34 



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

 ment of the knife, as in the paraffin 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. There are a number of automatic microtomes 

 specially designed for use \vith the freezing method. 



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- 

 ing ( 137, 146). If anise-seed oil is used, the sections are to be 

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



76. Rapid Method. Blocks of tissue 1 centimeter thick should 

 fix in 10% formalin 12 to 24 hours. If haste is a factor, take thinner 

 pieces and fix for 1 minute or more. Trim the block so that it is about 

 5 mm. thick; rinse in water for a few seconds, transfer to the freezing 

 microtome, freeze and section. 



Float the sections when cut from the knife into water from which 

 they may be gotten upon the slides by means of a camel's hair 

 brush. Drain off the water and press the sections out smooth by 

 means of blotting paper, filter paper or other absorbent paper. Cau- 

 tiously drop over the sections 95% and absolute alcohol and follow 

 this immediately with thin (% or /4%) celloidin solution ( 64) 

 which when it has partially evaporated out will serve to support the 

 section and- fasten it to the slide. It is now ready for staining. 

 ( 144). 



STAINING. 



77. Staining has for its first and primary purpose, the ren- 

 dering outlines and structures more distinct by giving them a color 

 contrast with their surroundings (color image). A second and 

 more important use is for the differentiation of particular struc- 

 tures or substances which by their selective staining facilitate the 

 histological analysis. Rational staining, like rational fixation, de- 

 pends upon the physics and chemistry of staining reactions; indeed, 

 in the demonstration of particular substances the fixation and stain- 

 ing should be determined by the mutual interdependence of their re- 

 actions, since they have the same purpose, the preservation and 

 demonstration of the substance sought for. 



