SECTION THREE 



Technique: 



{a) Wet Method 



1. Pieces of tissue not thicker than 5 mm. are fixed in the usual 

 manner. 



2. Wash in running water for the prescribed time for the parti- 

 cular fixative. If a fixative containing mercury has been used, 

 remove mercurial precipitate by the standard technique. 



3. Immerse the pieces of tissue for two hours in each of the 

 following: 50%, 70% and 90% alcohol. 



4. Immerse in absolute alcohol from two to sixteen hours, 

 according to the nature and thickness of the tissue. 



5. Immerse for twenty-four hours in a mixture consisting of 

 equal volumes of absolute alcohol and ether, in a stoppered wide- 

 mouth bottle, which must be absolutely dry. 



6. Impregnate with 2% Celloidin solution from five to seven 

 days. 



7. Transfer to 4% Celloidin for five to seven days. 



8. Impregnate with 8% Celloidin for three or four days. 



9. The tissue is then taken out of the Celloidin and put into a 

 mould made by folding a piece of writing paper, and the whole is 

 then placed in a desiccator and left for several days, lifting the 

 desiccator lid for a few seconds each day to accelerate the harden- 

 ing of the Celloidin. If, through shrinkage of the Celloidin during 

 this process, the tissue becomes exposed, pour on more Celloidin 

 solution to cover it. Hardening of the block may be hastened by 

 placing 1-2 ml. of chloroform in the bottom of the desiccator. 



The block is hard enough for sectioning when no impression is 

 left after pressing with the ball of the thumb. 



10. The base of the hardened Celloidin block is dipped into 8% 

 Celloidin then fixed to a roughened wooden or a vulcanite block by 

 pressing firmly, afterwards leaving for at least half an hour with a 

 weight on top. 



11. Expose to chloroform vapour for half an hour; then attach 

 the wooden or vulcanite block to the microtome holder, or store 

 the Celloidin block mounted on the wooden or vulcanite block in 

 80% alcohol until required for sectioning. 



487 



