segment of the epididymis, the histologic description of the epididymis consisting of an initial segment, middle segment and terminal segment provides a harmonized characterization that allows direct comparisons of homologous segments across species. Summary and conclusion The evolution of the epididymis from reptiles to mammals (Figure 1) entailed: Retention of the histologic initial segment. To varying degrees, elaboration of a histologic middle segment. An increase in the length, volume and size of the histologic terminal segment of scrotal mammals, whereby the lower extra-abdominal (scrotal) temperature increased oxygen availability to sustain and store more sperm, thus providing a physiologic mechanism for the evolution of descended testes in mammals. References Evolution of tetrapods Scrotum