Footnote 34: This upper portion of the kidneys is called Leydig's gland by Semper. It would be better to call it the Wolffian body, for I shall attempt to shew that it is homologous with the gland so named in Sauropsida and Mammalia. (3) The secondary duct (fig. 4, _d._) belonging to the lower portion of the kidneys opening into the former duct near its termination. (4) The segmental tubes (fig. 4, _st_) from whose convolutions and outgrowths the kidney is formed. They may be divided into two parts, according to the duct by which their secretion is carried off. In the male the following parts are present: (1) The Müllerian duct (fig. 5, _md._), consisting of a small remnant, attached to the liver, which represents the foremost end of the oviduct of the female. (2) The Wolffian duct (fig. 5, _wd_), which precisely corresponds to the Wolffian duct of the female, except that, in addition to functioning as the duct of the anterior part of the kidneys, it also serves to carry away the semen. In the female it is straight, but has in the adult male a very tortuous course (vide fig. 5). [Illustration: FIG. 5. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT MALE SELACHIAN. _md._ rudiment of Müllerian duct; _wd._ Wolffian duct, which also serves as vas deferens; _st._ segmental tubes. The ends of three of those which in the female open into the body-cavity, have in the male united with the testicular follicles, and serve to carry away the products of the testis; _d._ duct of the posterior segmental tubes; _t._ testis.] (3) the duct (fig. 5, _d._) of the posterior portion of the kidneys, which has the same relations as in the female. (4) The segmental tubes (fig. 5, _st._). These have the same relations as in the female, except that the most anterior two, three or more, unite with the testicular follicles, and carry away the semen into the Wolffian duct. * * * * * The mode of arrangement and the development of these parts suggest a number of considerations. In the first place it is important to notice that the segmental tubes develop primitively as completely independent organs[35], one of which appears in each segment. If embryology is in any way a repetition of ancestral history, it necessarily follows that these tubes were primitively independent of each other. Ancestral history, as recorded in development, is often, it is true, abridged; but it is clear that though abridgement might prevent a series of primitively separate organs from appearing as such, yet it would hardly be possible for a primitively compound organ, which always retained this condition, to appear during development as a series of separate ones. These considerations appear to me to prove that the segmented ancestors of vertebrates possessed a series of independent and segmental excretory organs.