I may, perhaps, at this point, be permitted to say a few words about my original account of the development of the Wolffian duct This account was incorrect, and based upon a false interpretation of an imperfect series of sections, and I took the opportunity, in a general account of the urinogenital system of Vertebrates, to point out my mistake[352]. Professor Semper has, however, subsequently done me the honour to discuss, at considerable length, my original errors, and to attempt to explain them. Since it appears to me improbable that the continuation of such a discussion can be of much general interest, it will suffice to say now, that both Professor Semper's and my own original statements on the development of the Wolffian duct were erroneous; but that both of us have now recognised our mistakes; and that the first morphologically correct account of the development was given by him.
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With reference to the formation of the urinal cloaca there is not much to say. The originally widely separated openings of the two Wolffian ducts gradually approximate in both sexes. By stage O (Pl. 19, fig. 1b) they are in close contact, and the lower ends of the two ducts actually coalesce at a somewhat later period, and open by a single aperture into the common cloaca. The papilla on which this is situated begins to make its appearance considerably before the actual fusion of the lower extremities of the two ducts.
Formation of Wolffian Body and Kidney proper.
Between stages L and M the hindermost ten or eleven segments of the primitive undivided excretory organ commence to undergo changes which result in their separation from the anterior segments as a distinct gland, which was spoken of in the description of the adult as the kidney proper, while the unaltered preceding segments of the kidney were spoken of as the Wolffian body.
It will be remembered that each segment of the embryonic kidney consists of four divisions, the last or fourth of which opens into the Wolffian duct. The changes which take place in the hindermost ten or eleven segments, and cause them to become distinguished as the kidney proper, concern alone the fourth division of each segment, which becomes prolonged backwards, and its opening into the Wolffian duct proportionately shifted. These changes affect the foremost segments of the kidney much more than the hindermost, so that the fourth division in the foremost segments becomes very much longer than in the hindermost, and at last all the prolongations of the kidney segments come to open nearly on the same level, close to the cloacal termination of the Wolffian duct (Pl. 21, fig. 8). The prolongations of the fourth division of the kidney-segments have already (p. [481]) been spoken of in the description of the adult as ureters, and this name will be employed for them in the present section.
The exact manner in which the changes, that have been briefly related, take place is rather curious, and very difficult to unravel without the aid of longitudinal sections. First of all, the junction between each segment of the kidney and the Wolffian duct becomes so elongated as to occupy the whole interval between the junctions of the two neighbouring segments. The original opening of each tube into the Wolffian duct is situated at the anterior end of this elongated attachment, the remaining part of the attachment being formed solely of a ridge of cells on the dorsal side of the Wolffian duct. The general character of this growth will be understood by comparing figs. 7a and 7b, Pl. 21—two longitudinal vertical sections through part of the kidneys. Fig. 7 a shews the normal junction of a segmental tube with the Wolffian duct in the Wolffian body, while in figure 7b (r.u) is shewn the modified junction in the region of the kidney proper in the same embryo. The latter of these figures (fig. 7b) appears to me to prove that the elongation of the attachments between the segmental tubes and Wolffian duct takes place entirely at the expense of the former. Owing to the length of this attachment, every transverse section through the kidney proper at this stage either presents a solid ridge of cells closely adhering to the dorsal side of the Wolffian duct, or else passes through one of the openings into the Wolffian duct.
During stage M the original openings of the segmental tubes into the Wolffian duct appear to me to become obliterated, and at the same time the lumen of each ureter is prolonged into the ridge of cells on the dorsal wall of the duct.
Both of these changes are illustrated in my figures. The fact of the obliteration of the original opening into the Wolffian duct is shewn in longitudinal section in Pl. 21, fig. 9, u, but more conclusively in the series of transverse sections represented on Pl. 21, figs. 3A, 3B, 3C. In the hindermost of these (3C) is seen the solid terminal point of a ureter, while the same ureter possesses a lumen in the two previous sections, but exhibits no signs of opening into the Wolffian duct. Sections may however be met with which appear to shew that in some instances the ureters still continue to open into the Wolffian duct, but these I find to be rare and inconclusive, and am inclined to regard them as abnormalities. The prolongation of the lumen of the ureters takes place in a somewhat peculiar fashion. The lumen is not, as might be expected, completely circumscribed by the wall of the ureter, but only dorsally and to the sides. Ventrally it is closed in by the dorsal wall of the Wolffian duct. In other words, each ureter is at first an incomplete tube. This peculiarity is clearly shewn in the middle figure of the series on Pl. 21, fig. 3B.
During stages M and N the ureters elongate considerably, and, since the foremost ones grow the most rapidly, they soon come to overlap those behind. As each ureter grows in length it remains an incomplete tube, and its lumen, though proportionately prolonged, continues to present the same general relations as at first. It is circumscribed by its proper walls only dorsally and laterally; its floor being formed in the case of the front ureter by the Wolffian duct, and in the case of each succeeding ureter by the dorsal wall of the ureter in front. This is most easily seen in longitudinal sections, and is represented on Pl. 21, fig. 9, or on a larger scale in fig. 9A. In the latter figure it is especially clear that while the wall on the dorsal side of the lumen of each ureter is continuous with the dorsal wall of the tubulus of its own segment, the wall on the ventral side is continuous with the dorsal wall of the ureter of the preceding segment. This feature in the ureters explains the appearance of transverse sections in which the ureters are not separate from each other, but form together a kind of ridge on the dorsal side of the Wolffian duct, in which there are a series of perforations representing the separate lumens of the ureters (Pl. 21, fig. 4). The peculiarities in the appearance of the dorsal wall of the Wolffian duct in fig. 9A, and the difference between the cells composing it and those of the ventral wall, become intelligible on comparing this figure with the representation of transverse section in figs. 3B and 3C, and especially in fig. 4. Most of the ureters continue to end blindly at the close of stage N, and appear to have solid posterior terminations like that of the Müllerian duct in Birds.