24 FIXING AGENTS. 



the formation of this precipitate may be entirely prevented by simply keep- 

 ing the preparations in the dark. The alcohol becomes yellow as usual (and 

 should be changed as often as this takes place), but no precipitate is formed. 

 If this precaution be taken, previous washing with water may be omitted, or 

 at all events greatly abridged. 



The brownish-green colour of chromic objects may be removed by treat- 

 ing them with peroxide of hydrogen (Unna, in Arch. f. mik. Anat., Bd. 

 xxx, 1887, p. 47 ; cf . Journ. Roy. Mie. Soc., 1887, p. 1060 ; and see the in- 

 structions for bleaching osmic acid preparations at the end of the last ). 



31. Chromo-acetic Acid (FLEMMING, Zellsbz. Kern. u. Zellth., 

 p. 382). 



Chromic acid . 0*2 to 0'25 per cent. 

 Acetic acid . . O'l per cent., in water. 



Flemming finds this the best reagent for the study of the 

 achromatic elements of karyokinesis. (Flemming wrote this- 

 in 1882, and I doubt whether it would now hold good.) Stain 

 with hasmatoxylin (the preparations are not favourable for 

 staining with safranin or other coal-tar colours). 



I can recommend as a good fixing and hardening mixture 

 for Annelids in general, and probably for other forms, the 

 following fluid due to EHLERS (I do not know whether it has 

 been published) : To 100 c.c. of chromic acid of 0*5 to 1 per 

 cent, add from 1 to 5 drops of glacial acetic acid. The pro- 

 portion of acetic acid indicated is sufficient to counteract any 

 tendency to shrinkage due to the chromic acid. 



32. Chromo-formic Acid (RABL, Morph. Jahrb., x, 1884, pp. 

 215, 216). Four or five drops of concentrated formic acid are 

 added to 200 c.c. of 0*33 per cent, chromic acid solution. The 

 mixture must be freshly prepared at the instant of using. Fix 

 for twelve to twenty-four hours, wash out with water, harden 

 in alcohol, stain with hsematoxylin or safranin. For the study 

 of karyokinesis. This is acknowledged to be one of the very 

 best reagents for the purpose. 



33. Chromic Acid and Spirit. A mixture of 2 parts of ^th 

 per cent, chromic acid solution with 1 part of methylated 

 spirit was much used by Klein in his investigations into the 

 structure of cells and nuclei, and found to give better results 

 than the ordinary reagents (including even osmic acid). 

 Hsematoxylin was used for staining. 



The addition of alcohol to augment the penetrating power 

 of chromic acid seems to be a step in the right direction, and 



