In my youngest example, in which the splitting had commenced, there were two separate ducts for only 14 sections, and in a slightly older one for about 18. In the second of these embryos the part of the segmental duct anterior to the front end of the Wolffian duct, which is converted directly into the oviduct, extended through 48 sections. In the space included in these 48 sections at least five, and I believe six, segmental tubes with openings into the body-cavity were present. These segmental tubes did not however unite with the oviduct, or at best, but one or two rudimentary junctions were visible, and the evidence of my earlier embryos appears to shew that the segmentaltubes in front of the Wolffian duct never become in the female united with the segmental duct. The anterior end of the Wolffian duct is very much smaller than the oviduct adjoining it, and as the reverse holds good in the male, an easy method is afforded of distinguishing the two sexes even at the earliest period of the formation of the Wolffian duct.
Hitherto merely the general features of the development of the oviduct and Wolffian duct have been alluded to, but a careful inspection of any good series of sections, shewing the junction of these two ducts, brings to light some features worth noticing in the formation of the oviduct. It might have been anticipated that, where the two ducts unite behind as the segmental duct, their lumens would have nearly the same diameter, but normally this appears to be far from the case.
To illustrate the formation of the oviduct I have represented a series of sections through a junction in an embryo in which the splitting into two ducts had only just commenced (Pl.21, fig. 1), but I have found that the features of this series of sections are exactly reproduced in other series in which the splitting has extended as far back as the end of the small intestine. In the series represented (Pl.21) 1A is the foremost section, and 1D the hindermost. In 1A the oviduct (od) is as large or slightly larger than the Wolffian duct (w.d), and in the section in front of this (which I have not represented) was considerably the larger of the two ducts. In 1B the oviduct has become markedly smaller, but there is no indication of its lumen becoming united with that of the Wolffian duct—the two ducts, though in contact, are distinctly separate. In 1C the walls of the two ducts have fused, and the oviduct appears merely as a ridge on the under surface of the Wolffian duct, and its lumen, though extremely minute,shews no sign of becoming one with that of the Wolffian duct. Finally, in 1D the oviduct can merely be recognised as a thickening on the under side of the segmental duct, as we must now call the single duct, but a slight bulging downwards of the lumen of the segmental duct appears to indicate that the lumens of the two ducts may perhaps have actually united. But of this I could not be by any means certain, and it seems quite possible that the lumen of the oviduct never does open into that of the segmental duct.
The above series of sections goes far to prove that the posterior part of the oviduct is developed as a nearly solid ridge split off from the under side of the segmental duct, into which at the utmost a very small portion of the lumen of the latter is continued. One instance has however occurred amongst my sections which probably indicates that the lumen of the segmental duct may sometimes, in the course of the formation of the oviduct and Wolffian duct, become divided into two parts, of which that for the oviduct, though considerably smaller than that for the Wolffian duct, is not so markedly so as in normal cases (Pl.21, fig. 2).
Professor Semper states that the lumen of the part of the oviduct split off from the hindermost end of the segmental duct becomes continuously smaller, till at last close to the cloaca it is split off as a solid rod of cells without a lumen, and thus it comes about that the oviduct, when formed, ends blindly, and does not open into the cloaca till the period of sexual maturity. My own sections do not include a series shewing the formation of a terminal part of the oviduct, but Semper's statements accord precisely with what might probably take place if my account of the earlier stages in the development of the oviduct is correct. The presence of a hymen in young female Elasmobranchii was first made known by Putmann and Garman[348], and subsequently discovered independently by Semper[349].
The Wolffian duct appears to receive its first segmental tube at its anterior extremity.
In the male the changes of the original segmental duct have a somewhat different character to those in the female, although there is a fundamental agreement between the two sexes. As in the female, a horizontal split makes its appearance a short way behind the front end of the segmental duct, and divides this into a dorsal Wolffian duct and a ventral Müllerian duct, the latter continuous with the anterior section of the segmental duct, which carries the abdominal opening. The differences in development between the two sexes are, in spite of a general similarity,very obvious. In the first place, the ventral portion split off from the segmental duct, instead of being as in the female larger in front than the Wolffian duct, is very much smaller; while behind it does not form a continuous duct, but in some parts a lumen is present, and in others again absent (Pl.21, fig. 6). It does not even form an unbroken cord, but is divided in disconnected portions. Those parts with a lumen do not appear to open into the Wolffian duct.
The process of splitting extends gradually backwards, so that there is a much longer rudimentary Müllerian duct by stage O than by stage N. By stage P the posterior portions of the Müllerian ducts have vanished. The anterior parts remain, as has been already stated, till adult life. A second difference between the male and female depends on the fact that, in the male, the splitting of the segmental duct into Müllerian duct and Wolffian duct never extends beyond the hinder extremity of the small intestine. A third and rather important point of difference consists in the splitting commencing far nearer the front end of the segmental duct in the male than in the female. In the female it was shewn that about 48 sections intervened between the front end of the segmental duct and the point where this became split, and that this region included five or six segmental tubes. In the male the homologous space only occupiesabout 7 to 12 sections, and does not contain the rudiment of more than a single segmental tube. Although my sections have not an absolutely uniform thickness, yet the above figures suffice to shew in a conclusive manner that the splitting of the segmental duct commences far further forwards in the male than in the female. This difference accounts for two facts which were mentioned in connection with the excretory organs of the adult,viz.(1) the greater length of the Wolffian body in the male than in the female, and (2) the fact that although a nearly similar number of segmental tubes persist in the adults of both sexes, yet that in the male there are five or six more segments in front of the first fully developed segmental opening than in the female.
The above description of the formation of the Müllerian duct in the male agrees very closely with that of Professor Semper for Acanthias. For Scyllium however he denies, as it appears tome erroneously, the existence of the posterior rudimentary parts of the Müllerian duct. He further asserts that the portions of the Müllerian duct with a lumen open into the Wolffian duct. The most important difference, however, between Professor Semper's and my own description consists in his having failed to note that the splitting of the segmental duct commences much further forwards in the male than in the female.
I have attempted to shew that the oviduct in the female, with the exception of the front extremity, is formed as a nearly solid cord split off from the ventral surface of the segmental duct, and not by a simple splitting of the segmental duct into two equal parts. If I am right on this point, it appears to me far easier to understand the relationship between the oviduct or Müllerian duct of Elasmobranchii and the Müllerian duct of Birds, than if Professor Semper's account of the development of the oviduct is the correct one. Both Professor Semper and myself have stated our belief in the homology of the ducts in the two cases, but we have treated their relationship in a very different way. Professor Semper[350]finds himself compelled to reject, on theoretical grounds, the testimony of recent observers on the development of the Müllerian duct in Birds, and to assert that it is formed out of the Wolffian duct, or, according to my nomenclature, 'the segmental duct.' In my account[351], the ordinary statements with reference to the development of the Müllerian duct in Birds are accepted; but it is suggested that the independent development of the Müllerian duct may be explained by the function of this duct in the adult having, as it were, more and more impressed itself upon the embryonic development, till finally all connection, even during embryonic life, between the oviduct and the segmental duct (Wolffian duct) became lost.
Since finding what a small portion of the segmental duct became converted into the Müllerian duct in Elasmobranchii, I have reexamined the development of the Müllerian duct in the Fowl, in the hope of finding that its posterior part might develop nearly in the same manner as in Elasmobranchii, at the expense of a thickening of cells on the outer surface of the Wolffian duct.I have satisfied myself, in conjunction with Mr Sedgwick, that this is not the case, and that the general account is in the main true; but at the same time we have obtained evidence which tends to shew that the cells which form the Müllerian duct are in part derived from the walls of the Wolffian duct. We propose giving a full account of our observations on this point, so that I refrain from mentioning further details here. It may however be well to point out that, apart from observations on the actual development of the Müllerian duct in the Bird, the fact of its abdominal opening being situated some way behind the front end of the Wolffian duct, is of itself a sufficient proof that it cannot be the metamorphosed front extremity of the Wolffian (= segmental) duct, in the same way that the abdominal opening of the Müllerian duct is the front extremity of the segmental duct in Elasmobranchii.
Although the evidence I can produce in the case of the Fowl of a direct participation of the Wolffian duct in the formation of the Müllerian is not of an absolutely conclusive kind, yet I am inclined to think that the complete independence of the two ducts, if eventually established as a fact, would not of itself be sufficient (as Semper is inclined to think) to disprove the identity of the Müllerian duct in Birds and Elasmobranchii.
We have, no doubt, almost no knowledge of the magnitude of the changes which can take place in the mode of development of the same organ in different types, yet this would have to be placed at a very low figure indeed in order to exclude the possibility of a change from the mode of development of the Müllerian duct in Elasmobranchii to that in Birds. We have, it appears to me, in the smallness of the portion of the segmental duct which goes to form the Müllerian duct in Elasmobranchii, evidence that a change has already appeared in this group in the direction of a development of the Müllerian duct independent of the segmental duct, and therefore of the Wolffian duct; and it has been in view of this consideration, that I have devoted so much attention to the apparently unimportant point of how much of the segmental duct was concerned in the formation of the Müllerian duct. An analogous change, in a somewhat different direction, would seem to be taking place in the development of the rudimentary Müllerian duct in the male Elasmobranchii.
It is, perhaps, just worth pointing out, that the blindness of the oviduct of female Elasmobranchii, and its mode of development from an imperfect splitting of the segmental duct, may probably be brought into connection with the blindness of the extremity of the Müllerian duct or oviduct which so often occurs in both sexes of Sturgeons (Accipenser).
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 theanterior 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. 7aand 7b,Pl.21—two longitudinal vertical sections through part of the kidneys. Fig. 7ashews 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 tubesand Wolffian duct takes placeentirely 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 inPl.21, fig. 9,u, but more conclusively in the series of transverse sections represented onPl.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,completelycircumscribed by the wall of the ureter, but onlydorsally 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 onPl.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 representedonPl.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.
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 inPl.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 inPl.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 onPl.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 tothis 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. 2sg.) gegen die Niere hin; da sie meist stark geneigt oder selbst geschlängelt (bei 6ctmlangen Embryonen) gegen die Niere zu verlaufen, wo sie sich an die primärenMalpighi'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).
And again:“Dieser[355]Gegensatz in der Umbildung der Segmentalgänge an der Hodenbasis scheint nun mit einem anderen Hand in Hand zu gehen. Es bildet sich nämlich am Innenrande der Niere durch Sprossung und Verwachsung der Segmentalgänge vor ihrer Insertion an das primäreMalpighi'sche Körperchen ein Canal beim Männchen aus, den ich alsNierenrandcanaloben bezeichnet habe. Ich habe denselben bei Acanthias Centrina (Taf.XXI.Fig. 13) und Mustelus (Taf.XV.Fig. 8) gefunden. Bei Centrina ist er ziemlich lang und vereinigt mindestens 7 Segmentalgänge, aber von diesen letzteren stehen nur 5 mit demHodennetz in Verbindung. Dort nun wo diese letzteren sich an den Nierenrandcanal ansetzen (Taf.XXI.Fig. 13sg.1-sg.5) findet sich jedesmal ein typisch ausgebildetesMalpighi'sches Körperchen, mit dem aber nun nicht mehr wie ursprünglich nur 2 Canäle verbunden sind (Taf.XXI.Fig. 14) sondern 3. Einer dieser letzteren ist derjenige Ast des Nierenrandcanals welcher die Verbindung mit dem nächst folgenden Segmentalgang zu besorgen hat. An den Stellen aber wo sich an den Nierenrandcanal die hinteren blind gegen den Hoden hin endenden Segmentalgänge ansetzen fehlen dieseMalpighi'schen Körperchen (Taf.XXI.Fig. 13sg7) vollständig. Auch bei Mustelus (Taf.XV.Figs. 8, 10) findet genau dasselbe Verhältniss statt; da aber hier nur 2 (oder 3) Segmentalgänge zu vasa efferentia umgewandelt werden, so stehen hier am kurzen Randcanal der Niere auch nur 2 oder 3Malpighi'sche Körperchen. Diese aber sind typisch ausgebildet”(Taf.XV.Fig. 10).
From these two extracts it is clear that Semper regards both the vasa efferentia, and central canal of the testis network, as well as the longitudinal canal of the Wolffian body, as products of the anterior segmental tubes.
The appearance of these various parts in the fully grown embryos or adults of such genera as Acanthias and Squatina strongly favours this view, but Semper appears to have worked out the development of these structures somewhat partially and by means of sections, a method not, in Scyllium at least, very suitable for this particular investigation. I myself at first unhesitatingly accepted Semper's views, and it was not till after the study of the paper of Dr Spengel on the Amphibian kidney that I came to have my doubts as to their accuracy. The arrangement of the parts in most Amphibians is strikingly similar to that in Elasmobranchii. From the testis come transverse canals corresponding with my vasa efferentia; these fall into a longitudinal canal of the kidneys, from which again, as in Squatina (Pl.20, fig. 8), Mustelus and Centrina, canals (the vasa efferentia of Spengel) pass off to Malpighian bodies. So far there is no difficulty, but Dr Spengel has made the extremely important discovery, that in young Amphibians each Malpighian body in the region of the generative ducts, in addition to receiving the vasa efferentia, is connected with a fully developed segmentaltube opening into the body-cavity. In Amphibians, therefore, it is improbable that the vasa efferentia are products of the open extremities of the segmental tubes, considering that these latter are found in their unaltered condition at the same time as the vasa efferentia. When it is borne in mind how strikingly similar in most respects is the arrangement of the testicular ducts in Amphibia and Elasmobranchii, it will not easily be credited that they develop in entirely different methods. Since then we find in Amphibians fully developed segmental tubes in the same segments as the vasa efferentia, it is difficult to believe that in Elasmobranchii the same vasa efferentia have been developed out of the segmental tubes by the obliteration of their openings.
I set myself to the solution of the origin of the vasa efferentia by means of surface views, after the parts had been made transparent in creosote, but I have met with great difficulties, and so far my researches have only been partially successful. From what I have been able to see of Squatina and Acanthias, I am inclined to think that the embryos of either of these genera would form far more suitable objects for this research than Scyllium. I have had a few embryos of Squatina which were unfortunately too old for my purpose.
Very early the vasa efferentia are fully formed, and their arrangement in an embryo eight centimetres long is shewn inPl.20, fig. 6,v.e. It is there seen that there are six if not seven vasa efferentia connected with a longitudinal canal along the base of the testes (Semper's central canal of the testis), and passing down like the segmental tubes to spaces between the successive segments of the Wolffian body. They were probably connected by a longitudinal canal in the Wolffian body, but this could not be clearly seen. In the segment immediately behind the last vas efferens was a fully developed segmental tube. This embryo clearly throws no light on the question at issue except that on the whole it supports Semper's views. I further failed to make out anything from an examination of still younger embryos.
In a somewhat older embryo there was connected with the anterior vas efferens a peculiar structure represented onPl.20, fig. 7,r.st? which strangely resembled the opening of an ordinary segmental tube, but as I could not find it in the younger embryo, this suggestion as to its nature, is, at the best,extremely hazardous. If, however, this body really is the remnant of a segmental opening, it would be reasonable to conclude that the vasa efferentia are buds from the segmental tubes as opposed to their openings; a mode of origin which is not incompatible with the discoveries of Dr Spengel. I have noticed a remnant, somewhat similar to that in the Scyllium embryo, close to the hindermost vas efferens in an embryo Squatina (Pl.20, fig. 8,r.st?).
With reference to the development of the longitudinal canal of the Wolffian body, I am without observations, but it appears to me to be probably a further development of the outgrowths of the vesicles of each segmental tube, which were described in connection with the development of the segmental tubes, p.492. Were an anterior outgrowth of one vesicle to meet and coalesce with the posterior outgrowth of the preceding vesicle, a longitudinal canal such as actually exists would be the result. The central canal of the base of the testes and the network connected with it in the adult (Pl.20, fig. 4), appear to be derivatives of the vasa efferentia.
I am thus compelled to leave open the question of the real nature of the vasa efferentia, but am inclined to regard them as outgrowths from the anterior segmental tubes, though not from their open terminations.
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My views upon the homologies of the various parts of the urinogenital system, the development of which has been described in the present chapter, have already been expressed in a paper on Urinogenital organs of Vertebrates[356]. Although Kölliker's[357]discovery of the segmental tubes in Aves, and the researches of Spengel[358], Gasser[359], Ewart[360]and others, have rendered necessary a few corrections in my facts, I still adhere in their entirety to the views expressed in that paper, and feel it unnecessary torepeat them in this place. I conclude the chapter with a résumé of the development of the urinogenital organs in Elasmobranchii from their first appearance to their permanent condition.
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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.
Two Sections of a Pristiurus Embryo with three visceral cleftsThe sections illustrate the development of the segmental duct (pd) or primitive duct of the kidneys. InA(the anterior of the two sections) this appears as a solid knob (pd) projecting towards the epiblast. InBis seen a section of the column which has grown backwards from the knob inA.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.
The sections illustrate the development of the segmental duct (pd) or primitive duct of the kidneys. InA(the anterior of the two sections) this appears as a solid knob (pd) projecting towards the epiblast. InBis seen a section of the column which has grown backwards from the knob inA.
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 tracesof 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.dis the segmental duct andoits abdominal opening.s.tpoints 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 coiledtubes, 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.
Diagram of the primitive condition of the Kidneypd.segmental duct. It opens atointo 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.
pd.segmental duct. It opens atointo 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.
The next important change concerns the segmental duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second in the male. The manner in which this takes place is diagrammatically represented in woodcut 6 by the clear linex, and in transverse section in woodcut 7. The resulting ducts are the (1) Wolffian duct dorsally, which remains continuous with the excretorytubules of the kidney, and ventrally (2) the oviduct or Müllerian duct in the female, and the rudiments of this duct in the male. In the female the formation of these ducts takes place by a nearly solid rod of cells, being gradually split off from the ventral side of all but the foremost part of the original segmental duct, with the short undivided anterior part of which duct it is continuous in front. Into it a very small portion of the lumen of the original segmental duct is perhaps continued (Pl.21, fig. 1A, etc.). The remainder of the segmental duct (after the loss of its anterior section and the part split off from its ventral side) forms the Wolffian duct. The process of formation of the ducts in the male chiefly differs from that in the female in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Müllerian duct, being shorter, and in the column of cells with which it is continuous being from the first incomplete.
Fig. 7.
Diagrammatic representation of a transverse section of a Scyllium Embryoillustrating the formation of the Wolffian and Müllerian ducts bythe longitudinal splitting of the segmental duct.
Diagrammatic representation of a transverse section of a Scyllium Embryomc.medullary canal;mp.muscle-plate;ch.notochord;ao.aorta;cav.cardinal vein;st.segmental tube. On the one side the section passes through the opening of a segmental tube into the body-cavity. On the other this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through;w.d.Wolffian duct;m.d.Müllerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate;gr.The germinal ridge with the thickened germinal epithelium;l.liver;i.intestine with spiral valve.
mc.medullary canal;mp.muscle-plate;ch.notochord;ao.aorta;cav.cardinal vein;st.segmental tube. On the one side the section passes through the opening of a segmental tube into the body-cavity. On the other this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through;w.d.Wolffian duct;m.d.Müllerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate;gr.The germinal ridge with the thickened germinal epithelium;l.liver;i.intestine with spiral valve.
The tubuli of the primitive excretory organ undergo further important changes. The vesicle at the termination of each segmental tube grows forwards towards the preceding tubulus, and joins the fourth section of it close to the opening into the Wolffian duct (Pl.21, fig. 10). The remainder of the vesicle becomes converted into a Malpighian body. By the first of these changes a connection is established between the successive segments of the kidney, and though this connection is certainly lost (or only represented by fibrous bands) in the anterior part of the excretory organs in the adult, and very probably in the hinder part, yet it seems most probable that traces of it are to be found in the presence of the secondary Malpighian bodies of the majority of segments, which are most likely developed from it.
Up to this time there has been no distinction between the anterior and posterior tubuli of the primitive excretory organ which alike open into the Wolffian duct. The terminal division of the tubuli of a considerable number of the hindermost of these (ten or eleven in Scyllium canicula), either in some species elongate, overlap, and eventually open by apertures (not usually so numerous as the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the urinogenital cloaca, formed by the coalesced terminalparts of the Wolffian ducts, in the male; or in other species become modified in such a manner as to pour their secretion into a single duct on each side, which opens in a position corresponding with the numerous ducts of the other type (woodcut, fig. 8). It seems that both in Amphibians and Elasmobranchii the type with a single duct, or approximations to it, are more often found in the females than in the males. The subject requires however to be more worked out in Elasmobranchii[361]. In both groups the modified posterior kidney-segments are probably equivalent to the permanent kidney of the amniotic Vertebrates, and for this reason the numerous ducts of the first group or single duct of the second were spoken of as ureters. The anterior tubuli of the primitive excretory organ retain their early relation to the Wolffian duct, and form the Wolffian body.
The originally separate terminal extremities of the Wolffian ducts always coalesce, and form a urinal cloaca, opening by a single aperture situated at the extremity of a median papilla behind the anus. Some of the abdominal openings of the segmental tubes in Scyllium, or in other cases all the openings, become obliterated.
In the male the anterior segmental tubes undergo remarkable modifications. There appear to grow from the first three or four or more of them (though the point is still somewhat obscure) branches, which pass to the base of the testis and there unite into a longitudinal canal, form a network, and receive the secretion of the testicular ampullæ (woodcut 9,nt). These ducts, the vasa efferentia, carry the semen to the Wolffian body, but before opening into the tubuli of this they unite into thelongitudinal canal of the Wolffian body (l.c), from which pass off ducts equal in number to the vasa efferentia, each of which normally ends in a Malpighian body. From the Malpighian body so connected start the convoluted tubuli of what may be called the generative segments of the Wolffian body along which the semen is conveyed to the Wolffian duct (v.d). The Wolffian duct itself becomes much contorted and acts as vas deferens.
Fig. 8.
Diagram of the arrangement of the Urinogenital Organs in an adultFemale Elasmobranch.