The Works of Francis Balfour 1-7
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol I. Separate Memoirs (1885)
VII. On the Origin and History of the Urinogenital Organs of Vertebrates
RECENT discoveries 2 as to the mode of development and anatomy of the urinogenital system of Selachians, Amphibians, and Cyclostome fishes, have greatly increased our knowledge of this system of organs, and have rendered more possible a comparison of the types on which it is formed in the various orders of vertebrates.
1 From the Journal of Anatomy and Physiology, Vol. X. 1875.
a The more important of these are :
Semper Ueber die Stammverwandtschaft der Wirbelthiere u. Anneliden. 6V//tralblatt f. Med. Wiss. 1874, No. 35.
Semper Segmentalorgane bei ausgewachsenen Haien. Centralblatt f. Med. IViss. 1874, No. 52.
Semper Das Urogenitalsystem der hoheren Wirbelthiere. Cenlralblatt f. Med. Wiss. 1874, No. 59.
Semper Stammesverwandschaft d. Wirbelthiere u. Wirbellosen. Arbeiten aits Zool. Zootom. Inst, Wurzburg. II Band.
Semper Bildung u. Wachstum der Keimdriisen bei den Plagiostomen. Centralblatt f. Med. Wiss. 1875, No. 12.
Semper Entw. d. Wolf. u. Mull. Gang. Centralblatt f. Med. Wiss. 1875, No. 29.
Alex. Schultz Phylogenie d. Wirbelthiere. Centralblatt f. Med. Wiss. 1874, No. 51.
Spengel Wimpertrichtern i. d. Amphibienniere. Centralblatt f. Med. Wiss. 1875, No. 23.
Meyer Anat. des Urogenitalsystems der Selachier u. Amphibien. Sitzb. Naturfor. Gesellschaft. Leipzig, 30 April, 1875.
F. M. Balfour Preliminary Account of development of Elasmobranch fishes. Quart. Journ. of Micro. Science, Oct. 1874. (This edition, Paper V. p. 60 et seq.}
W. Muller Persistenz der Urniere bei Myxine glutinosa. Jenaische Zeitschrijt,
1873 W. Muller Urogenilalsystem d. Amphioxus u. d. Cyclostomen. Jenaische Zeit schrift, 1875.
Alex. Gott-e Entwickelungsgeschichte der Unke {Bombinator ignciis].
136 THE URINOGENITAL ORGANS OF VERTEBRATES.
The following paper is an attempt to give a consecutive history of the origin of this system of organs in vertebrates and of the changes which it has undergone in the different orders.
For this purpose I have not made use of my own observations alone, but have had recourse to all the Memoirs with which I am acquainted, and to which I have access. I have commenced my account with the Selachians, both because my own investigations have been directed almost entirely to them, and because their urinogenital organs are, to my mind, the most convenient for comparison both with the more complicated and with the simpler types.
On many points the views put forward in this paper will be found to differ from those which I expressed in my paper (loc. cit^) which give an account of my original 1 discovery of the segmental organs of Selachians, but the differences, with the exception of one important error as to the origin of the Wolffian duct, are rather fresh developments of my previous views from the consideration of fresh facts, than radical changes in them.
In Selachian embryos an intermediate cell-mass, or middle plate of mesoblast is formed, as in birds, from a partial fusion of the somatic and splanchnic layers of the mesoblast at the outer border of the protovertebrae. From this cell-mass the whole of the urinogenital system is developed.
At about the time when three visceral clefts have appeared, there arises from the intermediate cell-mass, opposite the fifth protovertebra, a solid knob, from which a column of cells grows backwards to opposite the position of the future anus (fig. i,/<^.).
This knob projects outwards toward the epiblast, and the column lies at first between ^he mesoblast and epiblast. The knob and column do not long remain solid. The knob becoming hollow acquires a wide opening into the pleuroperitoneal or body cavity, and the column a lumen ; so that by the time that five visceral clefts have appeared, the two together form a
1 These organs were discovered independently by Professor Semper and myself. Professor Semper's preliminary account appeared prior to my own which was published (with illustrations) in the Quarterly Journal of Mic. Science. Owing to my being in South America, I did not know of Professor Semper's investigations till several months after the publication of my paper.
THE URINOGENITAL ORGANS OF VERTEBRATES.
137
FlG. I. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL
CLEFTS.
The sections are to shew 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 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 ; me. 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. somatopleura ; sp. splanchnopleura ; //. pleuroperitoneal or body cavity ; ep. epiblast ; al. alimentary canal.
duct closed behind, but communicating in front by a wide opening with the pleuroperitoneal cavity.
Before these changes are accomplished, a series of solid 1 outgrowths of elements of the 'intermediate cell- mass' appear at the uppermost corner of the body-cavity. These soon become hollow and appear as involutions from the body-cavity, curling round the inner and dorsal side of the previously formed duct.
One involution of this kind makes its appearance for each protovertebra, and the first belongs to the protovertebra immediately behind the anterior end of the duct whose development has just been described. In Pristiurus there are in all 29 of these at this period. The last two or three arise from that portion of the body-cavity, which at this stage still exists behind the anus. The first-formed duct and the subsequent involutions are the rudiments of the whole of the urinary system.
1 These outgrowths are at first solid in both Pristiurus, Scyllium and Torpedo, but in Torpedo attain a considerable length before a lumen appears in them.
138 THE URINOriENITAL ORGANS OF VERTEBRATES.
The duct is the primitive duct of the kidney 1 ; I shall call it in future the segmental duct ; and the involutions are the commencements of the segmental tubes which constitute the body of the kidney. I shall call them in future segmental tubes
Soon after their formation the segmental tubes become convoluted, and their blind ends become connected with the segmental duct of the kidney. At the same time, or rather before this, the blind posterior termination of each of the segmental ducts of the kidneys unites with and opens into one of the horns of the cloaca. At this period the condition of affairs is represented in fig. 2.
FIG. i. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN A
SELACHIAN 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 Miillerian duct below ; st. segmental tubes. They open at one end into the body-cavity, and at the other into the segmental duct.
There is at pd, the segmental duct of the kidneys, opening in front (p) into the body-cavity, and behind into the cloaca, and there are a series of convoluted segmental tubes (st), each opening at one end into the body-cavity, and at the other into the duct (pd).
The next important change which occurs is the longitudinal division of the segmental duct of the kidneys into Miiller's duct, or the oviduct, and the duct of the Wolffian bodies or Leydig's duct. The splitting 2 is effected by the growth of a wall of cells
1 This duct is often called either Miiller's duct, the oviduct, or the duct of the primitive kidneys ' Urnierengang.' None of these terms are very suitable. A justification of the name I have given it will appear from the facts given in the later parts of this paper. In my previous paper I have always called it oviduct, a name which is very inappropriate.
2 This splitting was first of all discovered and an account of it published by Semper ( Centralblatt f. Med. \Viss. 1875, No. 29). I had independently made it out
THE URINOGENITAL ORGANS OF VERTEKRATES. 139
which divides the duct into two parts (fig. 3, wd. and md.). It takes place in such a way that the front end of the segmental duct, anterior to the entrance of the first segmental tube, together with the ventral half of the rest of the duct, is split off from its dorsal half as an independent duct (vide fig. 2, x).
The dorsal portion also forms an independent duct, and into it the segmental tubes continue to open. Such at least is the
FIG. 3. TRANSVERSE SECTION OF A SELACHIAN EMBRYO ILLUSTRATING THE
FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.
me. 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 Wolfnan duct has been cut through ; wd. Wolffian duct ; md, Miillerian duct. The Miillerian duct and the Wolffian duct together constitute the primitive segmental duct ; gr. The germinal ridge with the thickened germinal epithelium ; /. liver ; i. intestine with spiral valve.
for the female a few weeks before the publication of Semper's account but have not yet made observations about the point for the male.
My own previous account of the origin of the Wolffian duct (Quart. Journ. of Micros. Science, Oct. 1874, and this edition, Paper V.), is completely false, and was due to my not having had access to a complete series of my sections when I wrote the paper.
140 THE URINOGENITAL ORGANS OF VERTEBRATES.
method of splitting for the female for the male the splitting is according to Professor Semper, of a more partial character, and consists for the most part in the front end of the duct only being separated off from the rest. The result of these changes is the formation in both sexes of a fresh duct which carries off the excretions of the segmental involutions, and which I shall call the Wolffian duct while in the female there is formed another complete and independent duct, which I shall call the Miillerian duct, or oviduct, and in the male portions only of such a duct.
The next change which takes place is the formation of another duct from the hinder portion of the Wolffian duct, which receives the secretion of the posterior segmental tubes. This secondary duct unites with the primary or Wolffian duct near its termination, and the primary ducts of the two sides unite together to open to the exterior by a common papilla.
Slight modifications of the posterior terminations of these ducts are found in different genera of Selachians (vide Semper, Centralblatt filr Med. Wiss. 1874, No. 59), but they are of no fundamental importance.
These constitute the main changes undergone by the segmental duct of the kidneys and the ducts derived from it ; but the segmental tubes also undergo important changes. In the majority of Selachians their openings into the body-cavity, or, at any rate, the openings of a large number of them, persist through life ; but the investigations of Dr Meyer 1 render it very probable that the small portion of each segmental tube adjoining the opening becomes separated from the rest and becomes converted into a sort of lymph organ, so that the openings of the segmental tubes in the adult merely lead into lymph organs and not into the gland of the kidneys.
These constitute the whole changes undergone in the female, but in the male the open ends of a varying number (according to the species) of the segmental tubes become connected with the testis and, uniting with the testicular follicles, serve to carry away the seminal fluid 2 . The spermatozoa have therefore to
1 Sitzen. der Naturfor. Gesdlschaft, Leipzig, 30 April, 1875.
2 We owe to Professor Semper the discovery of the arrangement of the seminal ducts. Centralblatt f. Med. Wiss. 1875, No. 12.
THE URINOGENITAL ORGANS OF VERTEBRATES.
141
pass through a glandular portion of the kidneys before they enter the Wolffian duct, by which they are finally carried away to the exterior.
In the adult female, then, there are the following parts of the urinogenital system (fig. 4) :
(i) The oviduct, or Miiller's duct (fig. 4, md.}, split off from the segmental duct of the kidneys. Each oviduct opens at its upper end into the body-cavity, and behind the two oviducts have independent communications with the cloaca. The oviducts serve simply to carry to the exterior the ova, and have no communication with the glandular portion of the kidneys.
FIG. 4. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT FEMALE SELACHIAN.
md. Miillerian duct ; wd. Wolffian duct ; st. segmental tubes ; d. duct of the posterior segmental tubes ; ov. ovary.
(2) The Wolffian ducts (fig. 4, wd.) or the remainder of the segmental ducts of the kidneys. Each Wolffian duct ends blindly in front, and the two unite behind to open by a common papilla into the cloaca.
This duct receives the secretion of the whole anterior end of the kidneys 1 , that is to say, of all the anterior segmental tubes.
(3) The secondary duct (fig. 4, d.) belonging to the lower portion of the kidneys opening into the former duct near its termination.
(4) The segmental tubes (fig. 4. st) from whose convolutions and outgrowths the kidney is formed. They may be divided
1 This upper portion of the kidneys is called Leydig's gland by Semper. It would be better to call it the Wolffian body, for I shall attempt to shew that it is homologous with the gland so named in Sauropsida and Mammalia.
142 THE URINOGENITAL ORGANS. OF VERTEBRATES.
into two parts, according to the duct by which their secretion is carried off.
In the male the following parts are present :
(1) The Miillerian duct (fig. 5, md.), consisting of a small remnant, attached to the liver, which represents the foremost end of the oviduct of the female.
(2) The Wolffian duct (fig. 5, wd], which precisely corresponds to the Wolffian duct of the female, except that, in addition to functioning as the duct of the anterior part of the kidneys, it also serves to carry away the semen. In the female it is straight, but has in the adult male a very tortuous course (vide fig. 5).
FIG. 5. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT MALE SELACHIAN.
md. rudiment of Mullerian duct ; wd. Wolffian duct, which also serves as vas deferens ; st. segmental tubes. The ends of three of those which in the female open into the body-cavity, have in the male united with the testicular follicles, and serve to carry away the products of the testis ; d. duct of the posterior segmental tubes ; t. testis.
(3) the duct (fig. 5, d.} of the posterior portion of the kidneys, which has the same relations as in the female.
(4) The segmental tubes (fig. 5. st.}. These have the same relations as in the female, except that the most anterior two, three or more, unite with the testicular follicles, and carry away the semen into the Wolffian duct.
The mode of arrangement and the development of these parts suggest a number of considerations.
In the first place it is important to notice that the segmental tubes develope primitively as completely independent
THE URINOGENITAL ORGANS OF VERTEBRATES. 143
organs 1 , one of which appears in each segment. If embryology is in any way a repetition of ancestral history, it necessarily follows that these tubes were primitively independent of each other. Ancestral history, as recorded in development, is often, it is true, abridged ; but it is clear that though abridgement might prevent a series of primitively separate organs from appearing as such, yet it would hardly be possible for a primitively compound organ, which always retained this condition, to appear during development as a series of separate ones. These considerations appear to me to prove that the segmented ancestors of vertebrates possessed a series of independent and segmental excretory organs.
Both Professor Semper and myself, on discovering these organs, were led to compare them and state our belief in their identity with the so-called segmental organs of Annelids.
This view has since been fairly generally accepted. The segmental organs of annelids agree with those of vertebrates in opening at one end into the body-cavity, but differ in the fact that each also communicates with the exterior by an independent opening, and that they are never connected with each other.
On the hypothesis of the identity of the vertebrate segmental tubes with the annelid segmental organs, it becomes essential to explain how the external openings of the former may have become lost.
This brings us at once to the origin of the segmental duct of the kidneys, by which the secretion of all the segmental tubes was carried to the exterior, and it appears to me that a right understanding of the vertebrate urinogenital system depends greatly upon a correct view of the origin of this duct. I would venture to repeat the suggestion which I made in my original paper (he. cit.} that this duct is to be looked upon as the most anterior of the segmental tubes which persist in vertebrates.
1 Further study of my sections has shewn me that the initial independence of these organs is even more complete than might be gathered from the description in my paper (loc. cit.). I now find, as I before conjectured, that they at first correspond exactly with the muscle-plates, there being one for each muscle-plate. This can be seen in the fresh embryos, but longitudinal sections shew it in an absolutely demonstrable manner.
144 THE URINOGENITAL ORGANS OF VERTEBRATES.
In favour of this view are the following anatomical and embryological facts, (i) It developes in nearly the same manner as the other segmental tubes, viz. in Selachians as a solid outgrowth from the intermediate cell- mass, which subsequently becomes hollowed so as to open into the body-cavity : and in Amphibians and Osseous and Cyclostome fishes as a direct involution from the body-cavity. (2) In Amphibians, Cyclostomes and Osseous fishes its upper end develops a glandular portion, by becoming convoluted in a manner similar to the other segmental tubes. This glandular portion is often called either the head-kidney or the primitive kidney. It is only an embryonic structure, but is important as demonstrating the true nature of the primitive duct of the kidneys.
We may suppose that some of the segmental tubes first united, possibly in pairs, and that then by a continuation of this process the whole of them coalesced into a common gland. One external opening sufficed to carry off the entire secretion of the gland, and the other openings therefore atrophied.
This history is represented in the development of the dogfish in an abbreviated form, by the elongation of the first segmental tube (segmental duct of the kidney) and its junction with each of the posterior segmental tubes. Professor Semper looks upon the primitive duct of the kidneys as a duct which arose independently, and was not derived from metamorphosis of the segmental organs. Against this view I would on the one hand urge the consideration, that it is far easier to conceive of the transformation by change of function (comp. Dohrn, Functions^vechsel, Leipzig, 1875) of a segmental organ into a segmental duct, than to understand the physiological cause which should lead, in the presence of so many already formed ducts, to the appearance of a totally new one. By its very nature a duct is a structure which can hardly arise de novo. We must even suppose that the segmental organs of Annelids were themselves transformations of still simpler structures. On the other hand I would point to the development in this very duct amongst Amphibians and Osseous fishes of a glandular portion similar to that of a segmental tube, as an a posteriori proof of its being a metamorphosed segmental tube. The development in insects of a longitudinal tracheal duct by the coalescence of a
THE URINOGENITAL ORGANS OF VERTEBRATES. 145
series of transverse tracheal tubes affords a parallel to the formation of a duct from the coalescence of a series of segmental tubes.
Though it must be admitted that the loss of the external openings of the segmental organs requires further working out, yet the difficulties involved in their disappearance are not so great as to render it improbable that the vertebrate segmental organs are descended from typical annelidan ones.
The primitive vertebrate condition, then, is probably that of an early stage of Selachian development while there is as yet a segmental duct, the original foremost segmental tube opening in front into the body-cavity and behind into the cloaca ; with which duct all the segmental tubes communicate. Vide Fig. 2.
The next condition is to be looked upon as an indirect result of the segmental duct serving as well for the products of the generative organs as the secretions of the segmental tubes.
As a consequence of this, the segmental duct became split into a ventral portion, which served alone for the ova, and a dorsal portion which received the secretion of the segmental tubes. The lower portion, which we have called the oviduct, in some cases may also have received the semen as well as the ova. This is very possibly the case with Ceratodus (vide Giinther, Trans, of Royal Society, 1871), and the majority of Ganoids (Hyrtl, Denksckriften Wien, Vol. VIII.). In the majority of other cases the oviduct exists in the male in a completely rudimentary form ; and the semen is carried away by the -same duct as the urine.
In Selachians the transportation of the semen from the testis to the Wolffian duct is effected by the junction of the open ends of two or three or more segmental tubes with the testicular follicles, and the modes in which this junction is effected in the higher vertebrates seem to be derivatives from this. If the views here expressed are correct it is by a complete change of function that the oviduct has come to perform its present office. And in the bird and higher vertebrates no trace, or only the very slightest (vide p. 165) of the primitive urinary function is retained during embryonic or adult life.
The last feature in the anatomy of the Selachians which B. 10
146 THE URINOGENITAL ORGANS OF VERTEBRATES.
requires notice is the division of the kidney into two portions, an anterior and posterior. The anatomical similarity between this arrangement and that of higher vertebrates (birds, &c.) is very striking. The anterior one precisely corresponds, anatomically, to the Wolffian body, and the posterior one to the true permanent kidney of higher vertebrates : and when we find that in the Selachians the duct for the anterior serves also for the semen as does the Wolffian duct of higher vertebrates, this similarity seems almost to amount to identity. A discussion of the differences in development in the two cases will come conveniently with the account of the bird ; but there appear to me the strongest grounds for looking upon the kidneys of Selachians as equivalent to both the Wolffian bodies and the true kidneys of the higher vertebrates.
The condition of the urinogenital organs in Selachians is by no means the most primitive found amongst vertebrates.
The organs of both Cyclostomous and Osseous fishes, as well as those of Ganoids, are all more primitive ; and in the majority of points the Amphibians exhibit a decidedly less differentiated condition of these organs than do the Selachians.
In Cyclostomous fishes the condition of the urinary system is very simple. In Myxine (vide Joh. Muller My xinoid fishes, and Wilhelm Muller, Jenaische Zeitsckrift, 1875, Das Urogenitalsystem des A mphioxus u. d. Cyclostomeri) there is a pair of ducts which communicate posteriorly by a common opening with the abdominal pore. From these ducts spring a series of transverse tubules, each terminating in a Malpighian corpuscle. These together constitute the mass of the kidneys. About opposite the gall-bladder the duct of the kidney (the segmental duct) narrows very much, and after a short course ends in a largish glandular mass (the head-kidney), which communicates with the pericardial cavity by a number of openings.
In Petromyzon the anatomy of the kidneys is fundamentally the same as in Myxine. They consist of the two segmental ducts, and a number of fine branches passing off from these, which become convoluted but do not form Malpighian tufts. The head-kidney is absent in the adult.
W. Muller (loc. cit.} has given a short but interesting account of the development of the urinary system of Petromyzon. He
THE URINOGENITAL ORGANS OF VERTEBRATES. 147
finds that the segmental ducts develop first of all as simple involutions from the body-cavity. The anterior end of each then developes a glandular portion which comes to communicate by a number of openings with the body-cavity. Subsequently to the development of this glandular portion the remainder of the kidneys appears in the posterior portion of the body-cavity ; and before the close of embryonic life the anterior glandular portion atrophies.
The comparison of this system with that of a Selachian is very simple. The first developed duct is the segmentai duct of a Selachian, and the glandular portion developed at its anterior extremity, which is permanent in Myxine but embryonic in Petromyzon, is, as W. Miiller has rightly recognized, equivalent to the head-kidney of Amphibians, which remains undeveloped in Selachians. It is, according to my previously stated view, the glandular portion of the first segmental organ or the segmental duct. The series of orifices by which this communicates with the body-cavity are due to the division of the primary opening of the segmental duct. This is shewn both by the facts of their development in Petromyzon given by Muller, as well as by the occurrence of a similar division of the primary orifice in Amphibians, which is mentioned later in this paper. In a note in my original paper (loc. cit.} I stated that these openings were equivalent to the segmental involutions of Selachians. This is erroneous, and was due to my not having understood the description given in a preliminary paper of Muller (JenaiscJie Zeitschrift, 1873). The large development of this glandular mass in the Cyclostome and Osseous fishes and in embryo Amphibians, implies that it must at one time have been important. Its earlier development than the remainder of the kidneys is probably a result of the specialized function of the first segmental organ.
The remainder of the kidney in Cyclostomes is equivalent to the kidney of Selachians. Its development from segmental involutions has not been recognized. If these segmental involutions are really absent it may perhaps imply that the simplicity of the Cyclostome kidneys, like that of so many other of their organs, is a result of degeneration rather than a primitive condition.
JO 2
148 THE URINOGENITAL ORGANS OF VERTEBRATES.
In Osseous fishes the segmental duct of the kidneys developes, as the observations of Rosenberg 1 (" Teleostierniere," Inaug. Disser. Dorpat, 1867) and Oellacher (Zeitschrift fiir Wiss. Zool. 1873) clearly prove, by an involution from the body-cavity. This involution grows backwards in the form of a duct and opens into the cloaca. The upper end of this duct (the most anterior segmental tube) becomes convoluted, and forms a glandular body, which has no representative in the urinary apparatus of Selachians, but whose importance, as indicating the origin of the segmental duct of the kidneys, I have already insisted upon.
The rest of the kidney becomes developed at a later period, probably in the same way as in Selachians ; but this, as far as I know, has not been made out.
The segmental duct of the kidneys forms the duct for this new gland, as in embryo Selachians (Fig. 2), but, unlike what happens in Selachians, undergoes no further changes, with the exception of a varying amount of retrogressive metamorphosis of its anterior end. The kidneys of Osseous fish usually extend from just behind the head to opposite the anus, or even further back than this. They consist for the most part of a broader anterior portion, an abdominal portion reaching from this to the anus, and, as in those cases in which the kidneys extend further back than the anus, of a caudal portion.
The two ducts (segmental ducts of the kidneys) lie, as a rule, in the lower part of the kidneys on their outer borders, and open almost invariably into a urinary bladder. In some cases they unite before opening into the bladder, but generally have independent openings.
This bladder, which is simply a dilatation of the united lower ends of the primitive kidney-ducts, and has no further importance, is almost invariably present, but in many cases lies unsymmetrically either to the right or the left. It opens to the exterior by a very minute opening in the genito-urinary papilla, immediately behind the genital pore. There are, however, a few cases in which the generative and urinary organs have a
1 I am unfortunately only acquainted with Dr Rosenberg's paper from an abstract.
THE URINOGEN1TAL ORGANS OF VERTEBRATES. 149
common opening. For further details vide Hyrtl, Denk. der k. Akad. Wien, Vol. II.
It is possible that the generative ducts of Osseous fishes are derived from a splitting from the primitive duct of the kidney, but this is discussed later in the paper.
In Osseous fishes we probably have an embryonic condition of the Selachian kidneys retained permanently through life.
In the majority of Ganoids the division of the segmental duct of the kidney into two would seem to occur, and the ventral duct of the two (Miillerian duct), which opens at its upper end into the body-cavity, is said to serve as an excretory duct for both male and female organs.
The following are the more important facts which are known about the generative and urinary ducts of Ganoids.
In Spatularia (vide Hyrtl, Geschlechts u. Harnwerkzeuge bei den Ganoiden, DenkscJiriften der k. Akad. Wien, Vol. VIII.) the following parts are found in the female.
(1) The ovaries stretching along the whole length of the abdominal cavity.
(2) The kidneys, which are separate and also extend along the greater part of the abdominal cavity.
(3) The ureters lying on the outer borders of the kidneys. Each ureter dilates at its lower end into an elongated wide tube, which continues to receive the ducts from the kidneys. The two ureters unite before terminating and open behind the anus.
(4) The two oviducts (Mullerian ducts). These open widely into the abdominal cavity, at about two-thirds of the distance from the anterior extremity of the body-cavity. Each opens by a narrow pore into the dilated ureter of its side.
In the male the same parts are found as in the female, but Hyrtl found that the Mullerian duct of the left side at its entrance into the ureter became split into two horns, one of which ended blindly. On the right side the opening of the Mullerian duct was normal.
In the Sturgeon (vide J. Muller, Ban u. Grenzeu d. Ganoiden, Berlin Akad. 1844; Leydig, FiscJien u. Reptilicn, and Hyrtl, Ganoideit) the same parts are found as in Spatularia.
ISO THE URINOGENITAL ORGANS OF VERTEBRATES.
The kidneys extend along the whole length of the bodycavity ; and the ureter, which does not reach the whole length of the kidneys, is a thin-walled wide duct lying on the outer side. On laying it open the numerous apertures of the tubules for the kidney are exposed. The Miillerian duct, which opens in both sexes into the abdominal cavity, ends, according to Leydig, in the cases of some males, blindly behind without opening into the ureter, and Miiller makes the same statement for both sexes. It was open on both sides in a female specimen I examined 1 , and Hyrtl found it invariably so in both sexes in all the specimens he examined.
Both Rathke and Stannius (I have been unable to refer to the original papers) believed that the semen was carried off by transverse ducts directly into the ureter, and most other observers have left undecided the mechanism of the transportation of the semen to the exterior. If we suppose that the ducts Rathke saw really exist they might perhaps be supposed to enter not directly into the ureter, but into the kidney, and be in fact homologous with the vasa efferentia of the Selachians. The frequent blind posterior termination of the Miillerian duct is in favour of the view that these ducts of Rathke are really present.
In Polypterus (vide Hyrtl, Ganoideii) there is, as in other Ganoids, a pair of Miillerian ducts. They unite at their lower ends. The ureters are also much narrower than in previously described Ganoids and, after coalescing, open into the united oviducts. The urinogenital canal, formed by coalescence of the Miillerian ducts and ureters, has an opening to the exterior immediately behind the anus.
In Amia (vide Hyrtl) there is a pair of Miillerian ducts which, as well as the ureters, open into a dilated vesicle. This vesicle appears as a continuation of the Miillerian ducts, but receives a number of the efferent ductules of the kidneys. There is a single genito-urinary pore behind the anus.
In Ceratodus (Giinther, Phil. Trans. 1871) the kidneys are small and confined to the posterior extremity of the abdomen. The generative organs extend however along the greater part of
1 For this specimen I am indebted to Dr Giinther.
THE UR1NOGENITAL ORGANS OF VERTEBRATES. 151
the length of the abdominal cavity. In both male and female there is a long Mullerian duct, and the ducts of the two sides unite and open by a common pore into a urinogenital cloaca which communicates with the exterior by the same opening as the alimentary canal. In both sexes the Mullerian duct has a wide opening near the anterior extremity of the bodycavity. The ureters coalesce and open together into the % urinogenital cloaca dorsal to the Mullerian ducts. It is not absolutely certain that the semen is transported to the exterior by the Mullerian duct of the male, which is perhaps merely a rudiment as in Amphibia. Dr Gunther failed however to find any other means by which it could be carried away.
The genital ducts of Lepidosteus differ in important particulars from those of the other Ganoids (vide M tiller, loc. cit. and Hyrtl, loc, cit.}.
In both sexes the genital ducts are continuous with the investments of the genital organs.
In the female the dilated posterior extremities of the ureters completely invest for some distance the generative ducts, whose extremities are divided into several processes, and end in a different way on the two sides. A similar division and asymmetry of the ducts is mentioned by Hyrtl as occurring in the male of Spatularia, and it seems not impossible that on the hypothesis of the genital ducts being segmental tubes these divisions may be remnants of primitive glandular convolutions. The ureters in both sexes dilate as in other Ganoids at their posterior extremities, and unite with one another. The unpaired urinogenital opening is situated behind the anus. In the male the dilated portion of the ureters is divided into a series of partitions which are not present in the female.
Till the embryology of the secretory system of Ganoids has been worked out, the homologies of their generative ducts are necessarily a matter of conjecture. It is even possible that what I have called the Mullerian duct in the male is functionless, as with Amphibians, but that, owing to the true ducts of the testis having been overlooked, it has been supposed to function as the vas deferens. Giinther's (loc. cit.} injection experiments on Ceratodus militate against this view, but I do not think they can be considered as conclusive as long as the
152 THE URINOGENITAL ORGANS OF VERTEBRATES.
mechanism for the transportatiop of the semen to the exterior has not been completely made out. Analogy would certainly lead us to expect the ureter to serve in Ganoids as the vas deferens.
The position of the generative ducts might in some cases lead to the supposition that they are not Mullerian ducts, or, in other words, the most anterior pair of segmental organs but a pair of the posterior segmental tubes.
What are the true homologies of the generative ducts of Lepidosteus, which are continuous with the generative glands, is somewhat doubtful. It is very probable that they may represent the similarly functioning ducts of other Ganoids, but that they have undergone further changes as to their anterior extremities.
It is, on the other hand, possible that their generative ducts are the same structures as those ducts of Osseous fishes, which are continuous with the generative organs. These latter ducts are perhaps related to the abdominal pores, and had best be considered in connection with these; but a completely satisfactory answer to the questions which arise in reference to them can only be given by a study of their development.
In the Cyclostomes the generative products pass out by an abdominal pore, which communicates with the peritoneal cavity by two short tubes 1 , and which also receives the ducts of the kidneys.
Gegenbaur suggests that these are to be looked upon as Mullerian ducts, and as therefore developed from the segmental ducts of the kidneys. Another possible view is that they are the primitive external openings of a pair of segmental organs. In Selachians there are usually stated to be a pair of abdominal pores. In Scyllium I have only been able to find, on each side, a large deep pocket opening to the exterior, but closed below towards the peritoneal cavity, so that in it there seem to be no abdominal pores 2 . In the Greenland Shark (Lcemargns Borealis)
1 According to M tiller (Myxinoiden, 1845) there is in Myxine an abdominal pore with two short canals leading into it, and Vogt and Pappenheim (An. Sci. Nat. Part IV. Vol. xi.) state that in Petromyzon there are two such pores, each connected with a short canal.
2 My own rough, examination of preserved specimens was hardly sufficient to
THE URINOGENITAL ORGANS OF VERTEBRATES. 153
Professor Turner (Journal of Anat. and Phys. Vol. VIII.) failed to find either oviduct or vas deferens, but found a pair of large open abdominal pores, which he believes serve to carry away the generative products of both sexes. Whether the so-called abdominal pores of Selachians usually end blindly as in Scyllium, or, as is commonly stated, open into the body-cavity, there can be no question that they are homologous with true abdominal powers.
The blind pockets of Scyllium appear very much like the remains of primitive involutions from the exterior, which might easily be supposed to have formed the external opening of a pair of segmental organs, and this is probably the true meaning of abdominal pores. The presence of abdominal pores in all Ganoids in addition to true genital ducts and of these pockets or abdominal pores in Selachians, which are almost certainly homologous with the abdominal pores of Ganoids and Cyclostomes, and also occur in addition to true Miillerian ducts, speak strongly against the view that the abdominal pores have any relation to Miillerian ducts. Probably therefore the abdominal pores of the Cyclostomous fishes (which seem to be of the same character as other abdominal pores) are not to be looked on as rudimentary Miillerian ducts.
We next come to the question which I reserved while speaking of the kidneys of Osseous fishes, as to the meaning of their genital ducts.
In the female Salmon and the male and female Eel, the een c>
erative products are carried to the exterior by abdominal pores, and there are no true generative ducts. In the case of most other Osseous fish there are true generative ducts which are continuous with the investment of the generative organs 1 and
enable me to determine for certain the presence or absence of these pores. Mr Bridge, of Trinity College, has, however, since then commenced a series of investigations on this point, and informs me that these pores are certainly absent in Scyllium as well as in other genera.
1 The description of the attachment of the vas deferens to the testis in the Carp given by Vogt and Pappenheim (Ann. Scien. Nat. 1859) does not agree with what I found in the Perch (Perca fluvialis}. The walls of the duct are in the Perch continuous with the investment of the testis, and the gland of the testis occupies, as it were, the greater part of the duct ; there is, however, a distinct cavity corresponding to what Vogt and P. call the duct, near the border of attachment of the testis into
154 TH E URINOGENITAL ORGANS OF VERTEBRATES.
have generally, though not always, an opening or openings independent of the ureter close behind the rectum, but no abdominal pores are present. It seems, therefore, that in Osseous fish the generative ducts are complementary to abdominal pores, which might lead to the view that the generative ducts were formed by a coalescence of the investment of the generative glands with the short duct of abdominal pore.
Against this view there are, however, the following facts :
(1) In the cases of the salmon and the eel it is perfectly true that the abdominal pore exactly corresponds with the opening of the genital duct in other Osseous fishes, but the absence of genital ducts in these cases must rather be viewed, as Vogt and Pappenheim (loc. cit.) have already insisted, as a case of degeneration than of a primitive condition. The presence of genital ducts in the near allies of the Salmonidae, and even in the male salmon, are conclusive proofs of this. If we admit that the presence of an abdominal pore in Salmonidae is merely a result of degeneration, it obviously cannot be used as an argument for the complementary nature of abdominal pores and generative ducts.
(2) Hyrtl (Denkschriften der k. Akad. Wien, Vol I.) states that in Mormyrus oxyrynchus there is a pair of abdominal pores in addition to true generative ducts. If his statements are correct, we have a strong argument against the generative ducts of Osseous fishes being related to abdominal pores. For though this is the solitary instance of the presence of both a genital opening and abdominal pores known to me in Osseous fishes, yet we have no right to assume that the abdominal pores of Mormyrus are not equivalent to those of Ganoids and Selachians. It must be admitted, with Gegenbaur, that embryology alone can elucidate the meaning of the genital ducts of Osseous fishes.
In Lepidosteus, as was before mentioned, the generative ducts, though continuous with the investment of the generative bodies, unite with the ureters, and in this differ from the generative ducts of Osseous fishes. The relation, indeed, of the
which the seminal tubules open. I could find at the posterior end of the testis no central cavity which could be distinguished from the cavity of this duct.
THE URINOGENITAL ORGANS OF VERTEBRATES. 155
generative ducts of Lepidosteus to the urinary ducts is very similar to that existing in other Ganoid fishes ; and this, coupled with the fact that Lepidosteus possesses a pair of abdominal pores on each side of the anus 1 , makes it most probable that its generative ducts are true Miillerian ducts.
In the Amphibians the urinary system is again more primitive than in the Selachians.
The segmental duct of the kidneys is formed 2 by an elongated fold arising from the outer wall of the body-cavity, in the same position as in Selachians. This fold becomes constricted into a canal, closed except at its anterior end, which remains open to the body-cavity. This anterior end dilates, and grows out into two horns, and at the same time its opening into the body-cavity becomes partly constricted, and so divided into three separate orifices, one for each horn and a central one between the two. The horns become convoluted, blood channels appearing between their convolutions, and a special coil of vessels is formed arising from the aorta and projecting into the body-cavity near the openings of the convolutions. These formations together constitute the glandular portion 3 of the original anterior segmental tube or segmental duct of the kidneys. I have already pointed out the similarity which this organ exhibits to the head-kidneys of Cyclostome fishes in its mode of formation, especially with reference to the division of the primitive opening. The lower end of the segmental duct unites with a horn of the cloaca.
After the formation of the gland just described the remainder of the kidney is formed.
1 This is mentioned by Miiller (Ganoid fishes, Berlin Akad. 1844), Hyrtl (loc. tit.), and Gtinther (loc. cit.}, and through the courtesy of Dr Giinther I have had an opportunity of confirming the fact of the presence of the abdominal pores on two specimens of Lepidosteus in the British Museum.
2 My account of the development of these parts in Amphibians is derived for the most part from Gotte, Die antwickdungsgescMchte der Unke.
3 It is called Kopfniere (head-kidney), or Urniere (primitive kidney), by German authors. Leydig correctly looks upon it as together with the permanent kidney constituting the Urniere of Amphibians. The term Urniere is one which has arisen in my opinion from a misconception ; but certainly the Kopfniere has no greater right to the appellation than the remainder of the kidney.
156 THE URINOGENITAL ORGANS OF VERTEBRATES.
This arises in the same way as in Selachians. A series of involutions from the body-cavity are developed ; these soon form convoluted tubes, which become branched and interlaced with one another, and also unite with the primitive duct of the kidneys. Owing to the branching and interlacing of the primitive segmental tubes, the kidney is not divided into distinct segments in the same way as with the Selachians. The mode of development of these segmental tubes was discovered by Gotte. Their openings are ciliated, and, as Spengel (loc. cit.} and Meyer (loc. Y.) have independently discovered, persist in most adult Amphibians. As both these investigators have pointed out, the segmental openings are in the adult kidneys of most Amphibians far more numerous than the vertebral segments to which they appertain. This is .due to secondary changes, and is not tp be looked upon as the primitive state of things. At this stage the Amphibian kidneys are nearly in the same condition as the Selachian, in the stage represented in Fig. 2. In both there is the segmental duct of the kidneys, which is open in front, communicates with the cloaca behind, and receives the whole secretion from the kidneys. The parallelism between the two is closely adhered to in the subsequent modifications of the Amphibian kidney, but the changes are not completed so far in Amphibians as in Selachians. The segmental duct of the Amphibian kidney becomes, as in Selachians, split into a Miillerian duct or oviduct, and a Wolffian duct or duct for the kidney.
The following points about this are noteworthy :
(1) The separation of the two ducts is never completed, so that they are united together behind, and for a short distance, blend and form a common duct ; the ducts of the two sides so formed also unite before opening to the exterior.
(2) The separation of the two ducts does not occur in the form of a simple splitting, as in Selachians. But the efferent ductules from the kidney gradually alter their points of entrance into the primitive duct. Their poinfe of entrance become carried backwards further and further, and since this process affects the anterior ducts proportionally more than the posterior, the efferent ducts finally all meet and form a common' duct which unites with the Mullerian duct near its posterior ex
THE URINOGENITAL ORGANS OF VERTEBRATES. 157
tremity. This process is not always carried out with equal completeness. In the tailless Amphibians, however, the process is generally 1 completed, and the ureters (Wolffian ducts) are of considerable length. Bufo cinereus, in the male of which the Mullerian ducts are very conspicuous, serves as an excellent example of this.
In the Salamander (Salamandra maculosa), Figs. 6 and 7, the process is carried out with greater completeness in the female than in the male, and this is the general rule in Amphibians. In the male Proteus, the embryonic condition would seem to be retained almost in its completeness so that the ducts of the kidney open directly and separately into the still persisting primitive duct of the kidney. The upper end of the duct nevertheless extends some distance beyond the end of the kidney and opens into the abdominal cavity. In the female Proteus, on the other hand, the separation into a Mullerian duct and a ureter is quite complete. The Newt (Triton) also serves as an excellent example of the formation of distinct Mullerian and Wolffian ducts being much more complete in the female than the male. In the female Newt all the tubules from the kidney open into a duct of some length which unites with the Mullerian duct near its termination, but in the male the anterior segmental tubes, including those which, as will be afterwards seen, serve as vasa efferentia of the testis, enter the Mullerian duct directly, while the posterior unite as in the female into a common duct before joining the Mullerian duct. For further details as to the variations exhibited in the Amphibians, the reader is referred to Leydig, Anat. Untersuchung, Fischen u. Reptilien. Ditto, Lehrbuch der Histologie, Menschen u. Thiere. Von Wittich, Siebold u. Kolliker, Zeitschrift, Vol. IV. p. 125.
The different conditions of completeness of the Wolffian ducts observable amongst the Amphibians are instructive in reference to the manner of development of the Wolffian duct in Selachians. The mode of division in the Selachians of the segmental duct of the kidney into a Mullerian and Wolffian
1 In Bombinator igneus, Von Wittich stated that the embryonic condition was retained. Leydig, Anatom. d. Amphib. u. Reptilien, shewed that this is not the case, but that in the male the Mullerian duct is very small, though distinct.
158 THE URINOGENITAL ORGANS OF VERTEBRATES.
duct is probably to be looked upon as an embryonic abbreviation of the process by which these two ducts are formed in Amphibians. The fact that this separation into Miillerian and Wolffian ducts proceeds further in the females of most Amphibians than in the males, strikingly shews that it is the oviductal function of the Miillerian duct which is the indirect cause of its separation from the Wolffian duct. The Miillerian duct formed in the way described persists almost invariably in both sexes, and in the male sometimes functions as a sperm reservoir ; e.g. Bufo cinereus. In the embryo it carries at its upper end the glandular mass described above (Kopfniere), but this generally atrophies, though remnants of it persist in the males of some species (e.g. Salamandra). Its anterior end opens, in most cases by a single opening, into the perivisceral cavity in both sexes, and is usually ciliated. As the female reaches maturity, the oviduct dilates very much ; but it remains thin and inconspicuous in the male.
The only other developmental change of importance is the connection of the testes with the kidneys. This probably occurs in the same manner as in Selachians, viz. from the junction of the open ends of the segmental tubes with the follicles of the testes. In any case the vessels which carry off the semen constitute part of the kidney, and the efferent duct of the testis is also that of the kidney. The vasa efferentia from the testis either pass through one or two nearly isolated anterior portions of the kidney (Proteus, Triton) or else no such special portion of the kidney becomes separated from the rest, and the vasa efferentia enter the general body of the kidney.
In the male Amphibian, then, the urinogenital system consists of the following parts (Fig. 6) :
(1) Rudimentary Miillerian ducts, opening anteriorly into the body-cavity, which sometimes carry aborted Kopfnieren.
(2) The partially or completely formed Wolffian ducts (ureters) which also serve as the ducts for the testes.
(3) The kidneys, parts of which also serve as the vasa efferentia, and whose secretion, together with the testicular products, is carried off by the Wolffian ducts.
THE URINOGENITAL ORGANS OF VERTEBRATES. 159
(4) The united lower parts of Wolffian and Miillerian ducts which are really the lower unsplit part of the segmental ducts of the kidneys.
FIG. 6. DIAGRAM OF THE URINOGENITAL ORGANS OF A MALE SALAMANDER.
(Copied from Ley dig's Histologie des Menschen u. der Thiere.)
md. MUller's duct (rudimentary); y. remnant of the secretory portion of the segmental duct Kopfniere ; Wd. Wolffian duct ; a less complete structure in the male than in the female ; st. segmental tubes or kidney. The openings of these into the body-cavity are not inserted in the figure ; t. testis. Its efferent ducts form part of the kidney.
In the female, there are (Fig. 7)
(1) The Miillerian ducts which function as the oviducts.
(2) The Wolffian ducts.
(3) The kidneys.
(4) The united Miallerian and Wolffian ducts as in the male.
Wfif
m.d
FIG. 7. DIAGRAM OF THE URINOGENITAL ORGANS OF A FEMALE SALAMANDER.
(Copied from Ley dig's Histologie des Menschen u. der Thiere)
Md. Muller's duct or oviduct ; Wd. Wolman duct or the duct of the kidneys ; st. segmental tubes or kidney. The openings of these into the body-cavity are not inserted in the figure ; o. ovary.
The urinogenital organs of the adult Amphibians agree in almost all essential particulars with those of Selachians. The
l6o THE URINOGENITAL ORGANS OF VERTEBRATES.
ova are carried off in both by a specialized oviduct. The Wolffian duct, or ureter, is found both in Selachians and Amphibians, and the relations of the testis to it are the same in both, the vasa efferentia of the testes having in both the same anatomical peculiarities.
The following points are the main ones in which Selachians and Amphibians differ as to the anatomy of the urinogenital organs ; and in all but one of these, the organs of the Amphibian exhibit a less differentiated condition than do those of the Selachian.
(1) A glandular portion (Kopfniere) belonging to the first segmental organ (segmental duct of the kidneys) is found in all embryo Amphibians, but usually disappears, or only leaves a remnant in the adult. It has not yet been found in any Selachian.
(2) The division of the primitive duct of the kidney into the Miillerian duct and the Wolffian duct is not completed so far in Amphibians as Selachians, and in the former the two ducts are confluent at their lower ends.
(3) The permanent kidney exhibits in Amphibians no distinction into two glands (foreshadowing the Wolffian bodies and true kidneys of higher vertebrates), as it does in the Selachians.
(4) The Miillerian duct persists in its entirety in male Amphibians, but only its upper end remains in male Selachians.
(5) The openings of the segmental tubes into the bodycavity correspond in number with the vertebral segments in most Selachians, but are far more numerous than these in Amphibians. This is the chief point in which the Amphibian kidney is more differentiated than the Selachian.
The modifications in development which the urinogenital system has suffered in higher vertebrates (Sauropsida and Mammalia) are very considerable ; nevertheless it appears to me to be possible with fair certainty to trace out the relationship of its various parts in them to those found in the Ichthyopsida. The development of urinogenital organs has been far more fully worked out for the bird than for any other member of the amniotic vertebrates ; but, as far as we know,
THE URINOGENITAL ORGANS OF VERTEBRATES. l6l
there are no essential variations except in the later periods of development throughout the division. These later variations, concerning for the most part the external apertures of the various ducts, are so well known and have been so fully described as to require no notice here. The development of these parts in the bird will therefore serve as the most convenient basis for comparison.
In the bird the development of these parts begins by the appearance of a column of cells on the upper surface of the intermediate cell-mass (Fig. 8, W.d\ As in Selachians, the intermediate cell-mass is a group of cells between the outer edge of the protovertebrae and the upper end of the body cavity. The column of cells thus formed is the commencement of the duct of the Wolffian body. Its development is strikingly similar to that of the segmental duct of the kidney in Selachians. I shall attempt when I have given an account of the development of the Miillerian duct to speak of the relations between the Selachian duct and that of the bird.
Romiti (A rcJiiv f. Micr. Anaf.Vol.X.) has recently stated that the Wolffian duct developes as an involution from the body cavity. The fact that the specimens drawn by Romiti to support this view are too old to determine such a point, and the inspection of a number of specimens made by my friend Mr Adam Sedgwick of Trinity College, who, at my request, has been examining the urinogenital organs of the fowl, have led me to the conclusion that Romiti is in error in differing from his predecessors as to the development of the Wolffian duct. The solid string of cells to form the Wolffian duct lies at first close to the epiblast, but, by the alteration in shape which the protovertebrse undergo and the general growth of cells around it, becomes gradually carried downwards till it lies close to the germinal epithelium which lines the body cavity. While undergoing this change of position it also acquires a lumen, but ends blindly both in front and behind. Towards the end of the fourth day the Wolffian duct opens into a horn of the cloaca. The cells adjoining its inner border commence, as it passes down on the third day, to undergo histological changes, which, by the fourth day, result in the formation of a B. II
162 THE URINOGENITAL ORGANS OF VERTEBRATES.
FIG. 8. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO
FOWL OF 45 h. To SHEW THE MODE OF FORMATION OF THE WOLFFIAN
DUCT.
A. epiblast ; B. mesoblast ; C. hypoblast ; M.c. medullary canal; Pv. Protovertebrse ; W.d. Wolffian duct ; So. Somatopleure ; Sp. Splanchnopleure ; //. pleuroperitoneal cavity ; ch. note-chord ; ay. dorsal aorta ; v. blood-vessels.
THE UR1NOGENITAL ORGANS OF VERTEBRATES. 163
series of ducts and Malpighian tufts which form the mass of the Wolffian body 1 .
The Miillerian duct arises in the form of an involution, whether at first solid or hollow, of the germinal epithelium, and, as I am satisfied, quite independently of the Wolffian duct. It is important to notice that its posterior end soon unites with the Wolffian duct, from which however it not long after becomes separated and opens independently into the cloaca. The upper end remains permanently open to the body cavity, and is situated nearly opposite the extreme front end of the Wolffian body.
Between the 8oth and rooth hour of incubation the ducts of the permanent kidneys begin to make their appearance. Near its posterior extremity each Wolffian duct becomes expanded, and from the dorsal side of this portion a diverticulum is constricted off, the blind end of which points forwards. This is the duct of the permanent kidneys, and around its end the kidneys are found. It is usually stated that the tubules of the permanent kidneys arise as outgrowths from the duct, but this requires to be worked over again.
The condition of the urinogenital system in birds immediately after the formation of the permanent kidneys is strikingly similar to its permanent condition in adult Selachians. There is the Miillerian duct in both opening in front into the body cavity and behind into the cloaca. In both the kidneys consist of two parts an anterior and posterior which have been called respectively Wolffian bodies and permanent kidneys in birds and Leydig's glands and the kidneys in Selachians.
The duct of the permanent kidney, which at first opens into that of the Wolffian body, subsequently becomes further split off from the Wolffian duct, and opens independently into the cloaca.
1 This account of the origin of the Wolffian body differs from that given by Waldeyer, and by Dr Foster and myself (Elements of Embryology, Foster and Balfonr), but I have been led to alter my view from an inspection of Mr Sedgwick's preparations, and I hope to shew that theoretical considerations lead to the expectation that the Wolffian body would develop independently of the duct.
I I 2
1 64 THE URINOGENITAL ORGANS OF VERTEBRATES.
The subsequent changes of these parts are different in the two sexes.
In the female the Mullerian ducts 1 persist and become the oviducts. Their anterior ends remain open to the body cavity. The changes in their lower ends in the various orders of Sauropsida and Mammalia are too well known to require repetition here. The Wolffian body and duct atrophy: there are left however in many cases slight remnants of the anterior extremity of the body forming the parovarium of the bird, and also frequently remnants of the posterior portion of the gland as well as of the duct. The permanent kidney and its duct remain unaltered.
In the male the Mullerian duct becomes almost completely obliterated. The Wolffian duct persists and forms the vas deferens, and the anterior so-called sexual portion of the Wolffian body also persists in an altered form. Its tubules unite with the seminiferous tubules, and also form the epididymis. Unimportant remnants of the posterior part of the Wolffian body also persist, but are without function. in. both sexes the so-called permanent kidneys form the sole portion of the primitive uriniferous system which persists in the adult.
In considering the relations between the modes of development of the urinogenital organs of the bird and of the Selachians, the first important point to notice is, that whereas in the Selachians the segmental duct of the kidneys is first developed and subsequently becomes split into the Mullerian and Wolffian ducts ; in the bird these two ducts develope independently. This difference in development would be accurately described by saying that in birds the segmental duct of the kidneys developes as in Selachians, but that the Mullerian ductdevelopes independently of it.
Since in Selachians the Wolffian duct is equivalent to the segmental duct of the kidneys with the Mullerian removed from it, when in birds the Mullerian duct developes independently of the segmental kidney duct, the latter becomes the same as the Wolfftan duct.
1 The right oviduct atrophies in birds, and the left alone persists in the adult.
THE URINOGENITAL ORGANS OF VERTEBRATES. 165
The second mode of stating the difference in development in the two cases represents the embryological facts of the bird far better than the other method.
It explains why the Wolffian duct appears earlier than the Miillerian and not at the same time, as one might expect according to the other way of stating the case. If the Wolffian duct is equivalent to the segmental duct of Selachians, it must necessarily be the first duct to develope ; and not improbably the development of the Miillerian duct would in birds be expected to occur at the time corresponding to that at which the primitive duct in Selachians became split into two ducts.
It probably also explains the similarity in the mode of development of the Wolffian duct in birds and the primitive duct of the kidneys in Selachians.
This way of stating the case is also in accordance with theoretical conclusions. As the egg-bearing function of the Miillerian duct became more and more confirmed we might expect that the adult condition would impress itself more and more upon the embryonic development, till finally the Miillerian duct ceased to be at any period connected with the kidneys, and the history of its origin ceased to be traceable in its development. This seems to have actually occurred in the higher vertebrates, so that the only persisting connection between the Miillerian duct and the urinary system is the brief but important junction of the two at their lower ends on the sixth or seventh day. This junction justly surprised Waldeyer (Eierstock it. Ei, p. 129), but receives a complete and satisfactory explanation on the hypothesis given above.
The original development of the segmental tubes is in the bird solely retained in the tubules of the Wolffian body arising independently of the Wolffian duct, and I have hitherto failed to find that there is a distinct division of the Wolffian bodies into segments corresponding with the vertebral segments.
I have compared the permanent kidneys to the lower portion of the kidneys of Selachians. The identity of the anatomical condition of the adult Selachian and embryonic bird which has been already pointed out speaks strongly in favour of this view ; and when we further consider that the duct of
165 THE URINOGENITAL ORGANS OF VERTEBRATES.
the permanent kidneys is developed in nearly the same way as the supposed homologous duct in Selachians, the suggested identity gains further support. The only difficulty is the fact that in Selachians the tubules of the part of the kidneys under comparison develope as segmental involutions in point of time anteriorly to their duct, while in birds they develope in a manner not hitherto certainly made out but apparently in point of time posteriorly to their duct. But when the immense modifications in development which the whole of the gland of the excretory organ has undergone in the bird are considered, I do not think that the fact I have mentioned can be brought forward as a serious diffiulty.
The further points of comparison between the Selachian and the bird are very simple. The Miillerian duct in its later stages behaves in the higher vertebrates precisely as in the lower. It becomes in fact the oviduct in the female and atrophies in the male. The behaviour of the Wolffian duct is also exactly that of the duct which I have called the Wolffian duct in Ichthyopsida, and in the tubules of the Wolffian body uniting with the tubuli seminiferi we have represented the junction of the segmental tubes with the testis in Selachians and Amphibians. It is probably this junction of two independent organs which led Waldeyer to the erroneous view that the tubuli seminiferi were developed from the tubules of the Wolffian body.
With the bird I conclude the history of the origin of the urinogenital system of vertebrates. I have attempted, and I hope succeeded, in tracing out by the aid of comparative anatomy and embryology the steps by which a series of independent and simple segmental organs like those of Annelids have become converted into the complicated series of glands and ducts which constitute the urinogenital system of the higher vertebrates. There are no doubt some points which require further elucidation amongst the Ganoid and Osseous fishes. The most important points which appear to me still to need further research, both embryological and anatomical, are the abdominal pores of fishes, the generative ducts of Ganoids, especially Lepidosteus, and the generative ducts of Osseous fishes.
THE URINOGENITAL ORGANS OF VERTEBRATES. 167
The only further point which requires discussion is the embryonic layer from which these organs are derived.
I have shewn beyond a doubt (loc. cit^) that in Selachians these organs are formed from the mesoblast. The unanimous testimony of all the recent investigators of Amphibians leads to the same conclusion. In birds, on the other hand, various investigators have attempted to prove that these organs are derived from the epiblast. The proof they give is the following : the epiblast and mesoblast appear fused in the region of the axis cord. From this some investigators have been led to the conclusion that the whole of the mesoblast is derived from the upper of the two primitive embryonic layers. To these it may be replied that, even granting their view to be correct, it is no proof of the derivation of the urinogenital organs from the epiblast, since it is not till the complete formation of the three layers that any one of them can be said to exist. Others look upon the fusion of the two layers as a proof of the passage of cells from the epiblast into the mesoblast. An assumption in itself, which however is followed by the further assumption that it is from these epiblast cells that the urinogenital system is derived ! Whatever may have been the primitive origin of the system, its mesoblastic origin in vertebrates cannot in my opinion be denied.
Kowalewsky (Embryo. Stud, an Vermeil u. ArtJiropoda, Mem. Akad. St Petersbourg, 1871) finds that the segmental tubes of Annelids develope from the mesoblast. We must therefore look upon the mesoblastic origin of the excretory system as having an antiquity greater even than that of vertebrates.
