These considerations fully explain the secondary segmentation of the vertebræ by which they become opposite the inter-muscular septa. On the other hand, the primary segmentation is clearly a remnant of the time when no vertebral bodies were present, and has no greater morphological significance than the fact that the cells to form the unsegmented investment of the notochord were derived from the segmented muscle-plates, and only secondarily became fused into a continuous tube. _The Urinogenital System._ The first traces of the urinary system become visible at about the time of the appearance of the third visceral cleft. At about this period the somatopleure and splanchnopleure become more or less fused together at the level of the dorsal aorta, and thus, as has been already mentioned, each of the original plates of mesoblast becomes divided into a vertebral plate and lateral plate (Pl. 11, fig. 6). The mass of cells resulting from this fusion corresponds with Waldeyer's intermediate cell-mass in the Fowl. At about the level of the fifth protovertebra the first trace of the urinary system appears. From the intermediate cell-mass a solid knob grows outwards towards the epiblast (woodcut, fig. 4, _pd_). This knob consists at first of 20-30 cells, which agree in character with the neighbouring cells of the intermediate cell-mass, and are at this period rounded. It is mainly, if not entirely, derived from the somatic layer of the mesoblast. From this knob there grows backwards a solid rod of cells which keeps in very close contact with the epiblast, and rapidly diminishes in size towards its posterior extremity. Its hindermost part consists in section of at most one or two cells. It keeps so close to the epiblast that it might be supposed to be derived from that layer were it not for the sections shewing its origin from the knob above mentioned. We have in this rod the commencement of what I have elsewhere[224] called the segmental duct. Footnote 224: "Urinogenital Organs of Vertebrates," _Journ. of Anat. and Phys._ Vol. X. [This Edition, No. VII.] [Illustration: FIG. 4. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS. The sections are to shew the development of the segmental duct (_pd_) or primitive duct of the kidneys. In _A_ (the anterior of the two sections) this appears as a solid knob projecting towards the epiblast. In _B_ is seen a section of the column which has grown backwards from the knob in _A_. _spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._ notochord; _X._ string of cells below the notochord; _mp._ muscle-plate; _mp´._ specially developed portion of muscle-plate; _ao._ dorsal aorta; _pd._ segmental duct; _so._ somatopleura; _sp._ splanchnopleura; _pp._ pleuro-peritoneal or body-cavity; _ep._ epiblast; _al._ alimentary canal.]