Structural differentiation of the male genital ducts of the echidna (Tachyglossus aculeatus).

Seven ductuli efferentes radiate from the testis of the echidna and join the ductus epididymidis either directly or after joining one of their neighbours. They are pigmented brown and appear to be structurally and functionally similar to the ductuli efferentes of therian mammals. The epididymis is anatomically differentiated into a large head and small tail which appear to be, respectively, larger and smaller than the similar regions of the epididymis of scrotal mammals; they also contain, respectively, larger and smaller proportions of the animals' extragonadal spermatozoa. Only the head of the epididymis is adjacent to the testis: the tail and the ductus deferens are distal to the testis. The ductus epididymis is also histologically differentiated into two segments (initial and terminal segments) which correspond precisely with the anatomical differentiation. The initial segment is structurally similar to the initial segment of the epididymis of scrotal mammals (i.e. tall epithelium with long stereocilia, a farily homogeneous supranuclear cytoplasma containing Golgi apparatus and a low concentration of spermatozoa in the lumen). The terminal segment has adaptations of the duct (as in scrotal mammals) for the storage of spermatozoa such as a wide lumen containing a high concentration of spermatozoa, low epithelium and thick layers of periductal muscle. However, it is peculiar in that the duct epithelium is thrown into folds and it is involved in considerable apocrine secretion.