To the Editor.—
In the article "Histogenesis of Skin Appendage Tumors,"1 the authors state "By histochemical criteria, cylindroma did not show definite differentiation either in apocrine or eccrine direction." Histochemical data, published previously, are not in agreement with their statement.2-5 The interpretation of the periodic-acid Schiff (PAS) stain was expanded to evince that with a yellow counter stain (picric acid), there were two basic colors observed in mammalian tissue sections after diastase digestion. The two diastase resistant (D) colors were magenta (M) and red (R). If the next tissue section was stained with alcian blue (AB), the magenta substance was AB positive ( + ) and the red substance was AB negative ( - ). If alcian blue was used as a counter stain or if the PAS was used alone, the two colors were not discernible. The results of the PAS picric reaction under controlled conditions2,4 indicated acidic carbohydrates or carbohydrate protein