The next change which takes place is the formation of another duct from the hinder portion of the Wolffian duct, which receives the secretion of the posterior segmental tubes. This secondary duct unites with the primary or Wolffian duct near its termination, and the primary ducts of the two sides unite together to open to the exterior by a common papilla. Slight modifications of the posterior terminations of these ducts are found in different genera of Selachians (vide Semper, _Centralblatt für Med. Wiss_. 1874, No. 59), but they are of no fundamental importance. These constitute the main changes undergone by the segmental duct of the kidneys and the ducts derived from it; but the segmental tubes also undergo important changes. In the majority of Selachians their openings into the body-cavity, or, at any rate, the openings of a large number of them, persist through life; but the investigations of Dr Meyer[32] render it very probable that the small portion of each segmental tube adjoining the opening becomes separated from the rest and becomes converted into a sort of lymph organ, so that the openings of the segmental tubes in the adult merely lead into lymph organs and not into the gland of the kidneys. Footnote 32: _Sitzun. der Naturfor. Gesellschaft_, Leipzig, 30 April, 1875. These constitute the whole changes undergone in the female, but in the male the open ends of a varying number (according to the species) of the segmental tubes become connected with the testis and, uniting with the testicular follicles, serve to carry away the seminal fluid[33]. The spermatozoa have therefore to pass through a glandular portion of the kidneys before they enter the Wolffian duct, by which they are finally carried away to the exterior. Footnote 33: We owe to Professor Semper the discovery of the arrangement of the seminal ducts. _Centralblatt f. Med. Wiss._ 1875, No. 12. In the adult female, then, there are the following parts of the urinogenital system (fig. 4): (1) The oviduct, or Müller's duct (fig. 4, _md._), split off from the segmental duct of the kidneys. Each oviduct opens at its upper end into the body-cavity, and behind the two oviducts have independent communications with the cloaca. The oviducts serve simply to carry to the exterior the ova, and have no communication with the glandular portion of the kidneys. [Illustration: FIG. 4. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT FEMALE SELACHIAN. _md._ Müllerian duct; _wd._ Wolffian duct; _st._ segmental tubes; _d._ duct of the posterior segmental tubes; _ov._ ovary.] (2) The Wolffian ducts (fig. 4, _wd._) or the remainder of the segmental ducts of the kidneys. Each Wolffian duct ends blindly in front, and the two unite behind to open by a common papilla into the cloaca. This duct receives the secretion of the whole anterior end of the kidneys[34], that is to say, of all the anterior segmental tubes.