74 The Preparation of Microscope Slides 



This fluid may be used equally well with a hard-shelled arthropod or 

 with a hard-shelled seed and will penetrate rapidly enough to preserve 

 the whole. It cannot be used satisfactorily with any object containing fat, 

 which will be dissolved, and is usually employed only when the principal 

 interest of the worker is in nuclear fixation. 



Picric Acid Fixatives. Picric acid (actually trinitrophenol ) has been 

 widely used in the last quarter century as a component of fixatives. The 

 best-known formula undoubtedly is: 



Bouin's Fluid: 



Saturated aqueous solution of picric acid 75 ml 



40% formaldehyde 25 ml 



Glacial acetic acid 5 ml 



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

 widespread, 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 sections cut from materials fixed in Bouin's fluid fre- 

 quently show large vacuoles. It is also very difficult to wash the fluid 

 from the tissues; even small traces of picric acid interfere with staining. 



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

 figures but has been replaced largely for this purpose by: 



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 

 recommended as fixatives from time to time, but only two are of suffi- 

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



