There is at _pd_, the segmental duct of the kidneys, opening in front (_o_) into the body-cavity, and behind into the cloaca, and there are a series of convoluted segmental tubes (_st_), each opening at one end into the body-cavity, and at the other into the duct (_pd_). The next important change which occurs is the longitudinal division of the segmental duct of the kidneys into Müller's duct, or the oviduct, and the duct of the Wolffian bodies or Leydig's duct. The splitting[31] is effected by the growth of a wall of cells which divides the duct into two parts (fig. 3, _wd._ and _md._). It takes place in such a way that the front end of the segmental duct, anterior to the entrance of the first segmental tube, together with the ventral half of the rest of the duct, is split off from its dorsal half as an independent duct (vide fig. 2, _x_). Footnote 31: This splitting was first of all discovered and an account of it published by Semper (_Centralblatt f. Med. Wiss._ 1875, No. 29). I had independently made it out for the female a few weeks before the publication of Semper's account--but have not yet made observations about the point for the male. My own previous account of the origin of the Wolffian duct (_Quart. Journ. of Micros. Science_, Oct. 1874, and this edition, Paper V.), is completely false, and was due to my not having had access to a complete series of my sections when I wrote the paper. [Illustration: FIG. 3. TRANSVERSE SECTION OF A SELACHIAN EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MÜLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT. _mc._ medullary canal; _mp._ muscle-plate; _ch._ notochord; _ao._ aorta; _cav._ cardinal vein; _st._ segmental tube. On the one side the section passes through the opening of a segmental tube into the body-cavity. On the other this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through; _wd._ Wolffian duct; _md._ Müllerian duct. The Müllerian duct and the Wolffian duct together constitute the primitive segmental duct; _gr._ The germinal ridge with the thickened germinal epithelium; _l._ liver; _i._ intestine with spiral valve.] The dorsal portion also forms an independent duct, and into it the segmental tubes continue to open. Such at least is the method of splitting for the female--for the male the splitting is according to Professor Semper, of a more partial character, and consists for the most part in the front end of the duct only being separated off from the rest. The result of these changes is the formation in both sexes of a fresh duct which carries off the excretions of the segmental involutions, and which I shall call the Wolffian duct--while in the female there is formed another complete and independent duct, which I shall call the Müllerian duct, or oviduct, and in the male portions only of such a duct.