the histologic terminal segment is the sperm storage region of the epididymis in reptiles, monotremes and both metatherian and eutherian mammals (figure 1). the testicond epididymis (reptiles and monotremes) has a limited sperm storage capacity compared with the scrotal epididymis (metatherian and eutherian mammals), which has a much larger terminal segment to accommodate increased sperm storage. it is the cooler temperature of the scrotal epididymis that reduces oxidative respiration of sperm in the terminal segment, thereby increasing oxygen availability to store more sperm per unit volume of duct, thus informing the evolution of descended testes in mammals.