WASHING AND SECTION CUTTING 11 



is often very useful, as it combines the advantages of both 

 reagents. 



6. Picric Acid is a very valuable hardening reagent, of which 

 the best preparation is Kleinenberg's. Specimens should be left 

 in it from 12 to 24 hours. It is prepared thus : with 100 c.c. 

 of water make a cold saturated solution of picric acid : add 2 

 c.c. of concentrated nitric acid : filter, and add to the filtrate 

 three times its volume of water. 



VII. ON WASHING. 



Specimens that have been hardened in any of the preceding 

 reagents (except alcohol) should be placed in water on 

 removal from the reagent, then in 30 per cent, alcohol, then 

 in 50 per cent, alcohol. They should then be washed in alter- 

 nate strengths of 60 per cent, and 70 per cent, alcohol in order 

 to remove all traces of the hardening reagent, and should be 

 stored in 70 per cent, alcohol. The object of using graduated 

 strengths of alcohol is to avoid distortion of the tissues by too 

 sudden changes of specific gravity. These remarks apply also 

 to treatment before and after staining. 



VIII. -ON SECTION CUTTING. 



Many tissues and organs can only be studied satisfactorily by 

 cutting them into thin sections, and this method of investiga- 

 tion is of such importance as to require special notice. There 

 are three chief stages : Hardening, Imbedding, and Cutting, 

 which will be noticed in succession. 



A. Hardening. 



Before the object can be cut into sections it is necessary to 

 harden it ; this may be effected by freezing, but the more 

 usual plan is by means of reagents, which have been discussed 

 under previous headings. 



B. Staining. 



The hardened specimens, if not too large, may now be stained 

 with either haematoxylin, borax-carmine, or picro-carmine; they 

 should then be brought to 70 or 90 per cent, alcohol. If the 

 specimen is too large to stain whole, the sections must be 

 stained after they are cut (see p. 1 3). 



C. Imbedding. 

 The preparation of sections is greatly facilitated by imbedding 



