Most animal tissues require hardening before sections can be cut from them, and to secure this, they are immersed for varying periods in suitable reagents. Hardening agents can be divided into two types: (a) those—e.g. alcohol, nitric and picric acids, etc.—which do not interfere with the subsequent processes of staining the tissues and sections, and (b) those—e.g. cosmic and chromic acids, etc.—which do more or less affect the action of the staining reagents. In hardening tissues, it is most important to be sure that the specimens are thoroughly penetrated by the liquid, and to this end it is necessary to immerse them in an abundance of the liquid. If small pieces are used, and placed in about 100 times their bulk, this should lead to satisfactory results.
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