Ultrastructure of the ductus epididymidis of the echidna, Tachyglossus aculeatus.

It is concluded that the ductus epididymidis of the echidna is divided into only two structurally distinct segments which are each homogeneous along their length. The initial segment in the echidna is structurally very similar to the initial segment proper in the epididymis of all other mammals which have been studied, whereas the terminal segment is structurally quite different from the terminal segment in other mammals. The terminal segment in the echidna is involved in considerable apocrine secretion of highly membranous material. The secretions gradually degenerate and occupy a large proportion of the lumen of the duct.