The development of Leydig cells from the 1st to the 14th year was electron microscopically investigated in specimens of testicles. Leydig cells and their precursor were found in the interstitium. The precursors of the Leydig cells were usually spindle-shaped and contained few organelles and filamentous structures. Leydig cells were usually found in groups. The nucleus was round and eccentric. The most striking feature of Leydig cells was the abundance of smooth endoplasmic reticulum. It was tubular and vacuolar in appearance. Mitochondria possessed cristae of the tubular type and contained occasionally intramitochondrial granules. Glycogen granules and lipid droplets were noted in the cytoplasm. No Reinke crystalloids were found throughout this period. Leydig cells arose from mesenchymal cells via their precursors. Phases of regression of Leydig cells were observed twice, in the 2nd and 3rd years, and from the 9th to the 11 th year. In these periods the above-mentioned Leydig cells were seldom observed and only degenerating cells were noted. After the 13th year the Leydig cells increased in size and in number.
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