On the cytochemical demonstration of aldehydes.

IN studying the distribution in tissues of substances which react with reduced fuchsin, it is frequently necessary to distinguish between (a) substances which react as though they were free aldehydes; (A) substances which react as though they were aldehyde acetals; and (c) substances which react as aldehydes as a result of oxidation of precursors in the tissue. In a recent paper on the plasmal reaction, A. J. Cain (1949) remarks on one of my papers on liver aldehydes (Danielli, 1949): 'The technique proposed (p. 68) is open to serious objection in that the fixative used (and recommended, p. 70) contained acetic acid, and that the time of fixation is given as not less than two hours and not more than five days. . . . This suggests that it was the pseudoplasmal reaction that was studied, not the true plasmal reaction.' Thus Dr. Cain supposes (A) that by using an acid fixative I failed to distinguish between free and acetal aldehyde, and (B) that the duration of fixation introduced artefacts due to oxidation. Neither supposition is correct. With reference to (A): Dr. Cain is referring to a part of my paper in which I considered only free aldehyde, but on p. 68 I stated: 'In section 7 of this paper a procedure is given which distinguishes between (a) free aldehyde groups, (b) aldehyde groups bound as acetals, and (c) aldehyde groups liberated by oxidation', and on p. 73 I gave a detailed procedure: 'It may be necessary at times to discriminate between (a) free aldehydes, (b) aldehydes present as acetals, and (c) aldehydes liberated by oxidation. This can be done as follows, after fixation with neutral formaldehyde solution: