Section through the trunk of an advanced embryo of ScylliumDiagrammatic section through the trunk of an advanced embryo ofScyllium, to shew the position of the ribs.ao., aorta;c.sh., cartilaginous notochordal sheath;cv., cardinal vein;hp., hæmal process;k., kidney;l.s., ligamentum longitudinale superius;m.el., membrana elastica externa;na., neural arch;no., notochord;ll., lateral line;rb., rib;sp.c., spinal cord.
Diagrammatic section through the trunk of an advanced embryo ofScyllium, to shew the position of the ribs.
ao., aorta;c.sh., cartilaginous notochordal sheath;cv., cardinal vein;hp., hæmal process;k., kidney;l.s., ligamentum longitudinale superius;m.el., membrana elastica externa;na., neural arch;no., notochord;ll., lateral line;rb., rib;sp.c., spinal cord.
There is much, therefore, to be said in favour of Götte's view. At the same time, there is another possible interpretation of the facts which would admit the homology of the ribs as well as of the hæmal arches throughout the Pisces.
Let us suppose, to start with, that the primitive arrangement of the parts is more or less nearly that found inLepidosteus, where we have well-developed ribs in the region of the trunk, girthing the body-cavity, and uniting in the caudal region to form the ventral parts of the hæmal arches. It is easy to conceive that the ribs in the trunk might somewhat alter their position by passing into the muscles, along the inter-muscular septa, till they come to lie between the dorso-lateral and ventro-lateral muscles, as in Elasmobranchii.Lepidosteusitself affords a proof that such a change in the position of the ribs is not impossible, in that it differs from other Ganoids and from Teleostei in the fact that the free ends of the ribs leave the neighbourhood of the body-cavity and penetrate into the muscles.
If it be granted that the mere difference in position between the ribs of Ganoids and Elasmobranchii is not of itself sufficient to disprove their homology, let us attempt to picture what would take place at the junction of the trunk and tail in a type in which the ribs had undergone the above change in position. On nearing the tail it may be supposed that the ribs would gradually become shorter, and at the same time alter their position, till finally they shaded off into ordinary hæmal processes. If, however, the hæmal canal became prolonged forwards by the formation of some additional complete or nearly complete hæmal arches, an alteration in the relation of the parts would necessarily take place. Owing to the position of the ribs, these structures could hardly assist in the new formation of the anterior part of the hæmal canal, but the continuation forwards of the canal would be effected by prolongations of the hæmal processes supporting the ribs. The new arches so formed would naturally be held to be homologous with the hæmal arches of the tail, though really not so, while the true nature of the ribs would also be liable to be misinterpreted, in that the ribs would appear to be lateral outgrowths of the hæmal processes of a wholly different nature to the ventral parts of the hæmal arches of the tail.
In some Elasmobranchii, as shewn in the accompanying woodcut (fig. 2), in the transitional vertebræ between the trunk and the tail, the ribs are supported by lateral outgrowths of the hæmal processes, while the wholly independent prolongations ofthe hæmal processes appear to be about to give rise to the hæmal arches of the tail.
This peculiar state of things led Götte, and subsequently one of us, to deny for Elasmobranchii all homology between the ribs and any part of the hæmal arches of the tail; but in view of the explanation just suggested, this denial was perhaps too hasty.
Fig. 2.
Transverse section through the ventral part of the notochordTransverse section through the ventral part of the notochord, and adjoining structures of an advancedScylliumembryo at the root of the tail.Vb., cartilaginous sheath of the notochord;ha., hæmal process;r.p., process to which the rib is articulated;m.el., membrana elastica externa;ch., notochord;ao., aorta;V.cau., caudal vein.
Transverse section through the ventral part of the notochord, and adjoining structures of an advancedScylliumembryo at the root of the tail.
Vb., cartilaginous sheath of the notochord;ha., hæmal process;r.p., process to which the rib is articulated;m.el., membrana elastica externa;ch., notochord;ao., aorta;V.cau., caudal vein.
We are the more inclined to take this view because the researches of Götte appear to shew that an occurrence, in many respects analogous, has taken place in some Teleostei.
In Teleostei, Johannes Müller, and following him Gegenbaur, do not admit that the hæmal arches of the tail are in any part formed by the ribs. Gegenbaur (Elements ofComp. Anat., translation, p. 431) says,“In the Teleostei, the costiferous transverse processes”(what we have called the hæmal processes)“graduallyconverge in the caudal region, and form inferior arches, which are not homologous with those of Selachii and Ganoidei, although they also form spinous processes.”
The opposite view, that the hæmal arches of the tail in Teleostei contain parts serially homologous with the basal parts of the hæmal processes as well as with the ribs, has been also maintained by many anatomists,e.g., Meckel, Aug. Müller,&c., and has recently found a powerful ally in Götte.
In many cases, the relations of the parts appear to be fundamentally those found inLepidosteusandAmia, and Götte has shewn by his careful embryological investigations onEsoxandAnguilla, that in these two forms there is practically conclusive evidence that the ribs as well as the hæmal costiferous processes of Gegenbaur, which support them, enter into the formation of the hæmal arches of the tail.
In a great number of Teleostei,e.g., the Salmon and most Cyprinoids,&c., the hæmal arches in the region of transition from the trunk to the tail have a structure which at first sight appears to support Johannes Müller's and Gegenbaur's view. The hæmal processes grow larger and meet each other ventrally; while the ribs articulated to them gradually grow smaller and disappear.
The Salmon is typical in this respect, and has been carefully studied by Götte, who attempts to shew (with, in our opinion, complete success) that the anterior hæmal arches are really not entirely homologous with the true hæmal arches behind, but that in the latter, the closure of the arch below is effected by the hæmal spine, which is serially homologous with a pair of coalesced ribs, while in the anterior hæmal arches,i.e., those of the trunk, the closure of the arch is effected by a bridge of bone uniting the hæmal processes.
The arrangement of the parts just described, as well as the view of Götte with reference to them, will be best understood from the accompanying woodcut (fig. 3), copied from Götte's memoir.
Götte sums up his own results on this point in the following words (p. 138):“It follows from this, that the half rings, forming the hæmal canal in the hindermost trunk vertebræ of the Salmon, are not (with the exception of the last) completely homologouswith those of the tail, but are formed by a connecting piece between the basal stumps (hæmal processes), which originates as a paired median process of these stumps.”
The incomplete homology between the anterior hæmal arches and the true caudal hæmal arches which follow them is exactly what we suggest may be the case in Elasmobranchii, and if it be admitted in the one case, we see no reason why it should not also be admitted in the other.
Fig. 3.
Semi-diagrammatic transverse sectionsSemi-diagrammatic transverse sections through the first caudal vertebra (A), the last trunk vertebra (B), and the two trunk vertebræ in front (C and D), of a Salmon embryo of 2-3centims.(From Götte.)ub., hæmal arch;ub´., hæmal process;ub´´., rib;c., notochord;a., aorta;v., vein;h., connecting pieces between hæmal processes;u., kidney;d., intestine;sp´., hæmal spine;m´., muscles.
Semi-diagrammatic transverse sections through the first caudal vertebra (A), the last trunk vertebra (B), and the two trunk vertebræ in front (C and D), of a Salmon embryo of 2-3centims.(From Götte.)
ub., hæmal arch;ub´., hæmal process;ub´´., rib;c., notochord;a., aorta;v., vein;h., connecting pieces between hæmal processes;u., kidney;d., intestine;sp´., hæmal spine;m´., muscles.
If this admission is made, the only ground for not regarding the ribs of Elasmobranchii as homologous with those of Ganoids is their different position, and we have already attempted to prove that this is not a fundamental point.
The results of our researches appear to us, then, to leave two alternatives as to the ribs of Fishes. One of these, which may be called Götte's view, may be thus stated:—The hæmal archesare homologous throughout the Pisces: in Teleostei, Ganoidei, and Dipnoi[527], the ribs, placed on the inner face of the body-wall, are serially homologous with the ventral parts of the hæmal arches of the tail; in Elasmobranchii, on the other hand, the ribs are neither serially homologous with the hæmal arches of the tail nor homologous with the ribs of Teleostei and Ganoidei, but are outgrowths of the hæmal processes into the space between the dorso-lateral and ventro-lateral muscles, which may perhaps have their homologues in Teleostei and Ganoids in certain accessory processes of the vertebræ.
The other view, which we are inclined to adopt, and the arguments for which have been stated in the preceding pages, is as follows:—The Teleostei, Ganoidei, Dipnoi, and Elasmobranchii are provided with homologous hæmal arches, which are formed by the coalescence below the caudal vein of simple prolongations of the primitive hæmal processes of the embryo. The canal enclosed by the hæmal arches can be demonstrated embryologically to be the aborted body-cavity.
In the region of the trunk the hæmal processes and their prolongations behave somewhat differently in the different types.
In Ganoids and Dipnoi, in which the most primitive arrangement is probably retained, the ribs are attached to the hæmal processes, and are placed immediately without the peritoneal membrane at the insertions of the intermuscular septa. These ribs are in many instances (Lepidosteus,Acipenser), and very probably in all, developed continuously with the hæmal processes, and become subsequently segmented from them. They are serially homologous with the ventral parts of the hæmal arches of the tail, which, like them, are in many instances (Ceratodus,Lepidosteus,Polypterus, and to some extent inAmia) segmented off from the basal parts of the hæmal arches.
In Teleostei the ribs have the same position and relations as those in Ganoids and Dipnoi, but their serial homology with the ventral parts of the hæmal processes of the tail, is often (e.g., the Salmon) obscured by some of the anterior hæmal arches in the posterior part of the trunk being completed, not by the ribs, butby independent outgrowths of the basal parts of the hæmal processes.
In Elasmobranchii a still further divergence from the primitive arrangement is present. The ribs appear to have passed outwards along the intermuscular septa into the muscles, and are placed between the dorso-lateral and ventro-lateral muscles (a change of position of the ribs of the same nature, but affecting only their ends, is observable inLepidosteus). This change of position, combined probably with the secondary formation of a certain number of anterior hæmal arches similar to those in the Salmon, renders their serial homology with the ventral parts of the hæmal processes of the tail far less clear than in other types, and further proof is required before such homology can be considered as definitely established.
This is not the place to enter into the obscure question as to how far the ribs of the Amphibia and Amniota are homologous with those of Fishes. It is to be remarked, however, that the ribs of the Urodela (1) occupy the same position in relation to the muscles as the Elasmobranch ribs, (2) that they are connected with the neural arches, and (3) that they coexist in the tail with the hæmal arches, and seem, therefore, to be as different as possible from the ribs of the Dipnoi.
Part IV.—The skeleton of the ventral lobe of the tail fin, and its bearing on the nature of the tail fin of the various types of Pisces.
In the embryos or larvæ of all the Elasmobranchii, Ganoidei, and Teleostei which have up to this time been studied, the unpaired fins arise as median longitudinal folds of the integument on the dorsal and ventral sides of the body, which meet at the apex of the tail. The tail at first is symmetrical, having a form which has been called diphycercal or protocercal. At a later stage, usually, though not always, parts of these fins atrophy, while other parts undergo a special development and constitute the permanent unpaired fins.
Since the majority of existing as well as extinct Fishes are provided with discontinuous fins, those forms, such as the Eel (Anguilla), in which the fins are continuous, have probably revertedto an embryonic condition: an evolutional process which is of more frequent occurrence than has usually been admitted.
In the caudal region there is almost always developed in the larvæ of the above groups a special ventral lobe of the embryonic fin a short distance from the end of the tail. In Elasmobranchii and Chondrostean Ganoids the portion of the embryonic tail behind this lobe persists through life, and a special type of caudal fin, which is usually called heterocercal, is thus produced. This type of caudal fin appears to have been the most usual in the earlier geological periods.
Simultaneously with the formation of the ventral lobe of the heterocercal caudal fin, the notochord with the vertebral tissues surrounding it, becomes bent somewhat dorsalwards, and thus the primitive caudal fin forms a dorsally directed lobe of the heterocercal tail. We shall call this part the dorsal lobe of the tail-fin, and the secondarily formed lobe the ventral lobe.
LepidosteusandAmia(Wilder,No.15) amongst the bony Ganoids, and, as has recently been shewn by A. Agassiz[528], most Teleostei acquire at an early stage of their development heterocercal caudal fins, like those of Elasmobranchii and the Chondrostean Ganoids; but in the course of their further growth the dorsal lobe partly atrophies, and partly disappears as such, owing to the great prominence acquired by the ventral lobe. A portion of the dorsally flexed notochord and of the cartilage or bone replacing or investing it remains, however, as an indication of the original dorsal lobe, though it does not project backwards beyond the level of the end of the ventral lobe, which in these types forms the terminal caudal fin.
The true significance of the dorsally flexed portion of the vertebral axis was first clearly stated by Huxley[529], but as A. Agassiz has fairly pointed out in the paper already quoted, this fact does not in any way militate against the view put forward by L. Agassiz that there is a complete parallelism between the embryonic development of the tail in these Fishes and the palæontological development of this organ. We thinkthat it is moreover convenient to retain the term homocercal for those types of caudal fin in which the dorsal lobe has atrophied so far as not to project beyond the ventral lobe.
We have stated these now well-known facts to enable the reader to follow us in dealing with the comparison between the skeleton supporting the fin-rays of the ventral lobe of the caudal fin, and that supporting the fin-rays of the remaining unpaired fins.
It has been shewn that inLepidosteusthe unpaired fins fall into two categories, according to the nature of the skeletal parts supporting them. The fin-rays of the true ventral lobe of the caudal fin are supported by the spinous processes of certain of the hæmal arches. The remaining unpaired fins, including the anal fin, are supported by the so-called interspinous bones, which are developed independently of the vertebral column and its arches.
The question which first presents itself is, how far does this distinction hold good for other Fishes? This question, though interesting, does not appear to have been greatly discussed by anatomists. Not unfrequently the skeletal supports of the ventral lobe of the caudal fin are assumed to be the same as those of the other fins.
Davidoff[530], for instance, in speaking of the unpaired fins of Elasmobranch embryos, says (p. 514):“The cartilaginous rays of the dorsal fins agreed not only in number with the spinous processes (as indeed is also found in the caudal fin of the full-grown Dog-fish),”&c.
Thacker[531], again, in his memoir on the Median and Paired Fins, states at p. 284:“We shall here consider the skeleton of the dorsal and anal fins alone. That of the caudal fin has undergone peculiar modifications by the union of fin-rays with hæmal spines.”
Mivart[532]goes into the question more fully. He points out (p. 471) that there is an essential difference between the dorsal and ventral parts of the caudal fin in Elasmobranchii, in that inthe former the radials are more numerous than the vertebræ and unconformable to them, while in the latter they are equal in number to the vertebræ and continuous with them.“This,”he goes on to say,“seems to point to a difference in nature between the dorsal and ventral portions of the caudal fin, in at least most Elasmobranchii.”He further points out thatPolyodonresembles Elasmobranchii. As to Teleostei, he does not express himself decidedly except in the case ofMuræna, to which we shall return.
Mivart expresses himself as very doubtful as to the nature of the supports of the caudal fin, and thinks“that the caudal fin of different kinds of Fishes may have arisen in different ways in different cases.”
An examination of the ventral part of the caudal fin in various Ganoids, Teleostei, and Elasmobranchii appears to us to shew that there can be but little doubt that, in the majority of the members of these groups at any rate, and we believe in all, the same distinction between the ventral lobe of the caudal fin and the remaining unpaired fins is found as inLepidosteus.
In the case of most Elasmobranchii, a simple inspection of the caudal fin suffices to prove this, and the anatomical features involved in this fact have usually been recognized; though, in the absence of embryological evidence, the legitimate conclusion has not always been drawn from them.
The difference between the ventral lobe of the caudal fin and the other fins in the mode in which the fin-rays are supported is as obvious in Chondrostean Ganoids as it is in Elasmobranchii; it would appear also to hold good forAmia.Polypteruswe have had no opportunity of examining, but if, as there is no reason to doubt, the figure of its skeleton given by Agassiz (Poissons Fossiles) is correct, there can be no question that the ventral lobe of the caudal fin is supported by the hæmal arches, and not by interspinous bones. InCalamoicthys, the tail of which we have had an opportunity of dissecting through the kindness of Professor Parker, the fin-rays of the ventral lobe of the true caudal fin are undoubtedly supported by true hæmal arches.
There is no unanimity of opinion as to the nature of the elements supporting the fin-rays of the caudal fin of Teleostei.
Huxley[533]in his paper on the development of the caudal fin of the Stickleback, holds that these elements are of the nature of interhæmal bones. He says (p. 39): "The last of these rings lay just where the notochord began to bend up. It was slightly longer than the bony ring which preceded it, and instead of having its posterior margin parallel with the anterior, it sloped from above downwards and backwards. Two short osseous plates, attached to the anterior part of the inferior surface of the penultimate ring, or rudimentary vertebral centrum, passed downwards and a little backwards, and abutted against a slender elongated mass of cartilage. Similar cartilaginous bodies occupy the same relation to corresponding plates of bone in the anterior vertebræ in the region of the anal fin; and it is here seen, that while the bony plates coalesce and form the inferior arches of the caudal vertebræ, the cartilaginous elements at their extremities become the interhæmal bones. The cartilage connected with the inferior arch of the penultimate centrum is therefore an“interhæmal”cartilage. The anterior part of the inferior surface of the terminal ossification likewise has its osseous inferior arch, but the direction of this is nearly vertical, and though it is connected below with an element which corresponds in position with the interhæmal cartilage, this cartilage is five or six times as large, and constitutes a broad vertical plate, longer than it is deep, and having its longest axis inclined downwards and backwards....
“Immediately behind and above this anterior hypural apophysis (as it may be termed) is another very much smaller vertical cartilaginous plate, which may be called the posterior hypural apophysis.”
We have seen that Mivart expresses himself doubtful on the subject. Gegenbaur[534]appears to regard them as hæmal arches.
The latter view appears to us without doubt the correct one. An examination of the tail of normal Teleostei shews that the fin-rays of that part of the caudal fin which is derived from the ventral lobe of the larva are supported by elements serially homologous with the hæmal arches, but in no way homologouswith the interspinous bones of the anal fin. The elements in question formed of cartilage in the larva, become ossified in the adult, and are known as the hypural bones. They may appear in the form of a series of separate hæmal arches, corresponding in number with the primitive somites of this region, which usually, however, atrophy in the adult, or more often are from the first imperfectly segmented, and have in the adult the form of two or three or even of a single broad bony plate. The transitional forms between this state of things and that, for instance, inLepidosteusare so numerous, that there can be no doubt that even the most peculiar forms of the hypural bones of Teleostei are simply modified hæmal arches.
This view of the hypural bones is, moreover, supported by embryological evidence, since Aug. Müller[535](p. 205) describes their development in a manner which, if his statements are to be trusted, leaves no doubt on this point.
There are a considerable number of Fishes which are not provided with an obvious caudal fin as distinct from the remaining unpaired fins,i.e.Chimæra, Eels, and various Eel-like forms amongst Teleostei, and the Dipnoi. Gegenbaur appears to hold that these Fishes ought to be classed together in relation to the structure of the caudal portion of their vertebral column, as he says on p. 431 of hisComparative Anatomy(English Translation):“In the Chimæræ, Dipnoi, and many Teleostei, the caudal portion of the vertebral column ends by gradually diminishing in size, but in most Fishes,&c.”
For our purpose it will, however, be advisable to treat them separately.
The tail of Chimæra appears to us to be simply a peculiar modification of the typical Elasmobranch heterocercal tail, in which the true ventral lobe of the caudal fin may be recognized in the fin-fold immediately in front of the filamentous portion of the tail. In the allied genusCallorhynchusthis feature is more distinct. The filamentous portion of the tail of Chimæra constitutes, according to the nomenclature adopted above, the true dorsal lobe, and may be partially paralleled in the filamentous dorsal lobe of the tail of the larvalLepidosteus(Plate 34, fig. 16).
The tail of the eel-like Teleostei is again undoubtedly a modification of the normal form of tail characteristic of the Teleostei, in which, however, the caudal fin has become very much reduced and merged into the prolongations of the anal and dorsal fins.
This can be very clearly seen in Siluroid forms with an Eel-like tail, such asCnidoglanis. Although the dorsal and ventral fins appear to be continuous round the end of the tail, and there is superficially no distinct caudal fin, yet an examination of the skeleton ofCnidoglanisshews that the end of the vertebral column is modified in the usual Teleostean fashion, and that the hæmal arches of the modified portion of the vertebral column support a small number of fin-rays; the adjoining ventral fin-rays being supported by independent osseous fin-supports (interspinous bones).
In the case of the Eel (Anguilla anguilla) Huxley (loc. cit.) long ago pointed out that the terminal portion of the vertebral column was modified in an analogous fashion to that of other Teleostei, and we have found that the modified hæmal arches of this part support a few fin-rays, though a still smaller number than inCnidoglanis. The fin-rays so supported clearly constitute an aborted ventral lobe of the caudal fin.
Under these circumstances we think that the following statement by Mivart (Zool. Trans.Vol.X., p. 471) is somewhat misleading:—
“As to the condition of this part (i.e.the ventral lobe of the tail-fin) in Teleosteans generally, I will not venture as yet to say anything generally,except that it is plain that in such forms as Muræna, the dorsal and ventral parts of the caudal fin are similar in nature and homotypal with ordinary dorsal and anal fins[536].”
The italicized portion of this sentence is only true in respect to that part of the fringe of fin surrounding the end of the body, which is not only homotypal with, but actually part of, the dorsal and anal fins.
Having settled, then, that the tails of Chimæra and of Eel-like Teleostei are simply special modifications of the typical form of tail of the group of Fishes to which they respectivelybelong, we come to the consideration of the Dipnoi, in which the tail-fin presents problems of more interest and greater difficulty than those we have so far had to deal with.
The undoubtedly very ancient and primitive character of the Dipnoi has led to the view, implicitly if not definitely stated in most text-books, that their tail-fin retains the character of the piscine tail prior to the formation of the ventral caudal lobe, a stage which is repeated embryologically in the pre-heterocercal condition of the tail in ordinary Fishes.
Through the want of embryological data, and in the absence of really careful histological examination of the tail of any of the Dipnoi, we are not willing to speak with very great confidence as to its nature; we are nevertheless of the opinion that the facts we can bring forward on this head are sufficient to shew that the tail of the existing Dipnoi is largely aborted, so that it is more or less comparable with that of the Eel.
We have had opportunities of examining the structure of the tail ofCeratodusandProtopterusin dissected specimens in the Cambridge Museum. The vertebral axis runs to the ends of the tail without shewing any signs of becoming dorsally flexed. At some distance from the end of the tail the fin-rays are supported by what are apparently segmented spinous prolongations of the neural and hæmal arches. The dorsal elements are placed above the longitudinal dorsal cord, and occupy therefore the same position as the independent elements of the neural arches ofLepidosteus. They are therefore to be regarded as homologous with the dorsal fin-supports or interspinous bones of other types. The corresponding ventral elements are therefore also to be regarded as interspinous bones.
In view of the fact that the fin-supports, whenever their development has been observed, are found to be formed independently of the neural and hæmal arches, we may fairly assume that this is also true for what we have identified as the interspinous elements in the Dipnoi.
The interspinous elements become gradually shorter as the end of the tail is approached, and it is very difficult from a simple examination of dissected specimens to make out how far any of the posterior fin-rays are supported by the hæmal arches only. To this question we shall return, but we may remarkthat, although there is a prolongation backwards of the vertebral axis beyond the last interspinous elements, composed it would seem of the coalesced neural and hæmal arches but without the notochord, yet by far the majority of the fin-rays which constitute the apparent caudal fin are supported by interspinous elements.
The grounds on which we hold that the tail of the Dipnoi is to be regarded as a degenerate rather than primitive type of tail are the following:—
(1) If it be granted that a diphycercal or protocercal form of tail must have preceded a heterocercal form, it is also clear that the ventral fin-rays of such a tail must have been supported, as inPolypterusandCalamoicthys, by hæmal arches, and not by interspinous elements; otherwise, a special ventral lobe, giving a heterocercal character to the tail, and provided with fin-rays supported only by hæmal arches, could never have become evolved from the protocercal tail-fin. Since the ventral fin-rays of the tail of the Dipnoi are supported by interspinous elements and not by hæmal arches, this tail-fin cannot claim to have the character ofthatprimitive type of diphycercal or protocercal tail from which the heterocercal tail must be supposed to have been evolved.
(2) Since the nearest allies of the Dipnoi are to be found inPolypterusand the Crossopterygidæ of Huxley, and since in these forms (as evinced by the structure of the tail-fin ofPolypterus, and the transitional type between a heterocercal and diphycercal form of fin observable in fossil Crossopterygidæ) the ventral fin-rays of the caudal fin were clearly supported by hæmal arches and not by interspinous elements, it is rendered highly probable that the absence of fin-rays so supported in the Dipnoi is a result of degeneration of the posterior part of the tail.
[We use this argument without offering any opinion as to whether the diphycercal character of the tail of many Crossopterygidæ is primary or secondary.]
(3) The argument just used is supported by the degenerate and variable state of the end of the vertebral axis in the Dipnoi—a condition most easily explained by assuming that the terminal part of the tail has become aborted.
(4) We believe that inCeratoduswe have been able to trace a small number of the ventral fin-rays supported by hæmal arches only, but these rays are so short as not to extend so far back as some of the rays attached to the interspinous elements in front. These rays may probably be interpreted, like the more or less corresponding rays in the tail of the Eel, as the last remnant of a true caudal fin.
The above considerations appear to us to shew with very considerable probability that the true caudal fin of the Dipnoi has become all but aborted like that of various Teleostei; and that the apparent caudal fin is formed by the anal and dorsal fins meeting round the end of the stump of the tail.
From the adult forms of Dipnoi we are, however, of opinion that no conclusion can be drawn as to whether their ancestors were provided with a diphycercal or a heterocercal form of caudal fin.
The general conclusions with reference to the tail-fin at which we have arrived are the following:—
(1) The ventral lobe of the tail-fin of Pisces differs from the other unpaired fins in the fact that its fin-rays are directly supported by spinous processes of certain of the hæmal arches instead of independently developed interspinous bones.
(2) The presence or absence of fin-rays in the tail-fin supported by hæmal arches may be used in deciding whether apparently diphycercal tail-fins are aborted or primitive.
[521]These specimens were given to us by Professor W. K. Parker, who received them from Professor Burt G. Wilder.
[522]Gegenbaur (No.6) takes a different view on this subject, as is clear from the following passage in this memoir (pp.369-370):—“Each vertebra ofLepidosteusthus consists of a section of the notochord, and of the cartilaginous tissue surrounding its sheath, which gives origin to the upper arches for the whole length of the vertebral column, and in the caudal region to that of the lower arches also.The latter do not however complete the enclosure of a lower canal, but this is effected by special independent elements, which are to be interpreted as homologues of the ribs.”(The italics are ours.) While we fully accept the homology between the ribs and the lower elements of the hæmal arches of the tail, the view expressed in the italicised section, to the effect that the lower parts of the caudal arches are not true hæmal arches but are independently formed elements, is entirely opposed to our observations, and has we believe only arisen from the fact that Gegenbaur had not the young larvæ to work with by which alone this question could be settled.
[523]“Beiträge zur vergl. Morphol. d. Skeletsystems d. Wirbelthiere.”Archiv f. Mikr. Anat.Vol.XVI.1879.
[524]“Beiträge z. vergl. Morph. d. Skeletsystems d. Wirbelthiere. II. Die Wirbelsäule u. ihre Anhänge.”Archiv f. Mikr. Anat.,Vol.XV., 1878, andVol.XVI., 1879.
[525]“Ueb. d. Entwick. d. Wirbelsäule d. Lepidosteus, mit. vergl. Anat. Bemerkungen.”Jenaische Zeitschrift, Bd.III., 1863.
[526]Comparative Embryology,Vol.II.,pp.462, 463 [the original edition].
[527]We find the serial homology of the ribs and ventral parts of the hæmal arches to be very clear inCeratodus. Wiedersheim states that it is not clear inProtopterus, although he holds that the facts are in favour of this view.
[528]“On the Young Stages of some Osseous Fishes.—I. The Development of the Tail,”Proc.of the American Academy of Arts and Sciences,Vol.XIII., 1877.
[529]“Observations on the Development of some Parts of the Skeleton of Fishes,”Quart. Journ. of Micr.Science,Vol.VII., 1859.
[530]“Beiträge z. vergl. Anat. d. hinteren Gliedmassen d. Fische,”Morph. Jahrbuch, Vol.V., 1879.
[531]Trans.of the ConnecticutAcad.,Vol.III., 1877.
[532]St George Mivart,“Fins of Elasmobranchii,”Zool. Trans.,Vol.X.
[533]“Observations on the Development of some parts of the Skeleton of Fishes,”Quart. Journ. Micr.Science,Vol.VII., 1859.
[534]Elements of Comparative Anatomy.(Translation), p. 431.
[535]“Beobachtungen zur vergl. Anat. d. Wirbelsäule,”Müller'sArchiv, 1853.
[536]The italics are ours.
I.—Anatomy.
The excretory organs ofLepidosteushave been described by Müller (No.13) and Hyrtl (No.11). These anatomists have given a fairly adequate account of the generative ducts in the female, and Hyrtl has also described the male generative ducts and the kidney and its duct, but his description is contradicted by our observations in some of the most fundamental points.
In the female example of 100.5centims.which we dissected, the kidney forms a paired gland, consisting of a narrow strip of glandular matter placed on each side of the vertebral column, onthe dorsal aspect of the body-cavity. It is covered on its ventral aspect by the oviduct and by its own duct, but is separated from both of these by a layer of the tough peritoneal membrane, through which the collecting tubes pass. It extends forwards from the anus for about three-fifths of the length of the body-cavity, and in our example had a total length of about 28centims.(Plate 39, fig. 60,k). Anteriorly the two kidneys are separated by a short interval in the median line, but posteriorly they come into contact, and are so intimately united as almost to constitute a single gland.
A superficial examination might lead to the supposition that the kidney extended forwards for the whole length of the body-cavity up to the region of the branchial arches, and Hyrtl appears to have fallen into this error; but what appears to be its anterior continuation is really a form of lymphatic tissue, something like that of the spleen, filled with numerous cells. This matter (Plate 39, fig. 60,ly.) continues from the kidney forwards without any break, and has a colour so similar to that of the kidney as to be hardly distinguishable from it with the naked eye. The true anterior end of the kidney is placed about 3centims.in front on the left side, and on the same level on the right side as the wide anterior end of the generative duct (Plate 39, fig. 60,od.). It is not obviously divided into segments, and is richly supplied with malpighian bodies.
It is clear from the above description that there is no trace of head-kidney or pronephros visible in the adult. To this subject we shall, however, again return.
As will appear from the embryological section, the ducts of the kidneys are probably simply the archinephric ducts, but to avoid the use of terms involving a theory, we propose in the anatomical part of our work to call them kidney ducts. They are thin-walled widish tubes coextensive with the kidneys. If cut open there may be seen on their inner aspect the numerous openings of the collecting tubes of the kidneys. They are placed ventrally to and on the outer border of the kidneys (Plate 39, fig. 60,s.g.). Posteriorly they gradually enlarge, and approaching each other in the median line, coalesce, forming an unpaired vesicle or bladder (bl.)—about 6centims.long in our example—opening by a median pore on a more or lessprominent papilla (u.g.) behind the anus. The dilated portions of the two ducts are called by Hyrtl the horns of the bladder.
The sides of the bladder and its so-called horns are provided with lateral pockets into which the collecting tubes of the kidney open. These pockets, which we have found in two female examples, are much larger in the horns of the bladder than in the bladder itself. Similar pockets, but larger than those we have found, have been described by Hyrtl in the male, but are stated by him to be absent in the female. It is clear from our examples that this is by no means always the case.
Hyrtl states that the wide kidney ducts, of which his description differs in no material point from our own, suddenly narrow in front, and, perforating the peritoneal lining, are continued forwards to supply the anterior part of the kidney. We have already shewn that the anterior part of the kidney has no existence, and the kidney ducts supplying it are, according to our investigations, equally imaginary.
It was first shewn by Müller, whose observations on this point have been confirmed by Hyrtl,&c., that the ovaries ofLepidosteusare continuous with their ducts, forming in this respect an exception to other Ganoids.
In our example ofLepidosteusthe ovaries (Plate 39, fig. 60,ov.) were about 18centims.in length. They have the form of simple sacks, filled with ova, and attached about their middle to their generative duct, and continued both backwards and forwards from their attachment into a blind process.
With reference to these sacks Müller has pointed out—and the importance of this observation will become apparent when we deal with the development—that the ova are formed in the thickness of the inner wall of the sack. We hope to shew that the inner wall of the sack is alone equivalent to the genital ridge of, for instance, the ovary ofScyllium. The outer aspect of this wall—i.e., that turned towards the interior of the sack—is equivalent to the outer aspect of the Elasmobranch genital ridge, on which alone the ova are developed[537]. The sack into which the ova fall is, as we shall shew in the embryological section, a special section of the body-cavity shut off from the remainder,and the dehiscence of the ova into this cavity is equivalent to their discharge into the body-cavity in other forms.
The oviduct (Plate 39, fig. 60,od.) is a thin-walled duct of about 21centims.in length in the example we are describing, continuous in front with the ovarian sack, and gradually tapering behind, till it ends (od´.) by opening into the dilated terminal section of the kidney duct on the inner side, a short distance before the latter unites with its fellow. It is throughout closely attached to the ureter and placed on its inner, and to some extent on its ventral, aspect. The hindermost part of the oviduct which runs beside the enlarged portion of the kidney duct—that portion called by Hyrtl the horn of the urinary bladder—is so completely enveloped by the wall of the horn of the urinary bladder as to appear like a projection into the lumen of the latter structure, and the somewhat peculiar appearance which it presents in Hyrtl's figure is due to this fact. In our examples the oviduct was provided with a simple opening into the kidney duct, on a slight papilla; the peculiar dilatations and processes of the terminal parts of the oviduct, which have been described by Hyrtl, not being present.
The results we have arrived at with reference to the male organs are very different indeed from those of our predecessor, in that we findthe testicular products to be carried off by a series of vasa efferentia, which traverse the mesorchium, and are continuous with the uriniferous tubuli; so that the semen passes through the uriniferous tubuli into the kidney duct and so to the exterior. We have moreover been unable to find in the male a duct homologous with the oviduct of the female.
This mode of transportation outwards of the semen has not hitherto been known to occur in Ganoids, though found in all Elasmobranchii, Amphibia, and Amniota. It is not, however, impossible that it exists in other Ganoids, but has hitherto been overlooked.
Our male example of Lepidosteus was about 60centims.in length, and was no doubt mature. It was smaller than any of our female examples, but this according to Garman (vide, p. 361) is usual. The testes (Plate 39, fig. 58A.,t.) occupied a similar position to the ovaries, and were about 21centims.long. They were, as is frequently the case with piscine testes,divided into a series of lobes (10-12), and were suspended by a delicate mesentery (mesorchium) from the dorsal wall of the abdomen on each side of the dorsal aorta. Hyrtl (No.11) states that air or quicksilver injected between the limbs of the mesentery, passed into a vas deferens homologous with the oviduct which joins the ureter. We have been unable to find such a vas deferens; but we have found in the mesorchium a number of tubes of a yellow colour, the colour being due to a granular substance quite unlike coagulated blood, but which appeared to us from microscopic examination to be the remains of spermatozoa[538]. These tubes to the number of 40-50 constitute, we believe, the vasa efferentia. Along the line of suspension of the testis on its inner border these tubes unite to form an elaborate network of tubes placed on the inner face of the testis—an arrangement very similar to that often found in Elasmobranchii (videF. M. Balfour,Monograph on the Development of Elasmobranch Fishes, plate 20, figs. 4 and 8).
We have figured this network on the posterior lobe of the testis (fig. 58B), and have represented a section through it (fig. 59A,n.v.e.), and through one of the vasa efferentia (v.e.) in the mesorchium. Such a section conclusively demonstrates the real nature of these passages: they are filled with sperm like that in the body of the testis, and are, as may be seen from the section figured, continuous with the seminal tubes of the testis itself.
At the attached base of the mesorchium the vasa efferentia unite into a longitudinal canal, placed on the inner side of the kidney duct (Plate 39, fig. 58A,l.c., also shewn in section in Plate 39, fig. 59B,l.c.). From this canal tubules pass off which are continuous with the tubuli uriniferi, as may be seen from fig. 59B, but the exact course of these tubuli through the kidney could not be made out in the preparations we were able to make of the badly conserved kidney. Hyrtl describes the arrangement of the vascular trunks in the mesorchium in the following way (No.11, p. 6):“The mesorchium contains vascular trunks,viz., veins, which through their numerous anastomosesform a plexus at the hilus of the testis, whose efferent trunks, 13 in number, again unite into a plexus on the vertebral column, which is continuous with the cardinal veins.”The arrangement (though not the number) of Hyrtl's vessels is very similar to that of our vasa efferentia, and we cannot help thinking that a confusion of the two may have taken place; which, in badly conserved specimens, not injected with semen, would be very easy.
We have, as already stated, been unable to find in our dissections any trace of a duct homologous with the oviduct of the female, and our sections through the kidney and its ducts equally fail to bring to light such a duct. The kidney ducts are about 19centims.in length, measured from the genital aperture to their front end. These ducts are generally similar to those in the female; they unite about 2centims.from the genital pore to form an unpaired vesicle. Their posterior parts are considerably enlarged, forming what Hyrtl calls the horns of the urinary bladder. In these enlarged portions, and in the wall of the unpaired urinary bladder, numerous transverse partitions are present, as correctly described by Hyrtl, which are similar to those in the female, but more numerous. They give rise to a series of pits, at the blind ends of which are placed the openings of the kidney tubules. The kidney duct without doubt serves as vas deferens, and we have found in it masses of yellowish colour similar to the substance in the vasa efferentia identified by us as remains of spermatozoa.
II.—Development.
In the general account of the development we have already called attention to the earliest stages of the excretory system.
We may remind the reader that the first part of the system to be formed is the segmental or archinephric duct (Plate 36, figs. 28 and 29,sg.). This duct arises, as in Teleostei and Amphibia, by the constriction of a hollow ridge of the somatic mesoblast into a canal, which is placed in contiguity with the epiblast, along the line of junction between the mesoblastic somites and the lateral plates of mesoblast. Anteriorly the ductdoes not become shut off from the body-cavity, and also bends inwards towards the middle line. The inflected part of the duct is the first rudiment of the pronephros, and very soon becomes considerably dilated relatively to the posterior part of the duct.
The posterior part of each segmental duct acquires an opening into the cloacal section of the alimentary tract. Apart from this change, the whole of the ducts, except their pronephric sections, remain for a long time unaltered, and the next changes we have to speak of concern the definite establishment of the pronephros.
The dilated incurved portion of each segmental duct soon becomes convoluted, and by the time the embryo is about 10millims.in length, but before the period of hatching, an important change is effected in the relations of their peritoneal openings[539].
Instead of leading into the body-cavity, they open into an isolated chamber on each side (Plate 38, fig. 51,pr.c.), which we will call thepronephric chamber. The pronephric chamber is not, however, so far as we can judge, completely isolated from the body-cavity. We have not, it is true, detected with certainty at this stage a communication between the two; but in later stages, in larvæ of from 11 to 26millims., we have found a richly ciliated passage leading from the body-cavity into the pronephros on each side (Plate 38, fig. 52,p.f.p.). We have not succeeded in determining with absolute certainty the exact relations between this passage and the tube of the pronephros, but we are inclined to believe that it opens directly into the pronephric chamber just spoken of.
As we hope to shew, this chamber soon becomes largely filled by a vascular glomerulus. On the accomplishment of these changes, the pronephros is essentially provided with all the parts typically present in a segment of the mesonephros (woodcut, fig. 4). There is a peritoneal tube (f)[540], opening into a vesicle (v); from near the neck of the peritoneal tube therecomes off a convoluted tube (pr.n.), forming the main mass of the pronephros, and ending in the segmental duct (sd.).
Fig. 4.