Stannius alludes[328]to both these bodies, and though he does not contribute much to Leydig's previous statements, yet he accepts Leydig's position with reference to the relation of the sympathetic and suprarenal bodies[329].
The general text-books of Histology, Kölliker's work, and Eberth's article in Stricker'sHistology, do not give much information on this subject; but Eberth, without apparently having examined the point, questions the accuracy of Leydig's statements with reference to the anatomical relations of the sympathetic ganglia and suprarenal bodies.
The last author who has dealt with this subject is Professor Semper[330]. He records observations both on the anatomy and development of these organs. His anatomical observations are in the main confirmatory of those of Leydig, but he shews still more clearly than did Leydig the segmental arrangement of the suprarenal bodies. He definitely regards the interrenal and suprarenal bodies as parts of the same system, and states that in many forms they are continuous (p. 228):
“Hier freilich gehen sie bei manchen Formen...in einen Körper über, welcher zwischen den Enden d. beiden Nieren liegend dicht an der einfachen Caudalvene sitzt.”
With reference to their development he says:“They arise then also completely independently of the kidneys, as isolated segmentally arranged groups of mesoderm cells between the convolutions of the segmental organs; only anteriorly do they stretch beyond them, and extend quite up to the pericardium.”
To Semper's statements I shall return, but now pass on to my own observations. The paired suprarenal bodies are dealt with first.
The suprarenal bodies.
My observations on these bodies in the adult Scyllium have only been made with specimens hardened in chromic acid, and there are many points which deserve a fuller investigation than I have been able to give them.
The general position and relations of the suprarenal bodies have been fully given by Leydig and Semper, and I have nothing to add to their statements. They are situated on branches of the aorta, segmentally arranged, and extend on each side of the vertebral column from close behind the heart to the posterior part of the body-cavity. The anterior pair are the largest, and are formed apparently from the fusion of two bodies[331]. When these bodies are examined microscopically, their connection with the sympathetic ganglia becomes at once obvious. Bound up in the same sheath as the anterior one is an especially large ganglion already alluded to by Leydig, and sympathetic ganglia are more or less distinctly developed in connection with all the others. There is however considerable irregularity in the development and general arrangement of the sympathetic ganglia, which are broken up into a number of small ganglionic swellings, on some of which an occasional extra suprarenal body is at times developed. As a rule it may be stated that there is a much smaller ganglionic development in connection with the posterior suprarenal bodies than with the anterior.
The different suprarenal bodies exhibit variations in structure mainly dependent on the ganglion cells and nerves in them, and their typical structure is best exhibited in a posterior one, in which there is a comparatively small development of nervous elements.
A portion of a section through one of these is represented onPl.19, fig. 6, and presents the following features. Externally there is present a fibrous capsule, which sends in the septa, imperfectly dividing up the body into a series of alveoli or lobes. Penetrating and following the septa there is a rich capillary network. The parenchyma of the body itself exhibits a well-markeddistinction in the majority of instances into a cortical and medullary substance. The cortical substance is formed of rather irregular columnar cells, for the most part one row deep, arranged round the periphery of the body. Its cells measure on about an average .03Mm.in their longest diameter. The medullary substance is more or less distinctly divided into alveoli, and is formed of irregularly polygonal cells; and though it is difficult to give an estimate of their size on account of their irregularity, .021Mm.may be taken as probably about the diameter of an average cell. The character of the cortical and medullary cells is nearly the same, and the cells of the two strata appear rather to differ in shape than in any other essential point. The protoplasm of both has a markedly yellow tinge, giving to the suprarenal bodies a yellowish brown colour. The nuclei are small compared to the size of the cells, being about .009Mm.in both cortical and medullary cells. In the anterior suprarenal body there is a less marked distinction between the cortical and the medullary layers, and a less pronounced yellow coloration of the whole, than in the posterior bodies. The suprarenal bodies are often partially or completely surrounded by a lymphoid tissue, which is alluded to in the account of their development.
The most interesting features of my sections of the anterior bodies are the relations they bring to light between the sympathetic ganglia and the suprarenal bodies. In the case of one of the posterior suprarenal bodies, a small ganglion is generally found attached to both ends of the body, and invested in the same sheath; in addition to this a certain number of ganglion cells (very conspicuous by their size and other characters) are to be found scattered through the body. In the anterior suprarenal bodies the development of ganglion cells is very much greater. If a section is taken through the region where the large sympathetic ganglion (already mentioned) is attached to the body, one half of the section is composed mainly of sympathetic ganglion cells and nerve fibres, and the other of suprarenal tissue, but the former spread in considerable numbers into the latter. A transverse section through the suprarenal body in front of, or behind this point, is still more instructive. One of these is represented inPl.19, fig. 7. The suprarenal tissue is notinserted, but fills up the whole space within the outline of the body. At one point a nerve (n) is seen to enter. In connection with this are a number of ganglion cells, the exact distribution of which has been reproduced. They are scattered irregularly throughout the suprarenal body, but are more concentrated at the smaller than at the large end. It is this small end which, in succeeding sections, is entirely replaced by a sympathetic ganglion. Wavy fibres (which I take to be nervous) are distributed through the suprarenal body in a manner which, roughly speaking, is proportional to the number of ganglion cells. At the large end of the body, where there are few nerve cells, the typical suprarenal structure is more or less retained. Where the nerve fibres are more numerous at the small end of the section, they give to the tissue a somewhat peculiar appearance, though the individual suprarenal cells retain their normal structure. In a section of this kind the ganglion and nerves are clearly so intimately united with the suprarenal body as not to be separable from it.
The question naturally arises as to whether there are cells of an intermediate character between the ganglion cells and the cells of the suprarenal body. I have not clearly detected any such, but my observations are of too limited a character to settle the point in an adverse sense.
The embryological part of my researches on these bodies is in reality an investigation of later development of the sympathetic ganglia. The earliest stages in the development of these have already been given[332], and I take them up here as they appear during stage L, and shall confine my description to the changes they undergo in the anterior part of the trunk. They form during stage L irregular masses of cells with very conspicuous branches connecting them with the spinal nerves (Pl.18, fig. 3). There may be noticed at intervals solid rods of cells passing from the bodies to the aorta,Pl.18, fig. 2. These rods are the rudiments of the aortic branches to which the suprarenal bodies are eventually attached.
In a stage between M and N the trunks connecting these bodies with the spinal nerves are much smaller and less easy to see than during stage L. In some cases moreover the nervesappear to attach themselves more definitely to a central and inner part of the ganglia than to the whole of them. This is shewn inPl.19, fig. 8, and I regard it as the first trace of a division of the primitive ganglia into a suprarenal part and a ganglionic part. The branches from the aorta have now a definite lumen, and take a course through the centre of these bodies, as do the aortic branches in the adult.
By stage O these bodies have acquired a distinct mesoblastic investment, which penetrates into their interior, and divides it, especially in the case of the anterior bodies, into a number of distinct alveoli. These alveoli are far more distinct in some parts of the bodies than in others. The nerve-trunks uniting the bodies with the spinal nerves are (at least in specimens hardened in picric and chromic acids) very difficult to see, and I have failed to detect that they are connected with special parts of the bodies, or that the separate alveoli differ much as to the nature of their constituent cells. The aortic branches to the bodies are larger than in the previous stage, and the bodies themselves fairly vascular.
By stage Q (Pl.19, fig. 9) two distinct varieties of cells are present in these bodies. One of these is large, angular, and strikingly resembles the ganglion cells of the spinal nerves at the same period. This variety is found in separate lobules or alveoli on the inner border of the bodies. I take them to be true ganglion cells, though I have not seen them in my sections especially connected with the nerves. The cells of the second variety are also aggregated in special lobules, and are very markedly smaller than the ganglionic cells. They form, I imagine, the cells of the true suprarenal tissue. At this and the earlier stage lymphoid tissue, like that surrounding the suprarenal bodies in the adult, is found adjacent to these bodies.
Stage Q forms my last embryonic stage, and it may perhaps be asked on what grounds I regard these bodies as suprarenal bodies at all and not as simple sympathetic ganglia.
My determination mainly rests on three grounds: (1) That a branch from the aorta penetrates these bodies and maintains exactly the same relations to them that the same branches of the aorta do in the adult to the true suprarenal bodies. (2) That the bodies are highly vascular. (3) That in my last stage theybecome divided into a ganglionic and a non-ganglionic part, with the same relations as the ganglia and suprarenal tissue in the adult. These grounds appear to me to afford ample justification for my determinations, and the evidence adduced above appears to me to render it almost certain that the suprarenal tissue is a product of the primitive ganglion and not introduced from the mesoblast without, though it is not to be denied that a more complete investigation of this point than it has been possible for me to make would be very desirable.
Professor Semper states that he only made a very slight embryological investigation of these bodies, and probably has only carefully studied their later stages. He has accordingly overlooked the branches connecting them with the spinal nerves, and has not therefore detected the fact that they develop as parts of the sympathetic nervous system. I feel sure that if he re-examines his sections of younger embryos he will not fail to discover the nerve-branches described by me. His descriptions apart from this point accord fairly well with my own. The credit of the discovery that these bodies are really derivatives of the sympathetic nervous system is entirely Leydig's: my observations do no more than confirm his remarkable observations and well-founded conclusions.
Interrenal body.
My investigations on the interrenal body in the adult are even less complete than those on the suprarenal bodies. I find the body forming a small rod elliptical in section in the posterior region of the kidney between the dorsal aorta and unpaired caudal vein. Some little distance behind its front end (and probably not at its thickest point) it measured in one example, of which I have sections, a little less than a millimetre in its longest diameter. Anteriorly it overlaps the suprarenal bodies, and I failed to find any connection between them and it. On this point my observations do not accord with those of Professor Semper. I have however only been able to examine hardened specimens.
It is,videPl.18, fig. 8, invested by a fairly thick tunica propria, which sends in septa, dividing it into rather well-markedlobules or alveoli. These are filled with polygonal cells, which form the true parenchyma of the body. These cells are in my hardened specimens not conspicuous by the number of oil-globules they contain, as might have been expected from Leydig's description[333]. They are rather granular in appearance, and are mainly peculiar from the somewhat large size of the nucleus. The diameter of an average cell is about .015Mm., and that of the nucleus about .01 to .012. The nuclei are remarkably granular. The septa of the body are provided with a fairly rich capillary network.
At the first glance there is some resemblance in structure between the tissues of the suprarenal and interrenal bodies, but on a closer inspection this resemblance resolves itself into both bodies being divided up into lobules by connective-tissue septa. There is in the interrenal body no distinction between cortical and medullary layers as in the suprarenal. The cells of the two bodies have very different characters, as is demonstrated by a comparison of the relative diameters of the nuclei and the cells. The cells of the suprarenal bodies are considerably larger than those of the interrenal (.021 to .03 as compared to .015), yet the nuclei of the larger cells of the former body do not equal in size those of the smaller cells of the latter (.009 as compared to .01).
My observations both on the coarser anatomy and on the histology of the interrenal body in the adult point to its being in no way connected with the suprarenal bodies, and are thus in accordance with the earlier and not the later views of Leydig.
The embryology of this body (under the title of suprarenal body) was first described in my preliminary account of the development of the Elasmobranch Fishes[334]. A short account of its embryonic structure was given, and I stated that although I had not fully proved the point, yet I believed it to be derived from the wall of the alimentary canal. As will be shewn in the sequel this belief was ill-founded, and the organ in question is derived from the mesoblast. Allusion has also been made to itby Professor Semper, who figures it at an early stage of development, and implies that it arises in the mesoblast and in connection with the suprarenal body. It appears at stage K as a rod-like aggregate of mesoblast cells, rather more closely packed than their neighbours, between the two kidneys near their hinder ends (Plate 11, fig. 9a,su). The posterior and best marked part of it does not extend further forwards than the front end of the large intestine, and reaches backwards nearly as far as the hinder end of the kidneys. This part of the body lies between the caudal vein and dorsal aorta.
At about the point where the unpaired caudal vein divides into the two cardinals, the interrenal body becomes less well marked off from the surrounding tissue, though it may be traced forward for a considerable distance in the region of the small intestine. It retains up to stage Q its original extension, but the anterior part becomes quite definite though still of a smaller calibre than the posterior. In one of my examples of stage O the two divisions were separated by a small interval, and not as in other cases continuous. I have not determined whether this was an accidental peculiarity or a general feature. I have never seen any signs of the interrenal body becoming continuous with the suprarenal bodies, though, as in the adult, the two bodies overlap for a considerable distance.
The histology of the interrenal body in the embryonic periods is very simple. At first it is formed of cells differing from those around in being more circular and more closely packed. By stage L its cells have acquired a character of their own. They are still spherical or oval, but have more protoplasm than before, and their nucleus becomes very granular. At the same time the whole body becomes invested by a tunic of spindle-shaped mesoblast cells. By stage O it begins to be divided into a number of separate areas or lobes by septa formed of nucleated fibres. These become more distinct in the succeeding stages up to Q (Pl.18, fig. 7), and in them a fair number of capillaries are formed.
From the above description it is clear that embryology lends no more countenance than does anatomy to the view that the interrenal bodies belong to the same system as the suprarenal, and it becomes a question with which (if of either) of these twobodies the suprarenal bodies of the higher Vertebrata are homologous. This question I shall not attempt to answer in a definite way. My own decided belief is that the suprarenal bodies of Scyllium are homologous with the suprarenal bodies of Mammalia, and a good many points both in their structure and position might be urged in favour of this view. In the mean time, however, it appears to me better to wait before expressing a definite opinion till the embryonic development of the suprarenal bodies has been worked out in the higher Vertebrata.
EXPLANATION OF PLATE 19.
Complete List of Reference Letters.
Nervous System.
n.Nerve.spn.Spinal nerve.syg.Sympathetic ganglion.
Alimentary Canal.
cl.Cloaca.incl.Cloacal involution.œep.Œsophageal epithelium.pan.Pancreas.th.Thyroid body.
General.
abp.Abdominal pocket (pore).aur.Auricle.cav.Cardinal vein.cauv.Caudal vein.ly.Lymphoid tissue.mm.Muscles.od.Oviduct.pc.Pericardium.pp.body-cavity.sr.Suprarenal body.u.Ureter.vao.Ventral aorta (anterior continuation of bulbus arteriosus).ven.Ventricle.wd.Wolffian duct.
Figs. 1a, 1b, 1c. Three sections through the cloacal region of an embryo belonging to stage O. 1ais the anterior of the three sections. Zeiss A,ocul.2. Reduced one-third.
1ashews the cloacal involution at its deepest part abutting on the cloacal section of the alimentary tract.
1bis a section through a point somewhat behind this close to the opening of the Wolffian ducts into the cloaca.
1cshews the opening to the exterior in the posterior part of the cloaca, and also the rudiments of the two abdominal pockets (abp).
Fig. 2. Section through the cloacal region of an embryo belonging to stage P. Zeiss A,ocul.2.
The figure shews the solid anterior extremity of the cloacal involution.
Fig. 3. Longitudinal vertical section through the thyroid body in a stage between O and P. Zeiss a a,ocul.1.
The figure shews the solid thyroid body (th) connected in front with throat, and terminating below the bulbus arteriosus.
Fig. 4. Pancreas (pan) and adjoining part of the alimentary tract in longitudinal section, from an embryo between stages L and M. Zeiss A,ocul.2.
Fig. 5. Portion of liver network of stage L. Zeiss C,ocul.2. The section is intended to illustrate the fact that the tubules or cylinders of which the liver is composed are hollow and not solid. Between the liver tubules are seen blood spaces with distinct walls, and blood corpuscles in their interior.
Fig. 6. Section through part of one of the suprarenal bodies of an adult Scyllium hardened in chromic acid. Zeiss C,ocul.2. The section shews the columnar cells forming the cortex and the more polygonal cells of the medulla.
Fig. 7. Transverse section through the anterior suprarenal body of an adult Scyllium. Zeiss B,ocul.2. Reduced one-third. The tissue of the suprarenal body has not been filled in, but only the sympathetic ganglion cells which are seen to be irregularly scattered through the substance of the body. The entrance of the nerve (n) is shewn, and indications are given of the distribution of the nerve-fibres.
Fig. 8. Section through the sympathetic ganglion of a Scyllium embryo between stages M and N, shewing the connecting trunk between the suprarenal body and the spinal nerve (spn), and the appearance of an indication in the ganglion of a portion more directly connected with the nerve. Zeiss D,ocul.2.
Fig. 9. Section through one of the anterior sympathetic ganglia of an embryo of stage Q, shewing its division into a true ganglionic portion (syg), and a suprarenal body (sr). Zeiss C,ocul.2.
[314]From observations on the development of the heart in the Fowl, I have been able to satisfy myself that the epithelioid lining of the heart is derived from the splanchnic mesoblast. When the cavity of the heart is being formed by the separation of the splanchnic mesoblast from the hypoblast, a layer of the former remains close to the hypoblast, but connected with the main mass of the splanchnic mesoblast by protoplasmic processes. A second layer next becomes split from the splanchnic mesoblast, connected with the first layer by the above-mentioned protoplasmic processes. These two layers form the epithelioid lining of the heart; between them is the cavity of the heart, which soon loses the protoplasmic trabeculæ which at first traverse it.
[315]Bischoff has recently stated,Historisch-kritische Bemerkungen ii. d. Entwicklung d. Säugethiereier, that Götte has found a double formation of the heart in Bombinator. It may seem bold to question the accuracy of Bischoff's interpretation of writings in his own language, but I have certainly failed to gather this either from Dr Götte's text or figures.
[316]VideElements of Embryology, Foster and Balfour,pp.64-66.
[317]Professor Bischoff (loc. cit.) throws doubts upon the double formation of the heart, and supports his views by Dr Foster's and my failure to find any trace of a double formation of the heart in the chick. Professor Bischoff must, I think, have misunderstood our description, which contains a clear account of the double formation of the heart.
[318]Entwicklungsgeschichte d. Unke,pp.779, 780, 781.
[319]Journal of Anatomy and Physiology,Vol.X.p. 794.
[320]The morphological importance of this point is considerable. It proves, for instance, that the hæmal arches of the vertebræ in the tail (videpp.373and374) potentially, at any rate, encircle the gut and enclose the body-cavity as completely as the ribs which meet in the median ventral line may be said to do anteriorly.
[321]Memoirs of the American Academy of Arts and Sciences,Vol.IX.
[322]I may state that my determinations of the arrangement of the circulation were made by actual observation of the flow of the blood under the microscope.
[323]My figure may be compared with that of Leydig,Rochen und Haie, PlateIII.fig. 6. Leydig calls the arterial ring the sinus terminalis, and appears to regard it as venous, but his description is so short that this point is not quite clear.
[324]Rochen und Haie and Untersuchung. ü. Fische u. Reptilien.
[325]I do not feel sure that Leydig's unpaired suprarenal body is really my interrenal body, or at any rate it alone. The point could no doubt easily be settled with fresh specimens, but these I unfortunately cannot at present obtain. My doubts rest partly on the fact that, in addition to my interrenal body, other peculiar masses of tissue (which may be called lymphoid in lieu of a better name) are certainly present around some of the larger vessels of the kidneys which are not identical in structure and development with my interrenal body, and partly that Stannius' statements (to be alluded to directly) rather indicate the existence of a second unpaired body in connection with the kidneys, though I do not fully understand his descriptions.
[326]Fische u. Reptilien, p. 14.
[327]Rochen u. Haie, p. 18.
[328]Vergleichende Anatomie,II.Auflage.
[329]Stannius' description is not quite intelligible, but appears to point to the existence of a third kind of body connected with the kidney. From my own observations (videabove), I am inclined to regard it as probable that such a third body exists.
[330]“Urogenitalsystem d. Plagiostomen.”Arb. zool.-zoot. Inst. z. Würzburg,Vol.II.
[331]There is a very good figure of them in Semper's paper,Pl.XXI.fig. 3.
[332]Antea,pp.394-396.
[333]Perhaps the body I am describing is not identical with Leydig's posterior suprarenal body. I do not, as mentioned above, feel satisfied that it is so from Leydig's description.
[334]Quarterly Journal of Microscopic Science, October, 1874. [This editionNo.V.]
The earliest stages in the development of the excretory system have already been described in a previous chapter[335]of this memoir, and up to the present time no investigator, with the exception of Dr Alex. Schultz[336], has gone over the same ground. Dr Schultz' descriptions are somewhat brief, but differ from my own mainly in stating that the segmental duct arises from an involution instead of as a solid knob. This discrepancy is, I believe, due to Dr Schultz drawing his conclusions as to the development of the segmental duct from its appearance at a comparatively late stage. He appears to have been unacquainted with my earlier descriptions.
The adult anatomy and later stages in the development of the excretory organs form the subject of the present chapter, and stand in marked contrast to the earlier stages in that they have been dealt with in a magnificent monograph[337]by Professor Semper, whose investigations have converted this previously almost unknown field of vertebrate embryology into one of the most fully explored parts of the whole subject. Reference is frequently made to this monograph in the succeeding pages, but my references, numerous as they are, give no adequate idea of the completeness and thoroughness of Professor Semper's investigations. In Professor Semper's monograph are embodied the results of a considerable number of preliminary papers published by him in hisArbeitenand in theCentralblatt. The excretory organs of Elasmobranchii have also formed the subjectof some investigations by Dr Meyer[338]and by myself[339]. Their older literature is fully given by Professor Semper. In addition to the above-cited works, there is one other paper by Dr Spengel[340]on the Urinogenital System of Amphibians, to which reference will frequently be made in the sequel, and which, though only indirectly connected with the subject of this chapter, deserves special mention both on account of the accuracy of the investigations of which it forms the record, and of the novel light which it throws on many of the problems of the constitution of the urinogenital system of Vertebrates.
Excretory organs and genital ducts in the adult.
The kidneys of Scyllium canicula are paired bodies in contact along the median line. They are situated on the dorsal wall of the abdominal cavity, and extend from close to the diaphragm to a point a short way behind the anus. Externally, each appears as a single gland, but by the arrangement of its ducts may be divided into two distinct parts, an anterior and a posterior. The former will be spoken of as the Wolffian body, and the latter as the kidney, from their respective homology with the glands so named in higher Vertebrates. The grounds for these determinations have already been fully dealt with both by Semper[341]and by myself.
Externally both the Wolffian body and the kidney are more or less clearly divided into segments, and though the breadth of both glands as viewed from the ventral surface is fairly uniform, yet the hinder part of the kidney is very much thicker and bulkier than the anterior part and than the whole of the Wolffian body. In both sexes the Wolffian body is rather longer than the kidney proper. Thus in a male example, 33 centimetreslong, the two glands together measured 8¼ centimetres and the kidney proper only 3½. In the male the Wolffian bodies extend somewhat further forwards than in the female. Leaving the finer details of the glands for subsequent treatment, I pass at once to their ducts. These differ slightly in the two sexes, so that it will be more convenient to take the male and female separately.
A partly diagrammatic representation of the kidney and Wolffian body of the male is given onPl.20, fig. 1. The secretion of the Wolffian body is carried off by a duct,the Wolffian duct (w.d.), which lies on the ventral surface of the gland, and receives a separate ductule from each segment (Pl.20, fig. 5). The main function of the Wolffian duct in the male is, however, that of a vas deferens. The testicular products are brought to it through the coils of the anterior segments of the Wolffian body by a number of vasa efferentia, the arrangement of which is treated of onpp.487,488. The section of the Wolffian duct which overlies the Wolffian body is much contorted, and in adult individuals at the generative period enormously so. The duct often presents one or two contortions beyond the hind end of the Wolffian body, but in the normal condition takes a straight course from this point to the unpaired urinogenital cloaca, into which it falls independently of its fellow of the opposite side. It receives no feeders from the kidney proper.
The excretion of the kidney proper is carried off not by a single duct, but by a series of more or less independent ducts, which, in accordance with Prof. Semper's nomenclature, will be spoken of asureters. These are very minute, and their investigation requires some care. I have reason, from my examinations of this and other species of Elasmobranchii, to believe that they are, moreover, subject to considerable variations, and the following description applies to a definite individual. Nine or possibly ten distinct ureters, whose arrangement is diagrammatically represented in fig. 1,Pl.20, were present on each side. It will be noticed that, whereas the five hindermost are distinct till close to their openings into the urinogenital cloaca, the four anterior ones appear to unite at once into a single duct, but are probably only bound up in a common sheath. The ureters fall into the common urinogenital cloaca,immediately behind the opening of the Wolffian duct (so far as could be determined), by four apertures on each side. In a section made through the part of the wall of the cloaca containing the openings of the ureters of both sides, there were present on the left side (where the section passed nearer to the surface than on the right) four small openings posteriorly,viz.the openings of the ureters and one larger one anteriorly,viz.the opening of the Wolffian duct. On the other side of the section where the level was rather deeper, there were five distinct ducts cut through, one of which was almost on the point of dividing into two. This second section proves that, in this instance at least, the two ureters did not unite till just before opening into the urinogenital cloaca. The same section also appeared to shew that one of the ureters fell not into the cloaca but into the Wolffian duct.
As stated above both the Wolffian duct and the ureters fall into an unpaired urinogenital cloaca. This cloaca communicates at one end with the general cloaca by a single aperture situated at the point of a somewhat conspicuous papilla, just behind the anus (Pl.20, fig. 1,o), and on the other it opens freely into a pair of bladders, situated in close contact with each other, on the ventral side of the kidney (Pl.20, fig. 1,sb). To these bladders Professor Semper has given the nameuterus masculinus, from having supposed them to correspond with the lower part of the oviducts of the female. This homology he now admits to be erroneous, and it will accordingly be better to drop the name uterus masculinus, for which may be substitutedseminal bladder—a name which suits their function, since they are usually filled with semen at the generation season. The seminal bladders communicate with the urinogenital cloaca by wide openings, and it is on the borders of these openings that the mouths of the Wolffian duct and ureters must be looked for. My embryological investigations, though they have not been specially directed to this point, seem to shew that the seminal bladders do not arise during embryonic life, and are still absent in very young individuals. It seems probable that both the bladders and the urinogenital cloaca are products of the lower extremities of the Wolffian duct. The only other duct requiring any notice in the male is the rudimentary oviduct. As was firstshewn by Semper, rudiments of the upper extremities of the oviducts, with their abdominal openings, are to be found in the male in the same position as in the female, on the front surface of the liver.
In the female the same ducts are present as in the male,viz.the Wolffian duct and the ureters. The part of the Wolffian duct which receives the secretion of the Wolffian body is not contorted, but is otherwise similar to the homologous part of the Wolffian duct in the male. The Wolffian ducts of the two sides fall independently into an unpaired urinal cloaca, but their lower ends, instead of remaining simple as in the male, become dilated into urinary bladders.VidePl.20, fig. 2. There were nine ureters in the example dissected, whose arrangement did not differ greatly from that in the male—the hinder ones remaining distinct from each other, but a certain amount of fusion, the extent of which could not be quite certainly ascertained, taking place between the anterior ones. The arrangement of the openings of these ducts is not quite the same as in the male. A somewhat magnified representation of it is given inPl.20, fig. 3,o.u.The two Wolffian ducts meet at so acute an angle that their hindermost extremities are only separated by a septum. In the region of this septum on the inner walls of the two Wolffian ducts were situated the openings of the ureters, of which there were five on each side arranged linearly. In a second example, also adult, I found four distinct openings on each side similarly arranged to those in the specimen described. Professor Semper states that all the ureters in the female unite into asingle ductbefore opening into the Wolffian duct. It will certainly surprise me to find such great variations in different individuals of this species as is implied by the discrepancy between Professor Semper's description and my own.
The main difference between the ureters in the male and female consists in their falling into the urinogenital cloaca in the former and into the Wolffian duct in the latter. Since, however, the urinogenital cloaca is a derivative of the Wolffian duct, this difference between the two sexes is not a very important one. The urinary cloaca opens, in the female, into the general cloaca by a median papilla of somewhat smaller dimensions than the corresponding papilla in the male. Seminalbladders are absent in the female, though possibly represented by the bladder-like dilatations of the Wolffian duct. The oviducts, whose anatomy is too well known to need description, open independently into the general cloaca.
Since the publication of Professor Semper's researches on the urinogenital system of Elasmobranch fishes, it has been well known that, in most adult Elasmobranchii, there are present a series of funnel-shaped openings, leading from the perivisceral cavity, by the intermediation of a short canal, into the glandular tubuli of the kidney. These openings are called by Professor Semper,Segmentaltrichter, and by Dr Spengel, in his valuable work on the urogenital system of Amphibia,Nephrostomen. In the present work the openings will be spoken of as segmental openings, and the tubes connected with them as segmental tubes. Of these openings there are a considerable number in the adults of both sexes of Scy. canicula, situated along the inner border of each kidney. The majority of them belong to the Wolffian body, though absent in the extreme anterior part of this. In very young examples a few certainly belong to the region of the kidney proper. Where present, there is one for each segment[342]. It is not easy to make certain of their exact number. In one male I counted thirteen. In the female it is more difficult than in the male to make this out with certainty, but in one young example, which had left the egg but a short time, there appeared to be at least fourteen present. According to Semper there are thirteen funnels in both sexes—a number which fairly well agrees with my own results. In the male, rudiments of segmental tubes are present in all the anterior segments of the Wolffian body behind the vasa efferentia, but it is not till about the tenth segment that the first complete one is present. In the female a somewhat smaller number of the anterior segments, six or seven, are without segmental tubes, or only possess them in a rudimentary condition.
A typical segment of the Wolffian body or kidney, in the sense in which this term has been used above, consists of a number of factors, each of which will be considered in detail with reference to its variations. OnPl.20, fig. 5, is representeda portion of the Wolffian body with three complete segments and part of a fourth. If one of these be selected, it will be seen to commence with (1) a segmental opening, somewhat oval in form (st.o) and leading directly into (2) a narrow tube, the segmental tube, which takes a more or less oblique course backwards, and, passing superficially to the Wolffian duct (w.d), opens into (3) a Malpighian body (p.mg) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms the fourth section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather definite convolutions, and after uniting with tubuli from one or two (according to size of the segment) accessory Malpighian bodies (a.mg), smaller than the one into which the segmental tube falls, eventually opens by a (5) narrowish tube into the Wolffian duct at the posterior end of the segment. Each segment is completely isolated (except for certain rudimentary structures to be alluded to shortly) from the adjoining ones,and never has more than one segmental tube and one communication with the Wolffian duct.
The number and general arrangement of the segmental tubes have already been spoken of. Their openings into the body-cavity are, in Scyllium, very small, much more so than in the majority of Elasmobranchii. The general appearance of a segmental tube and its opening is somewhat that of a spoon, in which the handle represents the segmental tube, and the bowl the segmental opening. Usually amongst Elasmobranchii the openings and tubes are ciliated, but I have not determined whether this is the case in Scy. canicula, and Semper does not speak definitely on this point. From the segmental openings proceed the segmental tubes, which in the front segments have nearly a transverse direction, but in the posterior ones are directed more and more obliquely backwards. This statement applies to both sexes, but the obliquity is greater in the female than in the male.
As has been said, each segmental tube normally opens into a Malpighian body, from which again there proceeds the tubulus, the convolutions of which form the main mass of each segment. This feature can be easily seen in the case of the Malpighian bodies of the anterior part of the Wolffian gland in youngexamples, and sometimes fairly well in old ones, of either sex[343]. There is generally in each segment a second Malpighian body, which forms the commencement of a tubulus joining that from the primary Malpighian body, and, where the segments are larger, there are three, and possibly in the hinder segments of the Wolffian gland and segments of the kidney proper, more than three Malpighian bodies.
The accessory Malpighian bodies, or at any rate one of them, appear to have curious relations to the segmental tubes. The necks of some of the anterior segmental tubes (Pl.20, fig. 5) close to their openings into the primary Malpighian bodies are provided with a small knob of cells which points towards the preceding segment and is usually connected with it by a fibrous band. This knob is most conspicuous in the male, and in very young animals or almost ripe embryos. In several instances in a ripe male embryo it appeared to me to have a lumen, and to be continued directly forwards into the accessory Malpighian body of the preceding segment. One such case is figured in the middle segment onPl.20, fig. 5. In this embryo segmental tubes were present in the segments immediately succeeding those connected with the vasa efferentia, and at the same time these segments contained ordinary and accessory Malpighian bodies. The segmental tubes of these segments were not, however, connected with the Malpighian body of their proper segment, but instead, turned forwards and entered the segment in front of that to which they properly belonged. I failed to trace them quite definitely to the accessory Malpighian body of the preceding segment, but, in one instance at least, there appeared to me to be present a fibrous connection, which is shewn in the figure already referred to,Pl.20, fig. 5,r.st. In any case it can hardly be doubted that this peculiarity of the foremost segmental tubes is related to what would seem to be the normal arrangement in the next few succeeding segments, where each segmental tube is connected with a Malpighian body in its own segment, and more or less distinctly with an accessory Malpighian body in the preceding segment.
In the male the anterior segmental tubes, which even in the embryo exhibit signs of atrophy, become in the adult completely aborted (as has been already shewn by Semper), and remain as irregular tubes closed at both ends, which for the most part do not extend beyond the Wolffian duct (Pl.20, fig. 4,r.st). In the adult, the first two or three segments with these aborted tubes contain only accessory Malpighian bodies; the remaining segments, with aborted segmental tubes, both secondary and primary Malpighian bodies. In neither case are the Malpighian bodies connected with the aborted tubes.
The Malpighian bodies in Scyllium present no special peculiarities. The outer layer of their capsule is for the most part formed of flattened cells; but, between the opening of the segmental tube and the efferent tubulus of the kidney, their cells become columnar.VidePl.20, fig. 5. The convoluted tubuli continuous with them are, I believe, ciliated in their proximal section, but I have not made careful investigations with reference to their finer structure. Each segment is connected with the Wolffian duct by a single tube at the hinder end of the segment. In the kidney proper, these tubes become greatly prolonged, and form the ureters.
It has already been stated that the semen is carried by vasa efferentia from the testes to the anterior segments of the Wolffian body, and thence through the coils of the Wolffian body to the Wolffian duct. The nature of the vasa will be discussed in the embryological section of this chapter: I shall here confine myself to a simple description of their anatomical relations. The consideration of their connections naturally falls under three heads: (1) the vasa efferentia passing from the testes to the Wolffian body, (2) the mode in which these are connected with the Wolffian body, and (3) with the testis.
InPl.20, fig. 4, drawn for me from nature by my friend Mr Haddon, are shewn the vasa efferentia and their junctions both with the testes and the kidney. This figure illustrates better than any description the anatomy of the various parts. Behind there are two simple vasa efferentia (v.e.) and in front a complicated network of vasa, which might be regarded as formed of either two or four main vessels. It will be shewn in the sequel that it is really formed of four distinct vessels.Professor Semper states that there is but a single vas efferens in Scyllium canicula, a statement which appears to me unquestionably erroneous. All the vasa efferentia fall into alongitudinal duct(l.c), which is connected in succession with the several segments of the Wolffian body (one for each vas efferens) which appertain to the testis. The hind end of the longitudinal duct is simple, and ends blindly close to its junction with the last vas efferens; but in front, where the vasa efferentia are complicated, the longitudinal duct also has a complicated constitution, and forms a network rather than a simple tube. It typically sends off a duct to join the coils of the Wolffian body between each pair of vasa efferentia, and is usually swollen where this duct parts from it. A duct similar to this has been described by Semper asNierenrandcanalin several Elasmobranchii, but its existence is expressly denied in the case of Scyllium! It is usually found in Amphibia, as we know from Bidder and Spengel's researches. Spengel calls itLängscanal des Hoden; the vessels from it into the kidney he callsvasa efferentia, and the vessels to it, which I speak of as vasa efferentia, he callsQuercanale.
The exact mode of junction of the separate vasa efferentia with the testis is difficult to make out on account of the opacity of the basal portion of the testis. My figure shews that there is a network of tubes (formed of four main tubes connected by transverse branches) which is a continuation of the anterior vasa efferentia, and joined by the two posterior ones. These tubes receive the tubuli coming from the testicular ampullæ. The whole network may be called, with Semper, thetesticular network. While its general relations are represented in my figure, the opacity of the testes was too great to allow of all the details being with certainty filled in.
The kidneys of Scyllium stellare, as might be expected, closely resemble those of Scy. canicula. The ducts of the kidney proper, have, in the former species, a larger number of distinct openings into the urinogenital cloaca. In two male examples I counted seven distinct ureters, though it is not impossible that there may have been one or two more present. In one of my examples the ureters had seven distinct openings into the cloaca, in the other five openings. In a female I counted eleven ureters opening into the Wolffian duct by seven distinct openings.In the remaining parts of the excretory organs the two species of Scyllium resemble each other very closely.
As may be gathered from Prof. Semper's monograph, the excretory organs of Scyllium canicula are fairly typical for Elasmobranchii generally. The division into kidney and Wolffian body is universal. The segmental openings may be more numerous and larger,e.g.Acanthias and Squatina, or absent in the adult,e.g.Mustelus and Raja. Bladder-like swellings of the Wolffian duct in the female appear to be exceptional, and seminal bladders are not always present. The variations in the ureters and their openings are considerable, and in some cases all the ureters are stated to fall into a single duct, which may be spoken of as the ureterpar excellence[344], with the same relations to the kidneys as the Wolffian duct bears to the Wolffian body. In some cases Malpighian corpuscles are completely absent in the Wolffian body,e.g.Raja.
The vasa efferentia of the testes in Scyllium are very typical, but there are some forms in which they are more numerous as well as others in which they are less so. Perhaps the vasa efferentia are seen in their most typical form in Centrina as described and figured (Pl.XXI) by Professor Semper, or in Squatina vulgaris, as I find it, and have represented it onPl.20, fig. 8. From my figure, representing the anterior part of the Wolffian body of a nearly ripe embryo, it will be seen that there are five vasa efferentia (v.e) connected on the one hand with a longitudinal canal at the base of the testes (n.t) and on the other with a longitudinal canal in the Wolffian body. Connected with the second longitudinal canal are four Malpighian bodies, three of them stalked and one sessile; from which again proceed tubes forming the commencements of the coils of the anterior segments of the Wolffian body. These Malpighian bodies are clearly my primary Malpighian bodies, but there are in Squatina, even in the generative segments, secondary Malpighian bodies. What Semper has described for Centrina and one or two other genera, closely correspond with what is present in Squatina.
Development of the Segmental Tubes.
On p.345,et seq.an account was given of the first formation of the segmental tubes and the segmental duct, and the history of these bodies was carried on till nearly the period at which it is taken up in the exhaustive Memoir of Professor Semper. Though the succeeding narration traverses to a great extent the same ground as Semper's Memoir, yet many points are treated somewhat differently, and others are dealt with which do not find a place in the latter. In the majority of instances, attention is called to points on which my results either agree with, or are opposed to, those of Professor Semper.
From previous statements it has been rendered clear thatat firstthe excretory organs of Elasmobranchii exhibit no division into Wolffian body or kidney proper. Since this distinction is merely a question of the ducts, and does not concern the glandular tubuli, no allusion is made to its appearance in the present section, which deals only with the glandular part of the kidneys and not with their ducts.
Up to the close of stage K the urinogenital organs consist of a segmental duct opening in front into the body-cavity, and terminating blindly behind in close contact with the cloaca, and of a series of segmental tubes, each opening into the body-cavity on the inner side of the segmental duct, but ending blindly at their opposite extremities. It is with these latter that we have at present to deal. They are from the first directed obliquely backwards, and coil close round the inner and dorsal sides of the segmental duct. Where they are in contact (close to their openings into the body-cavity) with the segmental duct, the lumen of the latter diminishes and so comes to exhibit regular alternations of size. This is shewn inPl.12, fig. 18,s.d. At the points where the segmental duct has a larger lumen, it eventually unites with the segmental tubes.
The segmental tubes rapidly undergo a series of changes, the character of which may be investigated, either by piecing together transverse sections, or more easily from longitudinal and vertical sections. They acquire a Λ-shaped form with an anterior limb opening into the body-cavity and posterior limb, resting on adilated portion of the segmental duct. The next important change which they undergo consists in a junction being effected between their posterior limbs and the segmental duct. In the anterior part of the body these junctions appear before the commencement of stage L. A segmental tube at this stage is shewn in longitudinal section onPl.21, fig. 7a, and in transverse section onPl.18, fig. 2. In the former the actual openings into the body-cavity are not visible. In the transverse section only one limb of the Λ is met with on either side of the section; the limb opening into the body-cavity is seen on the left side, and that opening into the segmental duct on the right side. This becomes quite intelligible from a comparison with the longitudinal section, which demonstrates that it is clearly not possible to see more than a single limb of the Λ in any transverse section.
After the formation of their junctions with the segmental duct, other changes soon take place in the segmental tubes. By the close of stage L four distinct divisions may be noticed in each tube. Firstly, there is the opening into the body-cavity, with a somewhat narrow stalk, to which the name segmental tube will be strictly confined in the future, while the whole products of the original segmental tube will be spoken of as a segment of the kidney. This narrow stalk opens into a vesicle (Pl.18, fig. 2, and 21, fig. 6), which forms the second division. From the vesicle proceeds a narrower section forming the third division, which during stage L remains very short, though in later stages it grows with great rapidity. It leads into the fourth division, which constitutes the posterior limb of the Λ, and has the form of a dilated tube with a narrow opening into the segmental duct.
The subsequent changes of each segment do not for the most part call for much attention. They consist mainly in the elongation of the third division, and its conversion into a coiled tubulus, which then constitutes the main mass of each segment of the kidney. There are, however, two points of some interest,viz.(1) the formation of the Malpighian bodies, and (2) the establishment of the connection between each segmental tube and the tubulus of the preceding segment which was alluded to in the description on p.486. The development of theMalpighian body is intimately linked with that of the secondary connection between two segments. They are both products of the metamorphosis of the vesicle which forms the termination of the segmental tube proper.
At about stage O this vesicle grows out in two directions (Pl.21, fig. 10),viz.towards the segment in front (p.x) and posteriorly into the segment of which it properly forms a part (mg). That portion which grows backward remains continuous with the third division of its proper segment, and becomes converted into a Malpighian body. It assumes (Pl.21, figs. 6 and 10) a hemispherical form, while near one edge of it is the opening from a segmental tube, and near the other the opening leading into a tubulus of the kidney. The two-walled hemisphere soon grows into a nearly closed sphere, with a central cavity into which projects a vascular tuft. For this tuft the thickened inner wall of cells forms a lining, and at the same time the outer wall becomes thinner, and formed of flattened cells, except in the interval between the openings of the segmental tube and kidney tubulus, where its cells remain columnar.
The above account of the formation of the Malpighian bodies agrees very well with the description which Pye[345]has given of the formation of these bodies in the embryonic Mammalian kidney. My statements also agree with those of Semper, in attributing the formation of the Malpighian body to a metamorphosis of part of the vesicle at the end of the segmental tube. Semper does not however enter into full details on this subject.
The elucidation of the history of the second outgrowth from the original vesicle towards the preceding segment is fraught with considerable difficulties, which might no doubt be overcome by a patient investigation of ample material, but which I have not succeeded in fully accomplishing.
The points which I believe myself to have determined are illustrated by fig. 10,Pl.21, a longitudinal vertical section through a portion of the kidney between stages O and P. In this figure parts of three segments of the kidney are represented. In the hindermost of the three—the one to the right—thereis a complete segmental tube (s.t) which opens at its upper extremity into an irregular vesicle, prolongedbehindinto a body which is obviously a developing Malpighian body,m.g, and infrontinto a wide tube cut obliquely in the section and ending apparently blindly (p.x). In the preceding segment there is also a segmental tube (s.t) whose opening into the body-cavity passes out of the plane of the section, but which is again connected with a vesicle dilating behind into a Malpighian body (m.g) and in front into the irregular tube (p.x), as in the succeeding segment,but this tube is now connected(and this could be still more completely seen in the segment in front of this)with a vesicle which opens into the thick-walled collecting tube (fourth division) of the preceding segmentclose to the opening of the latter into the Wolffian duct. The fact that the anterior prolongation of the vesicle ends blindly in the hinder-most segment is due of course to its terminal part passing out of the plane of the section.Thus we have established between stages O and P a connection between each segmental tube and the collecting tube of the segment in front of that to which it properly belongs; and it further appears that in consequence of this each segment of the kidney contains two distinct coils of tubuli which only unite close to their common opening into the Wolffian duct!
This remarkable connection is not without morphological interest, but I am unfortunately only able to give in a fragmentary manner its further history. During the greater part of embryonic life a large amount of interstitial tissue is present in the embryonic kidneys, and renders them too opaque to be advantageously studied as a whole; and I have also, so far, failed to prepare longitudinal sections suitable for the study of this connection. It thus results that the next stage I have satisfactorily investigated is that of a nearly ripe embryo already spoken of in connection with the adult, and represented onPl.20, fig. 5. This figure shews that each segmental tube, while distinctly connected with the Malpighian body of its own segment, also sends out a branch towards the secondary Malpighian body of the preceding segment. This branch in most cases appeared to be rudimentary, and in the adult is certainly not represented by more than a fibrous band, but I fancy that Ihave been able to trace it (though not with the distinctness I could desire) in surface views of the embryonic kidney of stage Q.The condition of the Wolffian body represented onPl.20, fig. 5 renders it probable that the accessory Malpighian body in each segment is developed in connection with the anterior growth from the original vesicle at the end of the segmental tube of the succeeding segment.How the third or fourth accessory Malpighian bodies, when present, take their origin I have not made out. It is, however, fairly certain that they form the commencement of two additional coils which unite, like the coil connected with the first accessory Malpighian body, with the collecting tube of the primitive coil close to its opening into the Wolffian duct or ureter.
The connection above described between two successive kidney segments appears to have escaped Professor Semper's notice, though I fancy that the peculiar vesicle he describes,loc. cit.p. 303, as connected with the end of each segmental tube, is in some way related to it. It seems possible that the secondary connection between the segmental tube and the preceding segment may explain a peculiar observation of Dr Spengel[346]on the kidney of the tailless Amphibians. He finds that, in this group, the segmental tubes do not open into Malpighian bodies, but into the fourth division of the kidney tube. Is it not just possible that in this case the primitive attachment of the segmental tubes may have become lost, and a secondary attachment, equivalent to that above described, though without the development of a secondary Malpighian body, have been developed? In my embryos the secondary coil of the segmental tubes opens, as in the Anura, into the fourth section of a kidney tubulus.
Development of the Müllerian and Wolffian ducts.
The formation of the Müllerian and Wolffian ducts out of the original segmental duct has been dealt with in a masterly manner by Professor Semper, but though I give my entire assent to his general conclusions, yet there are a few points onwhich I differ from him. These are for the most part of a secondary importance; but they have a certain bearing on the homology between the Müllerian duct of higher Vertebrates and that of Elasmobranchii. The following account refers to Scy. canicula, but so far as my observations go, the changes in Scy. stellare are nearly identical in character.
I propose treating the development of these ducts in the two sexes separately, and begin with the female.
Shortly before stage N a horizontal split arises in the segmental duct[347], commencing some little distance from its anterior extremity, and extending backwards. This split divides the duct into a dorsal section and a ventral one. The dorsal section forms the Wolffian duct, and receives the openings of the segmental tubes, and the ventral one forms the Müllerian duct or oviduct, and is continuous with the unsplit anterior part of the primitive segmental duct, which opens into the body-cavity. The nature of the splitting may be gathered from the woodcut, fig. 6, p.511, wherexrepresents the line along which the segmental duct is divided. The splitting of the primitive duct extends slowly backwards, and thus there is for a considerable period a single duct behind, which bifurcates in front. A series of transverse sections through the point of bifurcation always exhibits the following features. Anteriorly two separate ducts are present, next two ducts in close juxtaposition, and immediately behind this a single duct. A series of sections through the junction of two ducts is represented on Plate 21, figs. 1A, 1B, 1C, 1D.