FIXATION AND FIXATIVES 11 



It is a great pity that the use of this fixative should have become so wide- 

 spread, for its only advantage is that objects may be left in it for a long time 

 without becoming unduly hard. It has the disadvantage that picric acid forms 

 water-soluble compounds with many substances found in a cell, so that sec- 

 tions cut from materials fixed in Bouin's fluid frequently show large vacuoles. 

 It is also very difficult to wash the fluid from the tissues; even small traces of 

 picric acid interfere with staining. Gilson's fluid (see p. 10) has all the advan- 

 tages, and none of the disadvantages, of Bouin's. 



Bouin's fluid was recommended by its inventor for the fixation of meiotic 

 figures but has been replaced largely for this purpose by: 

 Allen's Fluid: 



Water 75 ml. 



40% Formaldehyde 15 ml. 



Glacial acetic acid 10 ml. 



Picric acid 1 Gm. 



Chromic acid 1 Gm. 



Urea 1 Gm. 



Small pieces of tissues should be fixed overnight and then washed in 70 per 

 cent alcohol until no more color comes away. 



Other Fixative Mixtures. A great variety of other materials has been rec- 

 ommended as fixatives from time to time, but only two are of sufficient 

 interest to be worth repeating here. The first of these is: 



Kolmer's Fluid: 



Water 87 ml. 



Potassium dichromate 1.8 Gm. 



Uranyl acetate 0.75 Gm. 



40% Formaldehyde 3.6 ml. 



Glacial acetic acid 9 ml. 



Trichloroacetic acid 4.8 ml. 



This very interesting fixative was developed originally for the fixation of 

 whole eyes but it may be used very profitably in any place in which nerve 

 structures are to be examined subsequently. The salts of uranium are widely 

 used in fixatives intended solely for the central nervous system, but this par- 

 ticular formula is also useful for general purposes. Fixation should take place 

 overnight, and then the material should be washed in running water. 



Another very little known but admirable fixative is: 



