The Works of Francis Balfour 1-23
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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)
XXIII. On the nature of the organ in Adult Teleosteans and Ganoids which is usually regarded as the Head-kidney or Pronephros
WHILE working at the anatomy of Lepidosteus I was led to doubt the accuracy of the accepted accounts of the anterior part of the kidneys in this' 2 and in allied species of Fishes. In order to test my doubts I first examined the structure of the kidneys in the Sturgeon (Acipenser), of which I fortunately had a wellpreserved specimen.
The bodies usually described as the kidneys consist of two elongated bands, attached to the dorsal wall of the abdomen, and extending for the greater part of the length of the abdominal cavity. In front each of these bands first becomes considerably narrowed, and then expands and terminates in a great dilatation, which is usually called the head-kidney. Along the outer border of the hinder part of each kidney is placed a wide ureter, which ends suddenly in the narrow part of the body, some little way behind the head-kidney. To the naked eye" there is no distinction in structure between the part of the socalled kidney in front of the ureter and that in the region of the ureter. Any section through the kidney in the region of the ureter suffices to shew that in this part the kidney is really formed of uriniferous tubuli with numerous Malpighian bodies. Just in front, however, of the point where the ureter ends the true kidney substance rapidly thins out, and its place is taken by a peculiar tissue formed of a trabecular work filled with cells,
1 From the Quarterly Journal of Microscopical Science, Vol. XXII., 1882.
2 I am about to publish, in conjunction with Mr Parker, a full account of the anatomy and development of Lepidosteus [No. XXII. of this edition], and shall therefore in this paper make no further allusion to it.
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 849
which I shall in future call lymphatic tissue. Thus the wliole of that part of the apparent kidney in front of the ureter, including the whole of the so-called head-kidney, is simply a great mass of lymphatic tissue, and does not contain a single urinifcrous tubule or MalpigJdan body,
The difference in structure between the anterior and posterior parts of the so-called kidney, although not alluded to in most modern works on the kidneys, appears to have been known to Stannius, at least I so interpret a note of his in the second edition of his Comparative Anatomy, p. 263, where he describes the kidney of the Sturgeon as being composed of two separate parts, viz. a spongy vascular substance (no doubt the so-called headkidney) and a true secretory substance.
After arriving at the above results with reference to the Sturgeon I proceeded to the examination of the structure of the so-called head-kidney in Teleostei.
I have as yet only examined four forms, viz. the Pike (Esox lucius), the Smelt (Osmerus eperlanus], the Eel (Anguilla anguilld), and the Angler (Lophius piscatorius).
The external features of the apparent kidney of the Pike have been accurately described by Hyrtl 1 . He says: "The kidneys extend from the second trunk vertebra to the end of the abdominal cavity. Their anterior extremities, w r hich have the form of transversely placed coffee beans, are united together, and lie on the anterior end of the swimming bladder. The continuation of the kidney backwards forms two small bands, separated from each other by the whole breadth of the vertebral column. They gradually, however, increase in breadth, so that about the middle of the vertebral column they unite together and form a single symmetrical, keel-shaped body," &c.
The Pike I examined was a large specimen of about 58 centimetres in length, and with an apparent kidney of about 25^ centimetres. The relations of lymphatic tissue and kidney tissue were much as in the Sturgeon. The whole of the anterior swelling, forming the so-called head-kidney, together with a considerable portion of the part immediately behind, forming not far short of half the whole length of the apparent kidney,
1 "Das Uropoetische System der Knochenfische," Si'.z. d. Wieu. Akad., 1850.
850 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS.
was entirely formed of lymphatic tissue. The posterior part of the kidney was composed of true kidney substance, but even at 1 6 centimetres from the front end of the kidney the lymphatic tissue formed a large portion of the whole.
A rudiment of the duct of the kidney extended forwards for a short way into the lymphatic substance beyond the front part of the functional kidney.
In the Smelt (Osmerus eperlamis] the kidney had the typical Teleostean form, consisting of two linear bands stretching for the whole length of the body-cavity, and expanding into a great swelling in front on the level of the ductus Cuvieri, forming the so-called head-kidney. The histological examination of these bodies shewed generally the same features as in the case of the Sturgeon and Pike. The posterior part was formed of the usual uriniferous tubuli and Malpighian bodies. The anterior swollen part of these bodies, and the part immediately following, were almost wholly formed of a highly vascular lymphatic tissue ; but in a varying amount in different examples portions of uriniferous tubules were present, mainly, however, in the region behind the anterior swelling. In some cases I could find no tubules in the lymphatic tissue, and in all cases the number of them beyond the region of the well-developed part of the kidney was so slight, that there can be little doubt that they are functionless remnants of the anterior part of the larval kidney. Their continuation into the anterior swelling, when present, consisted of a single tube only.
In the Eel (Anguilla anguilla), which, however, I have not examined w r ith the same care as the Smelt, the true excretory part of the kidney appears to be confined to the posterior portion, and to the portion immediately in front of the anus, the whole of the anterior part of each apparent kidney, which is not swollen in front, being composed of lymphatic tissue.
LopJiius piscatorius is one of the forms which, according to Hyrtl 1 , is provided with a head-kidney only, i.e. with that part of the kidney which corresponds with the anterior swelling of the kidney of other types. For this reason I was particularly anxious to investigate the structure of its kidneys.
1 "Das Uropoetische System der Knochenfische," Sitz. d. Wien. Akad., 1850.
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 851
Each of these bodies forms a compact oval mass, with the ureter springing from its hinder extremity, situated in a forward position in the body-cavity. Sections through the kidneys shewed that they were throughout penetrated by uriniferous tubules, but owing to the bad state of preservation of my specimens I could not come to a decision as to the presence of Malpighian bodies. The uriniferous tubules were embedded in lymphatic tissue, similar to that which forms the anterior part of the apparent kidneys in other Teleostean types.
With reference to the structure of the Teleostean kidneys, the account given by Stannius is decidedly more correct than that of most subsequent writers. In the note already quoted he gives it as his opinion that there is a division of the kidney into the same two parts as in the Sturgeon, viz. into a spongy vascular part and a true secreting part ; and on a subsequent page he points out the absence or poverty of the uriniferous tubules in the anterior part of the kidney in many of our native Fishes.
Prior to the discovery that the larvae of Teleosteans and Ganoids were provided with two very distinct excretory organs, viz. a pronephros or head-kidney, and a mesonephros or Wolffian body, which are usually separated from each other by a more or less considerable interval, it was a matter of no very great importance to know whether the anterior part of the socalled kidney was a true excretory organ. In the present state of our knowledge the question is, however, one of considerable interest.
In the Cyclostomata and Amphibia the pronephros is a purely larval organ, which either disappears or ceases to be functionally active in the adult state.
, Rosenberg, to whom the earliest satisfactory investigations on the development of the Teleostean pronephros are due, stated that he had traced in the Pike (Esox Indus) the larval organ into the adult part of the kidney, called by Hyrtl the pronephros ; and subsequent investigators have usually assumed that the socalled head-kidney of adult Teleosteans and Ganoids is the persisting larval pronephros.
We have already seen that Rosenberg was entirely mistaken on this point, in that the so-called head-kidney of the adult is
852 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS.
not part of the true kidney. From my own studies on young Fishes I do not believe that the oldest larvae investigated by Rosenberg were sufficiently advanced to settle the point in question ; and, moreover, as Rosenberg had no reason for doubting that the so-called head-kidney of the adult was part of the excretory organ, he does not appear to have studied the histological structure of the organ which he identified with the embryonic pronephros in his oldest larva.
The facts to which I have called attention in this paper demonstrate that in the Sturgeon the larval pronephros undoubtedly undergoes atrophy before the adult stage is reached. The same is true for Lepidosteus, and may probably be stated for Ganoids generally.
My observations on Teleostei are clearly not sufficiently extensive to prove that the larval pronephros never persists in this group. They appear to me, however, to shew that in the normal types of Teleostei the organ usually held to be the pronephros is actually nothing of the kind.
A different interpretation might no doubt be placed upon my observations on Lophius piscatorius, but the position of the kidney in this species appears to me to be far from affording a conclusive proof that it is homologous with the anterior swelling of the kidney of more normal Teleostei.
When, moreover, we consider that Lophius, and the other forms mentioned by Hyrtl as being provided with a head-kidney only, are all of them peculiarly modified and specialized types of Teleostei, it appears to me far more natural to hold that their kidney is merely the ordinary Teleostean kidney, which, like many of their other organs, has become shifted in position, than to maintain that the ordinary excretory organ present in other Teleostei has been lost, and that a larval organ has been retained, which undergoes atrophy in less specialized Teleostei.
As the question at present stands, it appears to me that the probabilities are in favour of there being no functionally active remains of the pronephros in adult Teleostei, and that in any case the burden of proof rests with those who maintain that such remnants are to be foun,d.
The general result of my investigations is thus to render it probable that the pronephros, though found in the larvce or em
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 853
bryos of almost all the IchtJiyopsida, except the Elasmobranchii, is always a purely larval organ, which never constitutes an active part of the excretory system in the adult state.
This conclusion appears to me to add probability to the view of Gegenbaur that the pronephros is the primitive excretory gland of the Chordata ; and that the mesonephros or Wolffian body, by which it is replaced in existing Ichthyopsida, is phylogenetically a more recent organ.
In the preceding pages I have had frequent occasion to allude to the lymphatic tissue which has been usually mistaken for part of the excretory organ. This tissue is formed of trabecular work, like that of lymphatic glands, in the meshes of which an immense number of cells are placed, which may fairly be compared with the similarly placed cells of lymphatic glands. In the Sturgeon a considerable number of cells are found with peculiar granular nuclei, which are not found in the Teleostei. In both groups, but especially in the Teleostei, the tissue is highly vascular, and is penetrated throughout by a regular plexus of very large capillaries, which appear to have distinct walls, and which pour their blood into the posterior cardinal vein as it passes through the organ. The relation of this tissue to the lymphatic system I have not made out.
The function of the tissue is far from clear. Its great abundance, highly vascular character, and presence before the atrophy of the pronephros, appear to me to shew that it cannot be merely the non-absorbed remnant of the latter organ. From its size and vascularity it probably has an important function ; and from its structure this must either be the formation of lymph corpuscles or of blood corpuscles.
In structure it most resembles a lymphatic gland, though, till it has been shewn to have some relation to the lymphatic system, this can go for very little.
On the whole, I am provisionally inclined to regard it as a form of lymphatic gland, these bodies being not otherwise represented in fishes.
