of the epididymis has been extensively documented in several species homologous to the initial segment of mammals. The initial segment of the epididymis, first described in the guinea pig epididymis, is a histologically distinct region of tall pseudostratified columnar epithelium that receives spermatozoa from the ductuli efferentes (Figure 1). The epididymis is the primary sperm storage organ in male reptiles. In all reptiles and mammals the sperm storage region of the epididymis can objectively be identified as that distal extremity of the epididymis that exhibits a widened diameter of duct which contains additional layers of circumferential smooth muscle capable of contraction during ejaculation in direct continuity with the vas deferens (Figure 1). This sperm storage region has been described as the anatomical cauda epididymis or the histologic terminal segment of the epididymis. The caudal region of the reptilian epididymis, where sperm are stored, is an anatomical extension that narrows into a conical shape before forming the vas deferens. The coiled epididymal duct within the cauda epididymis does not appear to be particularly long, and so may be limited in its capacity to store sperm in comparison to scrotal mammals. Limited sperm storage in the reptilian epididymis may be circumvented by the ability of female reptiles to store viable spermatozoa within their reproductive tract for utilization months or years after insemination. A competing reproductive strategy to long-term sperm storage that explains the production of offspring after prolonged periods in the absence of males is facultative parthenogenesis. In either case, these female reproductive strategies may have evolved to counter limited sperm storage in the reptilian male epididymis. Structural differentiation of the epididymis in monotreme mammals The monotremes (short beaked echidna, long beaked echidna, platypus) are testicond seasonal breeding mammals that exhibit some characteristics of the reproductive tract found in reptiles (e.g. testicond, presence of a cloaca). The fully developed monotreme epididymis exhibits two anatomical regions, similar to some reptiles. The two anatomical regions of the monotreme epididymis closely correspond to just two histologic regions (Figure 1B), an initial segment and a terminal segment. Structural differentiation of the epididymis into just an initial segment and terminal segment, with no intervening middle segment, has also subsequently been observed as far back as the epididymis of sharks. In the monotreme echidna, the initial segment, where sperm undergo maturation, is much larger than the terminal segment (Figure 1B), the later segment being the sperm storage region