HISTOCHEMICAL .ISTOCHEMICAL and electron microscopic investigations have been useful in the determination of the histogenesis of many appendage tumors for two reasons. First, the immature cells of appendage tumors, like immature cells of cutaneous appendages of the human embryo, contain certain enzymes that are quite specific for each cutaneous appendage. Second, the electron microscopic examination of the appendage tumors reveals details which are not recognizable on examination with the light microscope. With these new methods, many conventional viewpoints of the light microscopist have been confirmed, whereas, some were disproved.1-4 In this review, the histogenesis of eccrine poroma, syringoma, eccrine spiradenoma, cylindroma, and calcifying epithelioma will be discussed. Materials and Methods All histochemical methods and materials used were the same as those used previously.5-12 Original electron micrographs used for Fig 2, 3, 6, 8, and 10 were taken with a high resolution electron microscope (Hitachi HU-11C) at accelerating voltage