io6 



hoi remove the alcohol by soaking it for several hours to i day in 

 water. Remove to a thick solution of gum arabic in water, in 

 which it may remain for about 24 hours. It is then ready to freeze 

 and cut. 



(b) Anise-seed oil. For this method the tissue should be first 

 dehydrated (§ 43). When dehydration is complete, transfer the 

 tissue to anise-seed oil, in which it may soak for 12-24 hours ; it is 

 then ready to freeze and cut. 



§ 63. Cutting. Place a drop of the solution of gum arabic 

 (or anise-seed oil) upop the object carrier of the freezing microtome 

 and turn on the carbonic acid (or ether) spray. When the mixture 

 begins to harden, place the object upon it in an abundance of the 

 solution and freeze it solid. Covering with an inverted cup hastens 

 the freezing. 



When the tissue is completely frozen, cut it with a straight 

 movement of the knife, as in the paraflBn method, holding it firmly 

 upon the knife rest and making the strokes as rapidly as possible, at 

 the same time rapidly raising the tissue a few microns at a time by 

 means of the microtome screw. For cutting frozen sections, a 

 strong, wedge-shaped knife must be used. 



The mass of sections is transferred to a dish of water in which 

 the gum arabic is dissolved away and the sections are ready for stain- 

 ii^S (§§ 7i~79)- If anise-seed oil is used, the sections are to be 

 transferred to 95% alcohol which will dissolve out the oil ; stain 

 (§§ 69-79). If the tissue has been stained in toto the sections may 

 be transferred to anise-seed oil (or other clearer) and mounted in 



balsam directly. 



STAINING AND MOUNTING. 



§ 64. The remaining steps in making permanent histological 

 preparations are usually done at one time and are conveniently con- 

 sidered together. In all the processes seemingly complicated, if it 

 is remembered that the succession of media, as in paraflBn imbed- 

 ding, depends on their miscibility, and the reason for the various 

 steps is recognized, much of the diflSculty in remembering the order 

 in which they come will be avoided. 



There are here to be considered, {a) Paraflfin and Collodion sec- 

 tions, {b) Free-hand and Frozen sections, (c) Isolation preparations. 

 Of these, the paraflfin and collodion sections require some prelimi- 



