1. Absolute alcohol 24 hours.
2. Equal parts absolute alcohol and ether 24 hours.
3. Thin celloidin for 1-3 days.
4. Medium celloidin for 1-3 days.
5. Thick celloidin for 1-3 days.
6. Block.
The tissue is blocked by removing it from the thick celloidin on a section-lifter and placing it on a block of vulcanized fiber or wood with enough of the thick celloidin about it to form a good matrix. The preparation is then allowed to evaporate in the air until the surface of the celloidin becomes firm (does not stick to the finger). The block is then placed in 80 per cent alcohol or pure chloroform until hard enough for cutting (1-24 hours). Cork should not be used for blocking, nor should wood unless the tannin has been removed by long treatment with alcohol-ether. The celloidin will adhere more firmly to the fiber block if the latter is dry, and if there is a sufficient layer of celloidin between the tissue and the block. The imbedded tissues may be preserved in 80 per cent alcohol for a long time, but gradually lose their staining power. They may also be kept dry by coating them with melted paraffin. The blocks when preserved in alcohol may be marked with an indelible pencil.
For imbedding large pieces of tissue in celloidin a glass dish may be filled with thick celloidin and the infiltrated tissue placed in it with cutting surface down. The celloidin is then allowed to evaporate slowly under a glass cover, and fresh celloidin may be added as shrinkage occurs. When well-hardened the celloidin is cut out of the dish and the block trimmed to the proper proportions, and attached directly to the object-holder of the microtome or to a block of wood by a few drops of thick celloidin, allowing it to dry for a minute or so and then immersing in 80 per cent alcohol. The block may be cut on the freezing microtome by soaking the hardened celloidin in water to remove the alcohol (when block sinks), then coating it with saturated gum arabic solution, and freezing.
Rapid Celloidin Method.
I have used the following method in my laboratory for a number of years as a regular procedure in practical diagnostic work, and the results have been uniformly good.
1. Fresh tissue cut thin, or uterine curettings, in absolute alcohol 1½ hours, three changes of fresh absolute during this time.
2. Alcohol-ether 1 hour.
3. Thin celloidin at incubator temperature 12 hours (over night).
4. Medium celloidin 1 hour.
5. Block from medium celloidin, evaporating celloidin by blowing, and building up matrix by adding successive layers of celloidin.