the status of the histologic middle segment of the epididymis in reptiles is incompletely defined (figure 1a). considering the wide variation in the anatomical structures of the four orders (crocodilia, sphenodontia, squamata, testudines) of reptilian epididymides and the paucity of histologic studies that correlate anatomical structure to histology, the evolution of the middle segment in reptiles, if present, remains to be delineated. in contrast, extensive studies of the echidna epididymis show that the monotreme epididymis lacks a middle segment. it is only in metatherian and eutherian mammals that a middle segment has been extensively documented.