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Résumé.—The first trace of the urinary system makes its appearance as a knob springing from the intermediate cell-mass opposite the fifth protovertebra (woodcut, fig. 5A, p.d). This knob is the rudiment of the abdominal opening of the segmental duct, and from it there grows backwards to the level of the anus a solid column of cells, which constitutes the rudiment of the segmental duct itself (woodcut, fig. 5B, p.d). The knob projects towards the epiblast, and the column connected with it lies between the mesoblast and epiblast. The knob and column do not long remain solid, but the former acquires an opening into the body-cavity continuous with a lumen, which makes its appearance in the latter.
Fig. 5.
Two Sections of a Pristiurus Embryo with three visceral clefts.
The sections illustrate 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 (pd) 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. somatopleure; sp. splanchnopleure; pp. pleuro-peritoneal or body-cavity; ep. epiblast; al. alimentary canal.
While the lumen is gradually pushing its way backwards along the solid rudiment of the segmental duct, the first traces of the segmental tubes, or proper excretory organs, make their appearance in the form of solid outgrowths of the intermediate cell-mass, which soon become hollow and open into the body-cavity. Their blind ends curl obliquely backwards round the inner and dorsal side of the segmental duct. One segmental tube makes its appearance for each protovertebra, commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a short way behind the anus. Soon after their formation the blind ends of the segmental tubes open into the segmental duct, and each of them becomes divided into four parts. These are (woodcut 7) (1) a section carrying the abdominal opening or segmental tube proper, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2) and terminating in (4), a wider portion opening into the segmental duct. At the same time, or shortly before this, each segmental duct unites with and opens into one of the horns of the cloaca, and also retires from its primitive position between the epiblast and mesoblast, and assumes a position close to the epithelium lining the body-cavity. The general features of the excretory organs at this period are diagrammatically represented on the woodcut, fig. 6. In this fig. p.d is the segmental duct and o its abdominal opening. s.t points to the segmental tubes, the finer details of whose structure are not represented in the diagram. The kidneys thus form at this period an unbroken gland composed of a series of isolated coiled tubes, one extremity of each of which opens into the body-cavity, and the other into the segmental duct, which forms the only duct of the kidney, and communicates at one end with the body-cavity, and at the other with the cloaca.
Fig. 6.
Diagram of the primitive condition of the Kidney in an Elasmobranch Embryo.