Illustration of the various phases of human spermatogenesis depicted around the columnar Sertoli cells (orange) the only non-germ cell type in the seminiferous epithelium. From left to right: spermatogonia and spermatocytes with round spermatids (yellow) and two spermatocyte meiotic division spindles; next the spermatids begin early stage of elongation; enlarging pachytene spermatocytes and elongated spermatids; finally, spermatogonia dividing into new spermatocytes, new spermatids form from meiotic spermatocytes and mature sperm being released from the epithelium. The complete process of spermatogenesis in man takes about 70 days. From Kollmann, J. (1907) Handatlas der Entwicklungsgeschichte des Menschen, vol 2, Fig. 458. G. Fischer, Jena.