The seminiferous tubules consist of a thin peripheral membrana propria upon which rests the seminal epithelium, which is made up of the essential semen forming cells, and the cells of Sertoli. The spermatogenic cells may be divided into three groups, from within outward: the peripheral single layer of small cuboidal spermatogonia; one or two rows of large spermatocytes; and three to five rows of spheroidal spermatids. The cells of Sertoli are more or less of the syncitial type,—large in size and irregular in outline. They occur at various intervals between the layers of spermatogenic cells, with their bases resting upon the membrana propria. Centrally they send out protoplasmic processes for variable distances,—some even reaching the border of the innermost cell layers. SPERMATOGENESIS: In this process, the primary germinal cells, the spermatogonia, divide to form the primary spermatocytes. Maturation consists of two cell divisions of the primary spermatocytes, and these in turn form four spermatids. During the process, the number of chromosomes is reduced to half the number characteristic of the species. The spermatids then become converted into mature spermatozoa. This mode of transformation may be seen in Plate I. In the process, the nucleus of the spermatid forms a large part of the head: the centrosome divides, part passing to the extremities of the neck. One centrosome becomes the anterior, and remains attached to the head, while the other divides to form the posterior centrosome. The latter is divided into the anterior part, and the posterior nodule or annular ring. Besides this, the posterior centrosome becomes elongated to form the axial filament, and the cytoplasm forms the sheaths of the neck and tail. The spiral filament of the connecting piece is derived from the cytoplasmic mitochondria. At this time, a large part of the cellular cytoplasm is cast off. Meanwhile, the spermatozoa sink their heads into the long protoplasmic processes of the Sertoli or “nurse” cells which furnish nutritive material for their complete development. Finally the adult cells are cast off into the lumen. The structure of the spermatozoa, and a discussion of the semen will be taken up later.