FIG. 1. The various steps of spermatogenesis in man. Labels from the top to the bottom: Ad, dark type A spermatogonium; Ap, pale type A spermatogonium; B, type B spermatogonium; M, mitoses of Ap or B spermatogonia; P1, preleptotene primary spermatocytes; L, Z, EP, MP and LM correspond to the various stages of the prophase of primary spermatocytes (i.e., leptotene, zygotene, early, mid and late pachytene); II, secondary spermatocytes, Div 1 and Div 2 correspond to maturation divisions of the first and secondary spermatocytes; in spermatocytes and spermatids the following elements are labelled: G, Golgi apparatus, n, nucleolus; N, nucleus, A, acrosome; c, centrioles; t, tail; m, mitochondria; RB, residual body. The spermatozoon is seen in side view (left) or face view (right).