By stage O all the ureters have become prolonged up to the cloacal end of the Wolffian duct, so that the anterior one has a length equal to that of the whole kidney proper. For the most part they acquire independent openings into the end section of the Wolffian duct, though some of them unite together before reaching this. The general appearance of the hindermost of them between stages N and O is shewn in longitudinal and vertical section in Pl. 21, fig. 8, u.
They next commence to develop into complete and independent tubes by their side walls growing inwards and meeting below so as to completely enclose their lumen. This is seen already to have occurred in most of the posterior ureters in Pl. 21, fig. 8.
Before stage P the ureters cease to be united into a continuous ridge, and each becomes separated from its neighbours by a layer of indifferent tissue: by this stage, in fact, the ureters have practically attained very nearly their adult condition. The general features of a typical section through them are shewn on Pl. 21, fig. 5. The figure represents the section of a female embryo, not far from the cloaca. Below is the oviduct (od). Above this again is the Wolffian duct (w.d), and still dorsal to this are four ureters (u). In female embryos more than four ureters are not usually to be seen in a single section. This is probably owing to the persistence, in some instances, of the intimate connection between the ureters found at an earlier stage of development, and results in a single ureter coming to serve as the collecting duct for several segments. A section through a male embryo of stage P would mainly differ from that through a female in the absence of the oviduct, and in the presence of probably six[353], instead of four, ureters.
The exact amount of fusion which takes place between the ureters, and the exact number of the ureters, cannot easily be determined from sections, but the study of sections is chiefly of value in shewing the general nature of the changes which take place in the process of attaining the adult condition.
It may be noticed, as a consequence of the above account, that the formation of the ureters takes place by a growth of the original segmental tubes, and not by a splitting off of parts of the wall of the Wolffian duct.
The formation of ureters in Scyllium, which has been only very cursorily alluded to by Professor Semper, appears to differ very considerably from that in Acanthias as narrated by him.
The Vasa Efferentia.
A comparison of the results of Professor Semper on Elasmobranchii, and Dr Spengel on Amphibians, suggests several interesting questions with reference to the development of the vasa efferentia, and the longitudinal canal of the Wolffian body.
Professor Semper was the first to describe the adult anatomy and development of vasa efferentia in Elasmobranchii, and the following extracts will fully illustrate his views with reference to them.
“In[354] dem frühesten Stadium finden sich wie früher angegeben ungefahr 34 Trichter in der Leibeshöhle, von diesen gehen die 27 hintersten in die persistirenden Segmentaltrichter über, von denen 4 beim erwachsenen Thiere auf dem Mesorchium stehen. Die übrigen 7 schliessen sich vollständig ab zu den erwähnten länglichen und später mannigfach auswachsenden varicösen Trichterblasen; von diesen sind es wiederum 3-4 welche untereinander in der Längsrichtung verwachsen und dadurch den in der Basis der Hodenfalte verlaufenden Centralcanal des Hodens bilden. Ehe aber diese Verwachsung zu einem mehr oder minder geschlängelten Centralcanal vollständig wird, hat sich einmal das Lumen der Trichterblasen fast vollständig geschlossen und ausserdem von ihnen aus durch Verwachsung und Knospung die erste Anlage des rete vasculosum Halleri gebildet (Taf. XX. Figs. 1, 2c). Es erstreckt sich nämlich mehr oder minder weit in die Genitalfalte hinein ein unregelmässiges von kleinen Zellen begränztes Canalnetz welches zweifellos mit dem noch nicht ganz vollständigen Centralcanale des Hodens (Taf. XX. Fig. 2c) in Verbindung steht. Von diesem letzteren aus gehen in regelmässigen Abständen die Segmentalgänge (Taf. XX. Fig. 2 sg.) gegen die Niere hin; da sie meist stark geneigt oder selbst geschlängelt (bei 6ctm langen Embryonen) gegen die Niere zu verlaufen, wo sie sich an die primären Malpighi'schen Körperchen und deren Bildungsblasen ansetzen, so kann ein verticaler Querschnitt auch nie einen solchen nun zum vas efferens gewordenen Segmentalgang seiner ganzen Länge nach treffen. Gegen die Trichterfurche zu aber steht namentlich am hinteren Theile der Genitalfalte der Centralcanal häufig noch durch einen kurzen Zellstrang mit dem Keimepithel der Trichterfurche in Verbindung; mitunter findet sich hier sogar noch eine kleine Höhlung, Rest des ursprünglich hier vorhandenen weiten Trichters” (Taf. XX. Fig. 3c).