osmic and acetic acid without chromic 

 acid (Eimer) do not give such good results as the chromo- 

 aceto-osmic acid mixture. Mixtures of picric acid with osmic 

 acid or with osmic and acetic acid (proportions of the latter 

 as in the chromo-acetic osmic mixture ( 35), but of picric acid 

 about 50 per cent.) fix quite as well as the chromic mixtures, 

 but precise staining is even more difficult than with pure 

 osmic acid preparations. Flemming concludes that the bene- 

 ficial effects of the osmium in all these mixtures are to be 

 ascribed to the instantaneous rapidity with which it kills, the 



