CUTTING AND MOUNTING SECTIONS. 295 



too much, and then after, say, twelve hours transferred to 

 strong, 90 per cent, and in some cases absolute, alcohol. 

 After being transferred to 90 per cent alcohol, the specimens 

 should be again placed in fresh alcohol, in roomy vials not 

 too closely crowded, and should rest on a mass of cotton 

 so as not to lie directly on the bottom of the bottle. 



Fixation of the Histological Elements. Two things, says 

 Lee, are implied by the word " fixing;" first, the rapid kill- 

 ing of the element, so that it may not have time to change 

 the form it had during life, but is fixed in death in the 

 attitude it normally had during life; and second, the hard- 

 ening of it to such a degree as may enable it to resist with- 

 out further change of form the action of the reagents with 

 which it may subsequently be treated. 



The most convenient fixing agents are picro-sulphuric 

 acid (Kleinenberg's or Mayer's formula) and corrosive 

 sublimate. After treating the structure with one of these, 

 it should be washed so as to remove from the tissues all 

 traces of the fixing reagent. If corrosive sublimate, or osmic 

 acid, or a solution into which chromic acid or a chromate 

 enters, has been used for fixing, the washing may be done 

 with water; but if picric acid has been used, the washing 

 should be done with alcohol. 



Dehydration. As soon as the fixing agent is removed, the 

 water of the tissues must be removed; this is done by plac- 

 ing the object gradually in alcohol, at first in 50 per cent 

 alcohol for two hours, 70 per cent for six to twenty-four 

 hours, 80 per cent several hours, 95 per cent two or three 

 hours, and absolute alcohol time enough for complete 

 saturation. 



For insects, especially, Mayer's fluid seems preferable. 

 He prepares it as follows: distilled water lOOvols., sulphuric 

 acid 2 vols., picric acid, as much as will dissolve; filter. 

 As this fluid does not diffuse very rapidly through thick 

 chitin, Mayer directs that insects should be opened with 

 scissors, and the body-cavity at once filled with the solution 

 by means of a pipette. A large quantity of the solution 



