a histologically distinct initial segment of the epididymis is widely observed in many species of reptiles and even as far back as sharks. a large initial segment is also present in the epididymis of the testicond monotreme echidna. furthermore, the scrotal epididymis of metatherian and eutherian mammals nearly all exhibit an initial segment. hence, the initial segment of the epididymis is well conserved in testicond vertebrates (reptiles, monotremes) and in scrotal metatherian and eutherian mammals (figure 1).