Diagram of a concept of the organisation of germ cells in the seminiferous epithelium of the human testis. The sequence of germ cell maturation is shown as a spiral gyrating inwards to the lumen of the seminiferous tubule. This pattern is found in the human and several no-human primates but uncommon in most other animals. Ad,Ap,B = type A dark/pale, and B spermatogonia; PL, L, Z, P, II = preleptotene, leptotene, zygotene, pachytene, meiosis II spermatocytes; S = various types of spermatids; Rb = spermatid residual body.