Stereomorphic profiles of three types of spermatogonia and their topographical distribution in the basal portion of the seminiferous epithelium of adolescent or adult rats were studied mainly with the scanning electron microscope. According to their cytological features, three types of spermatogonia (large-sized and long fusiform or polygonal; mediumsized and elliptical; small-sized and round) were discerned in both groups of rats. The basal areas of the three spermatogonia were relatively smaller in the adolescent rats than in the adult rats. The type of spermatogonia in the adolescent rats varied somewhat depending upon the diameter of the seminiferous tubules, but such a discrimination could not be made in the adult rats. In the adolescent rats, small cells were less frequently observed than large and medium cells. This ratio of appearance was less marked in the adult rats.
Clones from the same spermatogonial cells showed similar topography in both groups of rats as follows. The clones of a few medium cells were arranged singularly (As) or in pairs (Apr), but the clones of most medium cells appeared to form single cords (Aal) through intercellular bridges, while those of the small cells formed an open polygonal network as a large syncytium.