The epididymis, which is a tube that connects a testicle to a vas deferens in the male reproductive system, evolved by retention of the mesonephric duct during regression and replacement of the mesonephros with the metanephric kidney. Similarly, during embryological involution of the paired mesonephric kidneys, each mesonephric duct is retained to become the epididymis, vas deferens, seminal vesicle and ejaculatory duct (Wolffian duct). In reptiles and birds both the testes and excurrent ducts (efferent ducts, epididymis, vas deferens) occur in an intra-abdominal location (testicond). Primitive mammals, such as the monotremes (prototheria), also are testicond. Marsupial (metatheria) and placental (eutheria) mammals exhibit differing degrees of testicular descent into an extra-abdominal scrotum. In scrotal mammals the epididymis is attached to the testes in an extra-abdominal position where the cauda epididymis extends beyond the lowest extremity of the testis. Hence, the cauda epididymis is exposed to the coolest of temperatures compared to all other reproductive structures. Whereas testicond reptiles contain an excurrent duct system, they lack male reproductive glands (absent seminal vesicles, prostate, bulbourethral glands). Monotreme mammals are also testicond (like reptiles) and contain some, but not all (absent seminal vesicles) of the male reproductive glands observed in most metatherian and eutherian mammals. This combination of reptilian and mammalian structures within the monotreme reproductive tract has informed the evolution of the male reproductive tract in mammals. For example, the intra-abdominal low sperm storage capacity of the echidna (Tachyglossus aculeatus) epididymis informed the role of the epididymis as the prime mover in the evolution of descended testes in mammals as it relates to lower extra-gonadal temperatures enhancing epididymal sperm storage in scrotal mammals. Furthermore, the structure of the monotreme reproductive tract also informed prostate evolution in monotreme mammals. Structural differentiation of the epididymis in reptiles The reptilian testis and epididymis typically undergo seasonal recrudescence coupled to the breeding season. All reptiles retain their testes and excurrent ducts within the abdomen (testicond). Generally, the reptilian epididymis does not exhibit the same degree of anatomical regionalization compared to scrotal mammals (Figure 1). Indeed, the anatomical appearance of the epididymis of many reptiles appears much more similar to the epididymis of monotremes than scrotal mammals (Figure 1). Anatomically, the gross morphologic features of the reptilian epididymis can vary between species, with some species of reptiles exhibiting just two anatomical regions whereas others (snakes) may exhibit no observable regionalization of the epididymis. A reptilian histologic initial segment