The purpose of embedding is to give to a tissue a sufficient firmness to permit the cutting of thin sections. Two methods are commonly employed - one with celloidin, the other with paraffin.

Celloidin has the advantage of not requiring heat, and can be used for larger pieces of tissue. On the evaporation of the alcohol and ether a comparatively solid mass remains.

Paraffin can be used for small pieces of tissue only. It also renders the specimen brittle, so that it is frequently difficult to cut good sections. Although fluid when kept at the necessary heat, the paraffin becomes hard on cooling.

Celloidin Method

In this process two solutions of celloidin of different thicknesses are employed - one of the consistency of syrup, the other of that of molasses. These solutions are made by adding to a mixture of equal parts of absolute alcohol and ether enough celloidin to give the desired consistency. The specimens must be thoroughly dehydrated in absolute alcohol and then placed in equal parts of absolute alcohol and ether for twenty-four to forty-eight hours. This latter step is not essential, but is advisable. From the alcohol the specimens are left in the thin celloidin at least twenty-four hours and in thick celloidin for a like period. If there is no hurry, the longer the time in each celloidin solution, the better will be the result. They are then placed on blocks, covered with thick celloidin, and allowed to harden. In the course of a few minutes, when the block can be turned upside down without the specimen sliding off, they should be placed in 80 per cent, alcohol. After remaining there for several hours they are ready to cut.

The blocks best adapted for use are those made out of vulcanite or hard paraffin. The latter are particularly convenient. A square of hard paraffin is cut up into blocks of various sizes and the tops roughened with a knife so as to give a better surface for the celloidin to adhere to. Cork and wood are not well adapted, as after being in the alcohol for any length of time the tannic acid is extracted; it penetrates the specimen and interferes with its staining properties.

In cutting celloidin sections the knife is clamped at a very marked slant, so that as much of it as is possible will be used. The blade and the specimen should be kept constantly wet with 80 per cent, alcohol. As the sections are cut they are lifted off the knife with a camel's-hair brush and placed in a dish containing water. This causes them to flatten out.

After the staining has been completed the sections are passed through graded alcohols to remove the water and are then placed in some fluid that will clear them. Clove oil should not be used, as it dissolves the celloidin. Bergamot, cedar oil, creosote, and xylol, alone or in combination with 1 part of carbolic to 3 parts of xylol, do not affect the celloidin.

Summary:

1. Dehydration in absolute alcohol.

2. Absolute alcohol and ether, equal parts, one to three days.

3. Thin celloidin, one to five days.

4. Thick celloidin, one to five days.

5. Mount on block.

6. Alcohol (80 per cent.), twelve to twenty-four hours.

7. Cut on microtome.

8. Stain, dehydrate, and clear.

9. Mount in balsam.