OsO4 has many advantages over Carnoy's fixative mixture for the Feulgen nuclear staining in the protozoan Tokophrya infusionum.

While Carnoy's fluid used prior to the Feulgen reaction produces shrinkage of the macronucleus and coarse clumping of its chromatin bodies, OsO4 preserves faithfully the size and shape of the macronucleus and its chromatin material.

This finding seems to be of special importance in view of the fact that electron microscopy relies on OsO4 fixation. The satisfactory preservation of structured detail in Feulgen-stained preparations is of importance for the correlation of histochemical and morphological information.

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