This knob projects outwards toward the epiblast, and the column lies at first between the mesoblast and epiblast. The knob and column do not long remain solid. The knob becoming hollow acquires a wide opening into the pleuro-peritoneal or body-cavity, and the column a lumen; so that by the time that five visceral clefts have appeared, the two together form a duct closed behind, but communicating in front by a wide opening with the pleuro-peritoneal cavity. Before these changes are accomplished, a series of _solid_[29] outgrowths of elements of the 'intermediate cell-mass' appear at the uppermost corner of the body-cavity. These soon become hollow and appear as involutions from the body-cavity, curling round the inner and dorsal side of the previously formed duct. Footnote 29: These outgrowths are at first solid in both Pristiurus, Scyllium and Torpedo, but in Torpedo attain a considerable length before a lumen appears in them. One involution of this kind makes its appearance for each protovertebra, and the first belongs to the protovertebra immediately behind the anterior end of the duct whose development has just been described. In Pristiurus there are in all 29 of these at this period. The last two or three arise from that portion of the body-cavity, which at this stage still exists behind the anus. The first-formed duct and the subsequent involutions are the rudiments of the whole of the urinary system. The duct is the primitive duct of the kidney[30]; I shall call it in future _the segmental duct_; and the involutions are the commencements of the segmental tubes which constitute the body of the kidney. I shall call them in future _segmental tubes_. Footnote 30: This duct is often called either Müller's duct, the oviduct, or the duct of the primitive kidneys 'Urnierengang.' None of these terms are very suitable. A justification of the name I have given it will appear from the facts given in the later parts of this paper. In my previous paper I have always called it oviduct, a name which is very inappropriate. Soon after their formation the segmental tubes become convoluted, and their blind ends become connected with the segmental duct of the kidney. At the same time, or rather before this, the blind posterior termination of each of the segmental ducts of the kidneys unites with and opens into one of the horns of the cloaca. At this period the condition of affairs is represented in Fig. 2. [Illustration: FIG. 2. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN A SELACHIAN EMBRYO. _pd._ segmental duct. It opens at _o_ into the body-cavity and at its other extremity into the cloaca; _x._ line along which the division appears which separates the segmental duct into the Wolffian duct above and the Müllerian duct below; _st._ segmental tubes. They open at one end into the body-cavity, and at the other into the segmental duct.]