18 Animal Micrology 



Picric acid, or solutions containing it, must be washed in 

 strong alcohol (70 per cent.), never in water because the latter 

 seems to undo the work of fixation. 



Corrosive sublimate and mixtures containing it are washed out 

 in water or alcohol. A little tincture of iodine should be added 

 to the wash from time to time to insure the removal of all corro- 

 sive sublimate crystals. Sufficient iodine has been added when it 

 no longer loses its reddish color after being in contact with the 

 preparation for a short time. 



Osmic acid and mixtures containing it should be washed in 

 running water. 



DEHYDRATING 



While under certain circumstances objects may be mounted in 

 aqueous media for examination, in the majority of cases, especially 

 where the preparation is to be a permanent one, it has been found 

 best to remove all water from the tissues, that is, to dehydrate 

 them. This renders preservation more certain, and it is a neces- 

 sity, moreover, if the object is to be imbedded later in paraffin or 

 celloidin, for neither of these substances is miscible with water. 

 Because of its strong affinity for water and the ease with which it 

 may be manipulated, alcohol has come to be used universally for 

 this purpose. It completes the process of hardening at the same 

 time. The dehydration must be gradual. In tissues transferred 

 from water or aqueous solutions directly to strong alcohol (or vice 

 versa) violent diffusion currents are set up which produce serious 

 distortion of the tissue elements. For this reason a series of 

 alcohols of gradually increasing strength (e. g., 35-50-70-83-95 

 per cent.) is used. The more delicate the object, the closer should 

 be the grades of alcohol. 



PRESERVING 



After fixing and washing, the process of dehydration is begun 

 ordinarily and tissues are carried as far as 70 per cent, alcohol. 

 It is customary to leave them in alcohol of from 70 to 83 per 

 cent, strength until they are needed. They may remain here 

 indefinitely. If they are to be preserved for a long time (for 



