the epididymis of marsupials (metatherians) and placental mammals (eutherians) has undergone further structural differentiation compared to that observed in prototherian mammals (figure 1). in scrotal mammals, an initial segment is nearly always observed, however, additional histologically distinct regions have developed between the initial segment and the distal sperm storage region (terminal segment). these intervening histologic regions have been referred to as the middle segment. the histologic regions of the middle segment (figure 1c) can vary in number in metatherian and eutherian species of mammals. beyond the histologic regions of the middle segment, the sperm storage region (anatomical cauda, histologic terminal segment) of scrotal mammals has enlarged to accommodate enhanced storage of sperm (figure 1c). the storage of sperm in the scrotal epididymis is enhanced by cooler extra-abdominal temperatures. indeed, experimental reflection of one epididymis into the warmer temperature of an abdominal location reduced sperm storage capacity by 75% compared to the contralateral epididymis that remained in the scrotum. significantly, cooler scrotal temperatures reduces oxidative respiration of sperm, thereby increasing oxygen availability to store more sperm per unit volume of duct, which has informed the evolution of descended testes in mammals.