Chapter XII: The Organs of Excretion (2)

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. 7_a_ and 7_b_, 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 7_b_ (_r.u_) is shewn the modified junction in the region of the kidney proper in the same embryo. The latter of these figures (fig. 7_b_) 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.

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.

Footnote 353: This at least holds good for one of my embryos
at this stage, which is labelled Scy. canicula, but which may
possibly be Scy. stellare.

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, 2_c_). 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. 2_c_) 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 6{ctm} 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. 3_c_).

Footnote 354: _Loc. cit._ p. 364.

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 primäre _Malpighi_'sche Körperchen ein Canal beim Männchen aus, den ich als _Nierenrandcanal_ oben 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 dem Hodennetz in Verbindung. Dort nun wo diese letzteren sich an den Nierenrandcanal ansetzen (Taf. XXI. Fig. 13 sg.1-sg.5) findet sich jedesmal ein typisch ausgebildetes _Malpighi_'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 diese _Malpighi_'schen Körperchen (Taf. XXI. Fig. 13 _sg_7) 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 3 _Malpighi_'sche Körperchen. Diese aber sind typisch ausgebildet" (Taf. XV. Fig. 10).

Footnote 355: _Loc. cit._ p. 395.

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 segmental tube 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 in Pl. 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 on Pl. 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.

* * * * *

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 to repeat 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.

Footnote 356: _Journal of Anatomy and Physiology_, Vol. X.
[This edition, No. VII.]

Footnote 357: _Entwicklungsgeschichte des Menschen u. der
höheren Thiere._

Footnote 358: _Loc. cit._

Footnote 359: _Beiträge zur Entwicklungsg. d. Allantois d.
Müller'schen Gänge u. d. Afters._

Footnote 360: "Abdominal Pores and Urogenital Sinus of
Lamprey," _Journal of Anatomy and Physiology_, Vol. X. p. 488.

* * * * *

_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.

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.

DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN ELASMOBRANCH EMBRYO.

_pd._ segmental duct. It opens at _o_ into 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 line _x_, and in transverse section in woodcut 7. The resulting ducts are the (1) Wolffian duct dorsally, which remains continuous with the excretory tubules 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.

DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MÜLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.

_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 terminal parts 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.

Footnote 361: The reverse of the above rule is the case with
Raja, in the male of which a closer approximation to the
single-duct type is found than in the female.

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 the _longitudinal 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.

DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT FEMALE ELASMOBRANCH.

_m.d._ Müllerian duct; _w.d._ Wolffian duct; _s.t._ glandular tubuli; five of them are represented with openings into the body-cavity; _d._ duct of the posterior segmental tubes; _ov._ ovary.]

In the woodcuts, figs. 8 and 9, are diagrammatically represented the chief constituents of the adult urinogenital organs in the two sexes. In the adult female, fig. 8, there are present the following parts:

(1) The oviduct or Müllerian duct (_m.d_) split off from the segmental duct of the kidneys. Each oviduct opens at its anterior extremity into the body-cavity, and behind the two oviducts have independent communications with the general cloaca.

(2) The Wolffian ducts (_w.d_), the other product of the segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior segment of the Wolffian body on each side, and unite behind to open by a common papilla into the cloaca. The Wolffian duct receives the secretion of the anterior part of the primitive kidney which forms the Wolffian body.

(3) The ureter (_d_) which carries off the secretion of the kidney proper. It is represented in my diagram in its most rare and differentiated condition as a single duct.

(4) The glandular tubuli (_s.t_), some of which retain their original openings into the body-cavity, and others are without them. They are divided into two groups, an anterior forming the Wolffian body, which pour their secretion into the Wolffian duct, and a posterior group forming the kidney proper, which are connected with the ureter.

DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT MALE ELASMOBRANCH.

_m.d._ rudiment of Müllerian duct; _w.d._ Wolffian duct, marked _vd_ in front and serving as vas deferens; _st._ glandular tubuli; two of them are represented with openings into the body-cavity; _d._ ureter; _t._ testis; _nt._ central canal at the base of the testis; _VE._ vasa efferentia; _lc._ longitudinal canal of the Wolffian body.]

In the male the following parts are present (woodcut 9):

(1) The Müllerian duct (_md_), consisting of a small rudiment attached to the liver representing the foremost end of the oviduct of the female.

(2) The Wolffian duct (_w.d_) which precisely corresponds to the Wolffian duct of the female, but, in addition to functioning as the duct of the Wolffian body, also acts as a vas deferens (_vd_). In the adult male its foremost part has a very tortuous course.

(3) The ureter (_d_), which has the same fundamental constitution as in the female.

(4) The segmental tubes (_st_). The posterior of these have the same arrangement in both sexes, but in the male modifications take place in connection with the anterior ones to fit them to act as transporters of the testicular products.

Connected with the anterior ones there are present (1) the vasa efferentia (VE), united on the one hand with (2) the central canal in the base of the testis (_nt_), and on the other with the longitudinal canal of the Wolffian body (_l.c_). From the latter are seen passing off the successive tubuli of the anterior segments of the Wolffian body in connection with which Malpighian bodies are typically present, though not represented in my diagram.

_Postscript._

It was my original intention to have given an account of the development of the generative organs. In the course, however, of my work a number of novel and unexpected points turned up, which have considerably protracted my investigations, and it has appeared to me better no longer to delay the appearance of this monograph, but to publish elsewhere my results on the generative organs. In chapter VI. p. 349 _et seq._ the early stages of the generative organs are described, but in contemplation of the completion of the account no allusion was made to their literature, and more especially to Professor Semper's important contributions. I may perhaps say that I have been able to confirm the most important result to which he and other anatomists have nearly simultaneously arrived with respect to Vertebrates, viz. _that the primitive ova give rise to both the male and female generative products_.

EXPLANATION OF PLATES 20 AND 21.

COMPLETE LIST OF REFERENCE LETTERS.

_amg._ Accessory Malpighian body. _cav._ Cardinal vein. _ge._ Germinal epithelium. _k._ True kidney. _l.c._ Longitudinal canal of the Wolffian body connected with vasa efferentia. _mg._ Malpighian body. _nt._ Network and central canal at the base of the testis. _o._ External aperture of urinal cloaca. _od._ Oviduct or Müllerian duct of the female. _od´._ Müllerian duct of the male. _ou._ Openings of ureters in Wolffian duct in the female (fig. 3). _pmg._ Primary Malpighian body. _px._ Growth from vesicle at the end of a segmental tube to join the collecting tube of the preceding segment. _rst._ Rudimentary segmental tube. _ru._ Ureter commencing to be formed. _sb._ Seminal bladder. _sd._ Segmental duct. _st._ Segmental tube. _sto._ Opening of segmental tube into body-cavity. _sur._ Suprarenal body. _t._ Testis. _u._ Ureters. _ve._ Vas efferens. _wb._ Wolffian body. _wd._ Wolffian duct.

PLATE 20.

Fig. 1. Diagrammatic representation of excretory organs on one side of a male Scyllium canicula, natural size.

Fig. 2. Diagrammatic representation of the kidney proper on one side of a female Scyllium canicula, natural size, shewing the ducts of the kidney and the dilated portion of the Wolffian duct.

Fig. 3. Opening of the ureters into the Wolffian duct of a female Scyllium canicula. The figure represents the Wolffian ducts (_wd_) with ventral portion removed so as to expose their inner surface, and shews the junction of the two W. ducts to form the common urinal cloaca, the single external opening of this (_o_), and openings of ureters into one Wolffian duct (_ou_).

Fig. 4. Anterior extremity of Wolffian body of a young male Scyllium canicula shewing the vasa efferentia and their connection with the kidneys and the testis. The vasa efferentia and longitudinal canal are coloured to render them distinct. They are intended to be continuous with the uncoloured coils of the Wolffian body, though this connection has not been very successfully rendered by the artist.

Fig. 5. Part of the Wolffian body of a nearly ripe male embryo of Scyllium canicula as a transparent object. Zeiss a a, ocul. 3. The figure shews two segmental tubes opening into the body-cavity and connected with a primary Malpighian body, and also, by a fibrous connection, with a secondary Malpighian body of the preceding segment. It also shews one segmental tube (_rst_) imperfectly connected with the accessory Malpighian body of the preceding segment of the kidney. The coils of the kidney are represented somewhat diagrammatically.

Fig. 6. Vasa efferentia of a male embryo of Scyllium canicula eight centimetres in length. Zeiss a a, ocul. 2.

There are seen to be at the least six and possibly seven distinct vasa going to as many segments of the Wolffian body and connected with a longitudinal canal in the base of the testis. They were probably also connected with a longitudinal canal in the Wolffian body, but this could not be clearly made out.

Fig. 7. The anterior four vasa efferentia of a nearly ripe embryo. Connected with the foremost one is seen a body which looks like the remnant of a segmental tube and its opening (_rst?_).

Fig. 8. Testis and anterior part of Wolffian body of an embryo of Squatina vulgaris.

The figure is intended to illustrate the arrangement of the vasa efferentia. There are five of these connected with a longitudinal canal in the base of the testis, and with another longitudinal canal in the Wolffian body. From the second longitudinal canal there pass off four ducts to as many Malpighian bodies. Through the Malpighian bodies these ducts are continuous with the several coils of the Wolffian body, and so eventually with the Wolffian duct. Close to the hindermost vas efferens is seen a body which resembles a rudimentary segmental tube (_rst?_).

PLATE 21.

Figs. 1A, 1B, 1C, 1D. Four sections from a female Scyllium canicula of a stage between M and N through the part where the segmental duct becomes split into Wolffian duct and oviduct. Zeiss B, ocul. 2. 1A is the foremost section.

The sections shew that the oviduct arises as a thickening on the under surface of the segmental duct into which at the utmost a very narrow prolongation of the lumen of the segmental duct is carried. The small size of the lumen of the Wolffian duct in the foremost section is due to the section passing through nearly its anterior blind extremity.

Fig. 2. Section close to the junction of the Wolffian duct and oviduct in a female embryo of Scyllium canicula belonging to stage N. Zeiss B, ocul. 2.

The section represented shews that in some instances the formation of the oviduct and Wolffian duct is accompanied by a division of the lumen of the segmental duct into two not very unequal parts.

Figs. 3A, 3B, 3C. Three sections illustrating the formation of a ureter in a female embryo belonging to stage N. Zeiss B, ocul. 2.

3A is the foremost section.

The figures shew that the lumen of the developing ureter is enclosed in front by an independent wall (fig. 3A), but that further back the lumen is partly shut in by the subjacent Wolffian duct, while behind no lumen is present, but the ureter ends as a solid knob of cells without an opening into the Wolffian duct.

Fig. 4. Section through the ureters of the same embryo as fig. 3, but nearer the cloaca. Zeiss B, ocul. 2.

The figure shews the appearance of a transverse section through the wall of cells above the Wolffian duct formed by the overlapping ureters, the lumens of which appear as perforations in it. It should be compared with fig. 9A, which represents a longitudinal section through a similar wall of cells.

Fig. 5. Section through the ureters, the Wolffian duct and the oviduct of a female embryo of Scy. canicula belonging to stage P. Zeiss B, ocul. 2.

Fig. 6. Section of part of the Wolffian body of a male embryo of Scyllium canicula belonging to stage O. Zeiss B, ocul. 2.

The section illustrates (1) the formation of a Malpighian body (_mg_) from the dilatation at the end of a segmental tube, (2) the appearance of the rudiment of the Müllerian duct in the male (_od´_).

Figs. 7_a_, 7_b_. Two longitudinal and vertical sections through part of the kidney of an embryo between stages L and M. Zeiss B, ocul. 2.

7_a_ illustrates the parts of a single segment of the Wolffian body at this stage, vide p. 491. The segmental tube and opening are not in the plane of the section, but the dilated vesicle is shewn into which the segmental tube opens.

7_b_ is taken from the region of the kidney proper. To the right is seen the opening of a segmental tube into the body-cavity, and in the segment to the left the commencing formation of a ureter, vide p. 502.

Fig. 8. Longitudinal and vertical section through the posterior part of the kidney proper of an embryo of Scyllium canicula at a stage between N and O. Zeiss A, ocul. 2.

The section shews the nearly completed ureters, developing Malpighian bodies, &c.

Fig. 9. Longitudinal and vertical section through the anterior part of the kidney proper of the same embryo as fig. 8. Zeiss A, ocul. 2.

The figure illustrates the mode of growth of the developing ureters.

9A. More highly magnified portion of the same section as fig. 9.

Compare with transverse section fig. 4.

Fig. 10. Longitudinal and vertical section through part of the Wolffian body of an embryo of Scyllium canicula at a stage between O and P.

The section contains two examples of the budding out of the vesicle of a segmental tube to form a Malpighian body in its own segment and to unite with the tubulus of the preceding segment close to its opening into the Wolffian duct.

XI. ON THE PHENOMENA ACCOMPANYING THE MATURATION
AND IMPREGNATION OF THE OVUM[362].

Footnote 362: From the _Quarterly Journal of Microscopical
Science_, April, 1878.

The brilliant discoveries of Strasburger and Auerbach have caused the attention of a large number of biologists to be turned to the phenomena accompanying the division of nuclei and the maturation and impregnation of the ovum. The results of the recent investigations on the first of these points formed the subject of an article by Mr Priestley in the sixteenth volume of this Journal, and the object of the present article is to give some account of what has so far been made out with reference to the second of them. The matters to be treated of naturally fall under two heads: (1) the changes attending the ripening of the ovum, _which are independent of impregnation_; (2) the changes which are directly due to impregnation.

Every ovum as it approaches maturity is found to be composed (Fig. 1) of (1) a protoplasmic body or vitellus usually containing yolk-spherules in suspension; (2) of a germinal vesicle or nucleus, containing (3) one or more germinal spots or nucleoli. It is with the germinal vesicle and its contents that we are especially concerned. This body at its full development has a more or less spherical shape, and is enveloped by a distinct membrane. Its contents are for the most part fluid, but may be more or less granular. Their most characteristic component is, however, a protoplasmic network which stretches from the germinal spot to the investing membrane, but is especially concentrated round the former (Fig. 1). The germinal spot forms a nearly homogeneous body, with frequently one or more vacuoles. It occupies an often excentric position within the germinal vesicle, and is usually rendered very conspicuous by its high refrangibility. In many instances it has been shewn to be capable of amoeboid movements (Auerbach, and Os. Hertwig), and is moreover more solid and more strongly tinged by colouring reagents than the remaining constituents of the germinal vesicle. These peculiarities have caused the matter of which it is composed to be distinguished by Auerbach and Hertwig as nuclear substance.

In many instances there is only one germinal spot, or one main spot, and two or three accessory smaller spots. In other cases, _e.g._ Osseous Fish, there are a large number of nearly equal germinal spots. The eggs which have been most investigated with reference to the changes of germinal vesicle are those with a single germinal spot, and it is with these that I shall have more especially to deal in the sequel.

The germinal vesicle occupies in the first instance a central position in the ovum, but at maturity is almost always found in close proximity to the surface. Its change of position in a large number of instances is accomplished during the growth of the ovum in the ovary, but in other cases does not take place till the ovum has been laid.

The questions which many investigators have recently set themselves to answer are the two following:--(1) What becomes of the germinal vesicle when the ovum is ready to be impregnated? (2) Is any part of it present in the ovum at the commencement of segmentation? According to their answers to these questions the older embryologists roughly fall into two groups: (1) By one set the germinal vesicle is stated to completely disappear and not to be genetically connected with the subsequent nuclei of the embryo. (2) According to the other set it remains in the ovum and by successive divisions forms the parent nucleus of all the nuclei in the body of the embryo. Though the second of these views has been supported by several very distinguished names the first view was without doubt the one most generally entertained, and Haeckel (though from his own observations he was originally a supporter of the second view) has even enunciated the theory that there exists an anuclear stage, after the disappearance of the germinal vesicle, which he regards as an embryonic repetition of the monad condition of the Protozoa.

While the supporters of the first view agree as to the disappearance of the germinal vesicle they differ considerably as to the manner of this occurrence. Some are of opinion that the vesicle simply vanishes, its contents being absorbed in the ovum; others that it is ejected from the ovum and appears as the _polar cell_ or _body_, or _Richtungskörper_ of the Germans--a small body which is often found situated in the space between the ovum and its membrane, and derives its name from retaining a constant position in relation to the ovum, and thus serving as a guide in determining the similar parts of the embryo through the different stages. The researches of Oellacher (15)[363] in this direction deserve special mention, as having in a sense formed the foundation of the modern views upon this subject. By a series of careful observations upon the egg of the trout and subsequently of the bird, he demonstrated that the germinal vesicle of the ovum, while still in the ovary, underwent partial degeneration and eventually became ejected. His observations were made to a great extent by means of sections, and the general accuracy of his results is fairly certain, but the nature of the eggs he worked on, as well as other causes, prevented his obtaining so deep an insight into the phenomena accompanying the ejection of the germinal vesicle as has since been possible. Lovén, Flemming (6), and others have been led by their investigations to adopt views similar in the main to Oellacher's. As a rule, however, it is held by believers in the disappearance of the germinal vesicle that it becomes simply absorbed, and many very accurate accounts, so far as they go, have been given of the gradual atrophy of the germinal vesicle. The description of Kleinenberg (14) for Hydra, and Götte for Bombinator, may perhaps be selected as especially complete in this respect; in both instances the germinal vesicle commences to atrophy at a relatively early period.

Footnote 363: The numbers appended to authors' names refer to
the list of publications at the end of the paper.

Coming to the more modern period the researches of five workers, viz. Bütschli, E. van Beneden, Fol, Hertwig, and Strasburger have especially thrown light upon this difficult subject. It is now hardly open to doubt that while part of the germinal vesicle is concerned in the formation of the polar cell or cells, when such are present, and is therefore ejected from the ovum, part also remains in the ovum and forms a nuclear body which will be spoken of as the _female pronucleus_, the fate of which is recorded in the second part of this paper. The researches of Bütschli and van Beneden have been especially instrumental in demonstrating the relation between the polar bodies and the germinal vesicle, and those of Hertwig and Fol, in shewing that part of the germinal vesicle remained in the ovum. It must not, however, be supposed that the results of these authors are fully substantiated, or that all the questions connected with these phenomena are settled. The statements we have are in many points opposed and contradictory, and there is much that is still very obscure.

In the sequel an account is first given of the researches of the above-named authors, followed by a statement of those results which appear to me the most probable.

The researches of van Beneden (3 and 4) were made on the ovum of the rabbit and of Asterias, and from his observations on both these widely separated forms he has been led to conclude that the germinal vesicle is either ejected or absorbed, but that it has in no case a genetic connection with the first segmentation sphere. He gives the following description of the changes in the rabbit's ovum. The germinal vesicle is enclosed by a membrane, and contains one main germinal spot, and a few accessory ones, together with a granular material which he calls _nucleoplasma_, which affects, as is usual in nuclei, a reticular arrangement. The remaining space in the vesicle is filled by a clear fluid. As the ovum approaches maturity the germinal vesicle assumes an excentric position, and fuses with the peripheral layer of the egg to constitute the _cicatricular lens_. The germinal spot next travels to the surface of the cicatricular lens and forms the _nuclear disc_: at the same time the membrane of the germinal vesicle vanishes though it probably unites with the nuclear disc. The nucleoplasma then collects into a definite mass and forms the nucleoplasmic body. Finally the nuclear disc assumes an ellipsoidal form and becomes the nuclear body. Nothing is now left of the original germinal vesicle but the nuclear body and the nucleoplasmic body both still situated within the ovum. In the next stage no trace of the germinal vesicle can be detected in the ovum, but outside it, close to the point where the modified remnants of the vesicle were previously situated, there is present a polar body which is composed of two parts, one of which stains deeply and resembles the nuclear body, and the other does not stain but is similar to the nucleoplasmic body. Van Beneden concludes that the polar bodies are the two ejected products of the germinal vesicle. In the case of Asterias, van Beneden has not observed the mode of formation of the polar bodies, and mainly gives an account of the atrophy of the germinal vesicle, but adds very little to what was already known to us from Kleinenberg's (14) earlier observations. He describes with precision the breaking up of the germinal spot into fragments and its eventual disappearance.

Though there are reasons for doubting the accuracy of all the above details on the ovum of the rabbit, nevertheless, the observations of van Beneden taken as a whole afford strong grounds for concluding that the formation of the polar cells is connected with the disappearance, partial or otherwise, of the germinal vesicle. A very similar account of the apparent disappearance of the germinal vesicle is given by Greeff (19) who states that the apparent disappearance of the germinal spot precedes that of the vesicle.

The observations of Bütschli are of still greater importance in this direction. He has studied with a view to elucidating the fate of the germinal vesicle, the eggs of Nephelis, Lymnæus, Cucullanus, and other Nematodes; and Rotifers. In all of these, with the exception of Rotifers, he finds polar bodies, and in this respect his observations are of value as tending to shew the widespread existence of these structures. Negative results with reference to the presence of the polar bodies have, it may be remarked, only a very secondary value. Bütschli has made the very important discovery that in perfectly ripe eggs of Nephelis, Lymnæus and Cucullanus and allied genera a _spindle_, similar to that of ordinary nuclei in the act of division, appears close to the surface of the egg. This spindle he regards as the metamorphosed germinal vesicle, and has demonstrated that it takes part in the formation of the polar cells. He states that the whole spindle is ejected from the egg, and that after swelling up and forming a somewhat spherical mass it divides into three parts.

In the Nematodes generally, Bütschli has been unable to find the spindle modification of the germinal vesicle, but he states that the germinal vesicle undergoes degeneration, its outline becoming indistinct and the germinal spot vanishing. The position of the germinal vesicle continues to be marked by a clear space which gradually approaches the surface of the egg. When it is in contact with the surface a small spherical body, the remnant of the germinal vesicle, comes into view, and eventually becomes ejected. The clear space subsequently disappears. This description of Bütschli resembles in some respects that given by van Beneden of the changes in the rabbit's ovum, and not impossibly refers to a nearly identical series of phenomena. The discovery by Bütschli of the spindle and its relation to the polar body has been of very great value.

The publications of van Beneden, and more especially those of Bütschli, taken by themselves lead to the conclusion that the whole germinal vesicle is either ejected or absorbed. Nearly simultaneously with their publications there appeared, however, a paper by Oscar Hertwig (11) on the eggs of one of the common sea urchins (_Toxopneustes lividus_), in which he attempted to shew that part of the germinal vesicle, at any rate, was concerned in the formation of the first segmentation nucleus. He believed (though he has himself now recognised that he was in error on the point) that no polar cell was formed in Toxopneustes, and that the whole germinal vesicle was absorbed, with the exception of the germinal spot which remained in the egg as the female pronucleus.

The following is the summary which he gives of his results, pp. 357-8.

"At the time when the egg is mature the germinal vesicle undergoes a
retrogressive metamorphosis and becomes carried towards the surface of
the egg by the contraction of the protoplasm. Its membrane becomes
dissolved and its contents disintegrated and finally absorbed by the
yolk. The germinal spot appears, however, to remain unaltered and to
continue in the yolk and to become the permanent nucleus of the ripe
ovum capable of impregnation."