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.