EMBEDDING 735 



in celloidin, but should be passed through the successive solutions in 

 a much more leisurely manner. Thus an eye requires two to three 

 weeks, a large piece of the central nervous system three to four weeks 

 for proper embedding. The tissue should now be fastened to a wooden 

 block and the celloidin hardened. Ordinary wood yields its resins to 

 the alcohol in which the blocks are to be kept ; the white pine blocks 

 which are commercially known as 'deck plugs' contain very little resin 

 and are admirably adapted for the purpose. Vulcanized fiber blocks are 

 still better, and glass blocks are also serviceable. The piece of tissue 

 should be so oriented upon the block that the future sections may be 

 cut nearly parallel to the block surface. Thick celloidin is then poured 

 over the tissue; a few moments' exposure to the air cements it firmly 

 to the block. After the celloidin has become firm by partial drying in 

 air, the block may be stored indefinitely in 70 per cent, alcohol (stronger 

 alcohol is apt to soften the celloidin), where the hardening is completed. 

 If it is desired to harden the celloidin rapidly for early cutting, the 

 block may be floated in a jar of chloroform, tissue down, for two or 

 three hours. 



The final step in the embedding process may be accomplished ad- 

 vantageously by allowing the tissue to remain in the slowly hardening 

 thick celloidin in a shallow dish. Care must be taken to begin this 

 step with sufficient celloidin to keep the tissue covered until the harden- 

 ing is completed. The tissue may then be cut from the firm celloidin 

 and blocked. 



Embedding in Paraffin. If sections thinner than 15 p. are de- 

 sired, paraffin embedding must be used; it is impossible to cut celloidin 

 sections with any great degree of certainty thinner than 10 p to 15 p., 

 Paraffin sections are readily cut at 5, and may be cut as thin as 2 /x. 

 The paraffin method is also to be selected for the rapid preparation 

 of tissues for sectioning, but it is only applicable to small pieces of 

 tissue. For large pieces better results will be obtained with celloidin. 

 Many methods for the employment of paraffin have been extolled; the 

 following can be recommended. The tissue, after fixation, should have 

 been hardened in alcohol. 



1. Dehydrate in absolute alcohol twenty-four to forty-eight hours; 

 v^ery small pieces of tissue (1 to 2 mm.) may be completely dehydrated 

 in three to six hours. 



2. Place in equal parts of absolute alcohol and xylol (xylcne), one 

 to three hours. 



3. Place in pure xylol until clear and translucent, one-half to two 



