29 



completely covered. When a film has formed, place in chloroform 

 as before. 



(b) In a paper box. When a box is required for imbedding 

 proceed as follows: The inside of the paper box should be slightly 

 oily to prevent the celloidin from sticking to it. Rub upon the 

 paper that is to be folded to form the box a little vaseline, and then 

 with a cloth or lens paper remove as much as possible. Fold the 

 paper into a box of convenient size and shape. Remove the object 

 from the thick celloidin and place it in the box, arranging it in the 

 manner wished with a view to sectioning it later. Pour over it 

 slowly, drop by drop or a little at a time, a 12% solution of celloidin 

 until the specimen is well covered and the box sufficiently filled. 

 It is better to have a deep layer over the specimen. The 12% solu- 

 tion does not afford the best mass for cutting, so that, with large 

 objects, it is better to allow the mass in the box to thicken by evap- 

 orating it slowly under a bell-jar (aquarium jar) until it has attained 

 such a consistency that it is no longer fluid. 



66. Hardening. When the celloidin mass is thick enough 

 so that it only dents when touched with the finger nail it is ready for 

 hardening. This may be done by pouring chloroform into the jar 

 in which the imbedded material is placed, covering from the air. 

 The chloroform vapor hardens the mass. W 7 hen it is well set it may 

 be transferred to a jar of the chloroform w r hich takes out the ether- 

 alcohol and hardens the celloidin mass, for which a few^ hours is 

 sufficient. Allow the chloroform to act for 6 to 24 hours. The 

 imbedding mass remains quite transparent when no water is present. 

 The hardening action of the chloroform may be quickened and 

 intensified by carefully heating the chloroform until bubbles of ether 

 begin to come from the specimen. Do not let the chloroform evapor- 

 ate. 



67. Alcohol hardening. When the celloidin mass is hard, 

 whether clear or not, it may either be transferred to alcohol of about 

 82% strength in which it is stored until cut, or it may be placed in 

 Clarifier (castor oil, 1 part; xylene, 3 or 4 parts). Alcohol of higher 

 percentage softens the mass; lower grades such as 67% usually in- 

 crease the hardness of the celloidin and in some cases are to be 

 recommended. 



The choice between alcohol and clarifier involves no decision of importance 

 in technique. The method of clarification has the advantage that the orientation 

 of the specimen in the microtome preparatory to cutting can be more perfectly 



