650 TECHNIQUE 



Embedding in Celloidin. Make a saturated solution of a little 

 celloidin (Schering's) in a mixture of equal parts of alcohol and 

 ether. The alcohol should contain no trace of copper sulfate. 

 This solution is for convenience known as number III and should 

 have a very thick, syrupy consistence. 



A small portion of number III is mixed with three to five times 

 its volume of the alcohol and ether mixture, to obtain number II, 

 which should have a somewhat viscid consistence. 



A second small portion of number III is diluted with ten to fif- 

 teen times its volume of the alcohol and ether, to produce celloidin 

 number I, which should have a thin, watery consistence. 



Small pieces of tissue which have been thoroughly hardened in 

 95 per cent, alcohol are treated as follows : 



1. Dehydrate in absolute alcohol, six to twenty-four hours. 



2. Place in the absolute alcohol and ether mixture, twelve to 

 twenty-four hours. 



3. Place in celloidin number I, twelve to twenty-four hours. 



4. Place in celloidin number II, twelve to twenty-four hours. 



5. Place in celloidin number III, twenty-four to forty-eight 

 hours, or longer. 



Pieces of tissue of considerable size may be satisfactorily em- 

 bedded 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. Ordi- 

 nary 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. The piece of tissue should now be properly ori- 

 ented upon the block, the future sections being cut nearly parallel 

 to the wooden surface. A bit of. the thick celloidin is poured over 

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

 to the block. As soon as the block can be inverted without dislodg- 

 ing the tissue, it is floated in a jar of chloroform, tissue down, for two 

 or three hours. It is now ready for cutting, but if this is not done at 

 once the blocks of tissue should be stored in a jar of 70 per cent, alco- 

 hol, in which they may be kept indefinitely. Stronger alcohol than 

 75 per cent, is apt to soften the celloidin and spoil the preparation. 



Embedding in Parafin. If sections thinner than 15 //. are 

 desired, parafin embedding must be used ; it is impossible to cut 



