[203]“Embryologische Mittheilungen.”Festschrift d. Naturfor. Gesell., Halle, 1879.[204]In Man (Kölliker) the trabeculæ form from the first a continuous plate in front of the pituitary space, and the latter very early acquires a cartilaginous floor.[205]The strongest evidence in favour of Huxley’s and Parker’s view of the nature of the columella is the fusion in the adult Sphenodon of the upper end of the hyoid with the columella (videHuxley,No.445). From an examination of a specimen in the Cambridge museum I do not feel satisfied that the fusion is not secondary, but have not been able to examine the junction of the hyoid and columella in section. For a different view to that of HuxleyvidePeters,“Ueb. d. Gehörknochelchen u. ihr Verhältniss zu. Zungenbeinbogen b. Sphenodon.”Berlin Monatsberichte,1874.[206]For some interesting remarks on the arrangement of these bones in Fishes,videBridge, “On the Osteology of Polyodon folium.”Phil. Trans., 1878.[207]It is not impossible that the solution of the difficulty about the præopercular is to be found by supposing that the præopercular as it exists in Teleostei is derived from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly than in Teleostei its primitive position.[208]The notochord appears also to enter into the posterior part of the region which ossifies as the basisphenoid.[209]According to Kölliker there are two centres in Man in both the basisphenoid and presphenoid.
[203]“Embryologische Mittheilungen.”Festschrift d. Naturfor. Gesell., Halle, 1879.
[204]In Man (Kölliker) the trabeculæ form from the first a continuous plate in front of the pituitary space, and the latter very early acquires a cartilaginous floor.
[205]The strongest evidence in favour of Huxley’s and Parker’s view of the nature of the columella is the fusion in the adult Sphenodon of the upper end of the hyoid with the columella (videHuxley,No.445). From an examination of a specimen in the Cambridge museum I do not feel satisfied that the fusion is not secondary, but have not been able to examine the junction of the hyoid and columella in section. For a different view to that of HuxleyvidePeters,“Ueb. d. Gehörknochelchen u. ihr Verhältniss zu. Zungenbeinbogen b. Sphenodon.”Berlin Monatsberichte,1874.
[206]For some interesting remarks on the arrangement of these bones in Fishes,videBridge, “On the Osteology of Polyodon folium.”Phil. Trans., 1878.
[207]It is not impossible that the solution of the difficulty about the præopercular is to be found by supposing that the præopercular as it exists in Teleostei is derived from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly than in Teleostei its primitive position.
[208]The notochord appears also to enter into the posterior part of the region which ossifies as the basisphenoid.
[209]According to Kölliker there are two centres in Man in both the basisphenoid and presphenoid.
The Pectoral girdle.
Pisces. Amongst Fishes the pectoral girdle presents itself in its simplest form in Elasmobranchii, where it consists of a bent band of cartilage on each side of the body, of somewhat variable form, meeting and generally uniting with its fellow ventrally. Its anterior border is in close proximity with the last visceral arch, and a transverse ridge on its outer and posterior border, forming the articular surface for the skeleton of the limb, divides it into a dorsal part, which may be called thescapula, and a ventral part which may be called thecoracoid.
In all the remaining groups of Fishes there is added to the cartilaginous band, which may wholly or partially ossify, an osseous support composed of a series of membrane bones.
In the types with such membrane bones the cartilaginous parts do not continue to meet ventrally, except in the Dipnoi where there is a ventral piece of cartilage, distinct from that bearing the articulation of the limb. The cartilage is moreover produced into two ventral processes, an anterior and a posterior, below the articulation of the limb; which may be called, in accordance with Gegenbaur’s nomenclature, thepræcoracoidandcoracoid. Of these the præcoracoid is far the mostprominent, and in the majority of cases the coracoid can hardly be recognised. The coracoid process is however well developed in the Selachioid Ganoids, and the Siluroid Teleostei. In Teleostei the scapular region often ossifies in two parts, the smaller of which is named by Parker præcoracoid, though it is quite distinct from Gegenbaur’s præcoracoid. The membrane bones, as they present themselves in their most primitive state in Acipenser and the Siluroids, are dermal scutes embracing the anterior edge of the cartilaginous girdle. In Acipenser there are three scutes on each side. A dorsal scute known as thesupra-clavicle, connected above with the skull by the post-temporal; a middle piece orclavicle, and a ventral orinfra-clavicle(interclavicle), which meets its fellow below.
In most Fishes the primitive dermal scutes have become subdermal membrane bones, and the infra-clavicle is usually not distinct, but the two clavicles form the most important part of the membranous elements of the girdle. Additional membrane bones (post-clavicles) are often present behind the main row.
The development of these parts in Fishes has been but little studied.
In Scyllium, amongst the Elasmobranchii, I find that each half of the pectoral girdle develops as a vertical bar of cartilage at the front border of the rudimentary fin,and externally to the muscle plates.
Before the tissue forming the pectoral girdle has acquired the character of true cartilage, the bars of the two sides meet ventrally by a differentiationin situof the mesoblastic cells, so that, when the girdle is converted into cartilage, it forms an undivided arc, girthing the ventral side of the body. There is developed in continuity with the posterior border of this arc on the level of the fin a horizontal bar of cartilage, which is continued backwards along the insertion of the fin, and, as will be shewn in the sequel, becomes the metapterygium of the adult (figs.344,bpand348,mp). With this bar the remaining skeletal elements of the fin are also continuous.
The foramina of the pectoral girdle are not in the first instance formed by absorption, but by the non-development of the cartilage in the region of pre-existing nerves and vessels.
The development of these parts in Teleostei has been recently investigated by ’Swirski (No.472) who finds in the Pike (Esox) that the cartilaginous pectoral girdle is at first continuous with the skeleton of the fin. It forms a rod with a dorsal scapular and ventral coracoid process. An independent mass of cartilage gives rise to a præcoracoid, which unites with the main mass, forming a triradiate bar like that of Acipenser or the Siluroids. The coracoid process becomes in the course of development gradually reduced.
’Swirski concludes that the so-called præcoracoid bar is to some extent a secondary element, and that the coracoid bar corresponds to the whole of the ventral part of the girdle of Elasmobranchii, but his investigations do not appear to me to be as complete as is desirable.
Amphibia and Amniota. The pectoral girdle contains a more or less constant series of elements throughout the Amphibia and Amniota; and the differences in structure between the shoulder girdle of these groups and that of Fishes are so great that it is only possible to make certain general statements respecting the homologies of the parts in the two sets of types.
The generally accepted view, founded on the researches of Parker, Huxley, and Gegenbaur, is to the effect that there is a primitively cartilaginous coraco-scapular plate, homologous with that in Fishes, and that the membrane bones in Fishes are represented by the clavicle and interclavicle in the Sauropsida and Mammalia, which are however usually admitted to be absent in Amphibia. These views have recently been challenged by Götte (No.466) and Hoffmann (No.467), on the ground of a series of careful embryological observations; and until the whole subject has been worked over by other observers it does not seem possible to decide satisfactorily between the conflicting views. It is on all hands admitted that the scapulo-coracoid elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half of each plate becomes the scapula, which may subsequently become divided into a supra-scapula and scapula proper; while the ventral half forms the coracoid, which is not always separated from the scapula, and is usually divided into a coracoid proper, a præcoracoid, and an epicoracoid. By the conversion of parts of the primitive cartilaginous plates into membranous tissue various fenestræ may be formed in the cartilage, and the barsbounding these fenestræ both in the scapula and coracoid regions have received special names; the anterior bar of the coracoid region, forming the præcoracoid, being especially important. At the boundary between the scapula and the coracoid, on the hinder border of the plate, is placed the glenoid articular cavity to carry the head of the humerus.
The grounds of difference between Götte and Hoffmann and other anatomists concern especially the clavicle and interclavicle. The clavicle is usually regarded as a membrane bone which may become to some extent cartilaginous. By the above anatomists, and by Rathke also, it is held to be at first united with the coraco-scapular plate, of which it forms the anterior limb, free ventrally, but united dorsally with the main part of the plate; and Götte and Hoffmann hold that it is essentiallya cartilage bone, which however in the majority of the Reptilia ossifies directly without passing through the condition of cartilage.
The interclavicle (episternum) is held by Götte to be developed from a paired formation at the free ventral ends of the clavicles, but he holds views which are in many respects original as to its homologies in Mammalia and Amphibia. Even if Götte’s facts are admitted, it does not appear to me necessarily to follow that his deductions are correct. The most important of these is to the effect that the dermal clavicle of Pisces has no homologue in the higher types. Granting that the clavicle in these groups is in its first stage continuous with the coraco-scapular plate, and that it may become in some forms cartilaginous before ossifying, yet it seems to me all the same quite possible that it is genetically derived from the clavicle of Pisces, but that it has to a great extent lost even in development its primitive characters, though these characters are still partially indicated in the fact that itusually ossifies very early and partially at least as a membrane bone[210].
In treating the development of the pectoral girdle systematically it will be convenient to begin with the Amniota, which may be considered to fix the nomenclature of the elements of the shoulder girdle.
Lacertilia. The shoulder girdle is formed as two membranous plates, from the dorsal part of the anterior border of each of which a bar projects (Rathke, Götte), which is free at its ventral end. This bar, which is usually (Gegenbaur, Parker) held to be independent of the remaining part of the shoulder girdle, gives rise to the clavicle and interclavicle. The scapulo-coracoid plate soon becomes cartilaginous, while at the same time the clavicular bar ossifies directly from the membranous state. The ventral ends of the two clavicular bars enlarge to form two longitudinally placed plates, which unite together and ossify as the interclavicle.
Parker gives a very different account of the interclavicle in Anguis. He states that it is formed of two pairs of bones ‘strapped on to the antero-inferior part of the præsternum,’ which subsequently unite into one.
Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of a triradiate cartilage on each side, with one dorsal and two ventral limbs. It is admitted on all hands that the dorsal limb is the scapular element, and the posterior ventral limb the coracoid; but, while the anterior ventral limb is usually held to be the præcoracoid, Götte and Hoffmann maintain that, in spite of its being formed of cartilage, it is homologous with the anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the homologue of the clavicle.
Parker and Huxley (doubtfully) hold that the three anterior elements of the ventral plastron (entoplastron and epiplastra) are homologous with the interclavicle and clavicles, but considering that these plates appear to belong to a secondary system of dermal ossifications peculiar to the Chelonia, this homology does not appear to me probable.
Aves. There are very great differences of view as to the development of the pectoral arch of Aves.
About the presence in typical forms of the coraco-scapular plate and two independent clavicular bars all authors are agreed. With reference to the clavicle and interclavicle Parker (No.468) finds that the scapular end of the clavicle attaches itself to and ossifies a mass of cartilage, which he regards as the mesoscapula, while the interclavicle is formed of a mass of tissue between the ends of the clavicles where they meet ventrally, which becomes the dilated plate at their junction.
Gegenbaur holds that the two primitive clavicular bars are simply clavicles, without any element of the scapula; and states that the clavicles are not entirely ossified from membrane, but that a delicate band of cartilage precedes the osseous bars. He finds no interclavicle.
Götte and Rathke both state that the clavicle is at first continuous with the coraco-scapular plate, but becomes early separated, and ossifies entirely as a membrane bone. Götte further states that the interclavicles are formed as outgrowths of the median ends of the clavicles, which extend themselves at an early period of development along the inner edges of the two halves of the sternum. They soon separate from the clavicles, which subsequently meet to form the furculum; while the interclavicular rudiments give rise, on the junction of the two halves of the sternum,to its keel, and to the ligamentconnecting the furculum with the sternum. The observations of Götte, which tend to shew the keel of the sternum is really an interclavicle, appear to me of great importance.
A præcoracoid, partially separated from the coracoid by a space, is present in Struthio. It is formed by a fenestration of a primitively continuous cartilaginous coracoid plate (Hoffmann). In Dromæus and Casuarius clavicles are present (fused with the scapula in the adult Dromæus), though absent in other Ratitæ (Parker, etc.).
Mammalia. The coracoid element of the coraco-scapular plate is much reduced in Mammalia, forming at most a simple process (except in the Ornithodelphia) which ossifies however separately[211].
With reference to the clavicles the same divergencies of opinion met with in other types are found here also.
The clavicle is stated by Rathke to be at first continuous with the coraco-scapular plate. It is however soon separated, and ossifies very early, in the human embryo before any other bone. Gegenbaur however shewed that the human clavicle is provided with a central axis of cartilage, and this observation has been confirmed by Kölliker, and extended to other Mammalia by Götte. The mode of ossification is nevertheless in many respects intermediate between that of a true cartilage bone and a membrane bone. The ends of the clavicles remain for some time, or even permanently, cartilaginous, and have been interpreted by Parker, it appears to me on hardly sufficient grounds, as parts of the mesoscapula and præcoracoid. Parker’s so-called mesoscapula may ossify separately. The homologies of the episternum are much disputed. Götte, who has worked out the development of the parts more fully than any other anatomist, finds that paired interclavicular elements grow out backwards from the ventral ends of the clavicles, and uniting together form a somewhat T-shaped interclavicle overlying the front end of the sternum. This condition is permanent in the Ornithodelphia, except that the anterior part of the sternum undergoes atrophy. But in the higher forms the interclavicle becomes almost at once divided into three parts, of which the two lateral remain distinct, while the median element fuses with the subjacent part of the sternum and constitutes with it the presternum (manubrium sterni). If Götte’s facts are to be trusted, and they have been to a large extent confirmed by Hoffmann, his homologies appear to be satisfactorily established. As mentioned onp.563Ruge (No.438) holds that Götte is mistaken as to the origin of the presternum.
Gegenbaur admits the lateral elements as parts of the interclavicle, while Parker holds that they are not parts of an interclavicle but are homologous with the omosternum of the Frog, which is however held by Götte to be a true interclavicle.
Amphibia. In Amphibia the two halves of the shoulder girdle are each formed as a continuous plate, the ventral or coracoid part of which is forked, and is composed of a larger posterior and a smaller anterior bar-like process, united dorsally. In the Urodela the two remain permanently free at their ventral ends, but in the Anura they become united, and the space between them then forms a fenestra. The anterior process is usually (Gegenbaur, Parker) regarded as the præcoracoid, but Götte has pointed out that in its mode of development it strongly resembles the clavicle of the higher forms, and behaves quite differently to the so-called præcoracoid of Lizards. It is however to be noticed that it differs from the clavicle in the fact that it is never segmented off from the coraco-scapular plate, a condition which has its only parallel in the equally doubtful case of the Chelonia. Parker holds that there is no clavicle present in the Amphibia, while Gegenbaur maintains that an ossification which appears in many of the Anura (though not in the Urodela) in the perichondrium on the anterior border of the cartilaginous bar above mentioned is the representative of the clavicle. Götte’s observations on the ossification of this bone throw doubt upon this view of Gegenbaur; while the fact that the cartilaginous bar may be completely enclosed by the bone in question renders Gegenbaur’s view, that there is present both a clavicle and præcoracoid, highly improbable.
No interclavicle is present in Urodela, but in this group and in a number of the Anura, a process grows out from the end of each of the bars (præcoracoids) which Götte holds to be the clavicles. The two processes unite in the median line, and give rise in front to the anterior unpaired element of the shoulder girdle (omosternum of Parker). They sometimes overlap the epicoracoids behind, and fusing with them bind them together in the median line. Parker who has described the paired origin of the so-called omosternum, holds that it is not homologous with the interclavicle, but compares it with his omosternum in Mammals.
Bibliography.
(463)Bruch. “Ueber die Entwicklung der Clavicula und die Farbe des Blutes.”Zeit. f. wiss. Zool., iv. 1853.(464)A. Dugès. “Recherches sur l'ostéologie et la myologie des Batraciens à leurs différens âges.”Mémoires des savants étrang. Académie royale des sciences de l'institut de France,Vol.VI. 1835.(465)C. Gegenbaur.Untersuchungen zur vergleichenden Anatomie der Wirbelthiere, 2 Heft.Schultergürtel der Wirbelthiere. Brustflosse der Fische.Leipzig, 1865.(466)A. Götte. “Beiträge z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere, Brustbein u. Schultergürtel.”Archiv f. mikr. Anat.Vol.XIV.1877.(467)C. K. Hoffmann. “Beiträge z. vergleichenden Anatomie d. Wirbelthiere.”Niederländisches Archiv f. Zool.,Vol.V. 1879.(468)W. K. Parker. “A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata.”Ray Society, 1868.(469)H. Rathke.Ueber die Entwicklung der Schildkröten.Braunschweig, 1848.(470)H. Rathke.Ueber den Bau und die Entwicklung des Brustbeins der Saurier, 1853.(471)A. Sabatier.Comparaison des ceintures et des membres antérieurs et postérieurs d. la Série d. Vertébrés.Montpellier, 1880.(472)Georg ’Swirski.Untersuch. üb. d. Entwick. d. Schultergürtels u. d. Skelets d. Brustflosse d. Hechts.Inaug. Diss. Dorpat, 1880.
Pelvic girdle.
Pisces. The pelvic girdle of Fishes is formed of a cartilaginous band, to the outer and posterior side of which the basal element of the pelvic fin is usually articulated. This articulation divides it into a dorsal iliac, and ventral pubic section. The iliac section never articulates with the vertebral column.
In Elasmobranchii the two girdles unite ventrally, but the iliac section is only slightly developed. In Chimæra there is a well developed iliac process, but the pubic parts of the girdle are only united by connective tissue.
In the cartilaginous Ganoids the pelvic girdle is hardly to be separated from the skeleton of the fin. It is not united with its fellow, and is represented by a plate with slightly developed pubic and iliac processes.
In the Dipnoi there is a simple median cartilage, articulated with the limb, but not provided with an iliac process. In bony Ganoids and Teleostei there is on each side a bone meeting its fellow in the ventral line, which is usually held to be the rudiment of the pelvic girdle; while Davidoff attempts to shew that it is the basal element of the fin, and that, except in Polypterus, a true pelvic girdle is absent in these types.
From my own observations I find that the mode of development of the pelvic girdle in Scyllium is very similar to that of the pectoral girdle. There is a bar on each side, continuous on its posterior border with the basal element of the fin (figs.345and347). This bar meets and unites with its fellow ventrally before becoming converted into true cartilage, and though the iliac process (il) is never very considerable, yet it is better developed in the embryo than in the adult, and is at first directed nearly horizontally forwards.
Amphibia and Amniota. The primitive cartilaginous pelvicgirdle of the higher types exhibits the same division as that of Pisces into a dorsal and a ventral section, which meet to form the articular cavity for the femur, known as theacetabulum. The dorsal section is always single, and is attached by means of rudimentary ribs to the sacral region of the vertebral column, and sometimes to vertebræ of the adjoining lumbar or caudal regions. It always ossifies as the ilium.
The ventral section is usually formed of two more or less separated parts, an anterior which ossifies as the pubis, and a posterior which ossifies as the ischium. The space between them is known as the obturator foramen. In the Amphibia the two parts are not separated, and resemble in this respect the pelvic girdle of Fishes. They generally meet the corresponding elements of the opposite side ventrally, and form a symphysis with them. The symphysis pubis, and symphysis ischii may be continuous (Mammalia, Amphibia).
The observations on the development of the pelvic girdle in the Amphibia and Amniota are nearly as scanty as on those of Fishes.
Amphibia. In the Amphibia (Bunge,No.473) the two halves of the pelvic girdle are formed as independent masses of cartilage, which subsequently unite in the ventral line.
In the Urodelous Amphibia (Triton) each mass is a simple plate of cartilage divided into a dorsal and ventral section by the acetabulum. The ventral parts, which are not divided into two regions, unite in a symphysis comparatively late.
The dorsal section ossifies as the ilium. The ventral usually contains a single ossification in its posterior part which forms the ischium; while the anterior part, which may be considered as representing the pubis, usually remains cartilaginous; though Huxley (No.475) states that it has a separate centre of ossification in Salamander, which however does not appear to be always present (Bunge). There is a small obturator foramen between the ischium and pubis, which gives passage to the obturator nerve. It is formed by the part of the tissue where the nerve is placed not becoming converted into cartilage.
There is a peculiar cartilage in the ventral median line in front of the pubis, which is developed independently of and much later than the true parts of the pelvic girdle. It may be called the præpubic cartilage.
Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two ventral (pubic and ischiad) processes. The acetabulum is placed on the outer side at the junction of the three processes, each of which maybe considered to have a share in forming it. The distal ends of the pubis and ischium are close together when first formed, but subsequently separate. Each of them unites at a late stage with the corresponding process of the opposite side in a ventral symphysis. A centre of ossification appears in each of the three processes of the primitive cartilage.
Aves. In Birds the parts of the pelvic girdle no longer develop as a continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the Duck, all the elements. The ilium early exhibits a short anterior process, but the pubis and ischium are at first placed with their long axes at right angles to that of the ilium, but gradually become rotated so as to lie parallel with it, their distal ends pointing backwards, and not uniting ventrally excepting in one or two Struthious forms.
Mammalia. In Mammalia the pelvic girdle is formed in cartilage as in the lower forms, but in Man at any rate the pubic part of the cartilage is formed independently of the remainder (Rosenberg). There are the usual three centres of ossification, which unite eventually into a single bone—the innominate bone. The pubis and ischium of each side unite with each other ventrally, so as completely to enclose the obturator foramen.
Huxley holds that the so-called marsupial bones of Monotremes and Marsupials, which as shewn by Gegenbaur (No.474) are performed in cartilage, are homologous with the præpubis of the Urodela; but considering the great gap between the Urodela and Mammalia this homology can only be regarded as tentative. He further holds that the anterior prolongations of the cartilaginous ventral ends of the pubis of Crocodilia are also structures of the same nature.
Bibliography.
(473)A. Bunge.Untersuch. z. Entwick. d. Beckengürtels d. Amphibien, Reptilien u. Vögel. Inaug. Diss.Dorpat, 1880.(474)C. Gegenbaur. “Ueber d. Ausschluss des Schambeins von d. Pfanne d. Hüftgelenkes.”Morph. Jahrbuch,Vol.II. 1876.(475)Th. H. Huxley. “The characters of the Pelvis in Mammalia, etc.”Proc. of Roy. Soc.,Vol.XXVIII. 1879.(476)A. Sabatier.Comparaison des ceintures et des membres antérieurs et postérieurs dans la Série d. Vertébrés.Montpellier, 1880.
Comparison of Pectoral and Pelvic girdles.
Throughout the Vertebrata a more or less complete serial homology may be observed between the pectoral and pelvic girdles.
In the cartilaginous Fishes each girdle consists of a continuous band, a dorsal and ventral part being indicated by the articulation of the fin; the former being relatively undeveloped in the pelvicgirdle, while in the pectoral it may articulate with the vertebral column. In the case of the pectoral girdle secondary membrane bones become added to the primitive cartilage in most Fishes, which are not developed in the case of the pelvic girdle.
In the Amphibia and Amniota the ventral section of each girdle becomes divided into an anterior and a posterior part, the former constituting the præcoracoid and pubis, and the latter the coracoid and ischium; these parts are however very imperfectly differentiated in the pelvic girdle of the Urodela. The ventral portions of the pelvic girdle usually unite below in a symphysis. They also meet each other ventrally in the case of the pectoral girdle in Amphibia, but in most other types are separated by the sternum, which has no homologue in the pelvic region, unless the præpubic cartilage is to be regarded as such. The dorsal or scapular section of the pectoral girdle remains free; but that of the pelvic girdle acquires a firm articulation with the vertebral column.
If the clavicle of the higher types is derived from the membrane bones of the pectoral girdle of Fishes, it has no homologue in the pelvic girdle; but if, as Götte and Hoffmann suppose, it is a part of the primitive cartilaginous girdle, the ordinary view as to the serial homologies of the ventral sections of the two girdles in the higher types will need to be reconsidered.
Limbs.
It will be convenient to describe in this place not only the development of the skeleton of the limbs but also that of the limbs themselves. The limbs of Fishes are moreover so different from those of the Amphibia and Amniota that the development of the two types of limb may advantageously be treated separately.
In Fishes the first rudiments of the limbs appear as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins.
These ridges are two in number on each side, an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected, but in some Elasmobranch embryos, more especially in Torpedo, they are connected together at their first developmentby a line of columnar epiblast cells[212]. This connecting line of columnar epiblast is a very transitory structure, and after its disappearance the rudimentary fins become more prominent, consisting (fig. 343,b) of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold and the fin becomes a simple ridge of mesoblast, covered by epiblast. The pectoral fins are usually considerably ahead of the pelvic fins in development.
Illustration: Figure 343Fig. 343. Section through the ventral part of the trunk of a young embryo of scyllium at the level of the umbilical cord.b.pectoral fin;ao.dorsal aorta;cav.cardinal vein;ua.vitelline artery;u.v.vitelline vein;al.duodenum;l.liver;sd.opening of segmented duct into the body cavity;mp.muscle plate;um.umbilical canal.
Fig. 343. Section through the ventral part of the trunk of a young embryo of scyllium at the level of the umbilical cord.b.pectoral fin;ao.dorsal aorta;cav.cardinal vein;ua.vitelline artery;u.v.vitelline vein;al.duodenum;l.liver;sd.opening of segmented duct into the body cavity;mp.muscle plate;um.umbilical canal.
For the remaining history it is necessary to confine ourselves to Scyllium as the only type which has been adequately studied.
The direction of the original ridge which connects the two fins of each side is nearly though not quite longitudinal, sloping somewhat obliquely downwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line a little way behind the anus.
The elongated ridge, forming the rudiment of each fin, gradually projects more and more, and so becomes broader in proportion to its length, but at the same time its actual attachment to the side of the body becomes shortened from behind forwards, so that what was originally theattached borderbecomes in part converted into theposterior border. This process is much more completely carried out in the case of the pectoral fins than in that of the pelvic, and the changes of form undergone by the pectoral fin in its development may be gathered fromfigs.344and348.
Before proceeding to the development of the skeleton of the fin it may be pointed out that the connection of the two rudimentary fins by a continuous epithelial line suggests the hypothesis that they are the remnants of two continuous lateral fins[213].
Shortly after the view that the paired fins were remnants of continuous lateral fins had been put forward in my memoir on Elasmobranch Fishes, two very interesting papers were published by Thacker (No.489) and Mivart (No.484) advocating this view on the entirely independent grounds of the adult structure of the skeleton of the paired fins in comparison with that of the unpaired fins[214].
The development of the skeleton has unfortunately not been as yet very fully studied. I have however made some investigations on this subject on Scyllium, and ’Swirski has also made some on the Pike.
In Scyllium the development of both the pectoral and pelvic fins is very similar.
In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girdle, and running backwards parallel to the long axis of the body. The outer side of this bar is continued into a plate whichextends into the fin, and which becomes very early segmented into a series of parallel rays at right angles to the longitudinal bar.
In other words, the primitive skeleton of both the fins consists of a longitudinal bar running along the base of the fin, and giving off at right angles series of rays which pass into the fin. The longitudinal bar, which may be called thebasipterygium, is moreover continuous in front with the pectoral or pelvic girdle as the case may be.
Illustration: Figure 344Fig. 344. Pectoral fin of a young embryo of Scyllium in longitudinal and horizontal section.The skeleton of the fin was still in the condition of embryonic cartilage.b.p.basipterygium (eventual metapterygium);fr.fin rays;p.g.pectoral girdle in transverse section;f.foramen in pectoral girdle;pc.wall of peritoneal cavity.
Fig. 344. Pectoral fin of a young embryo of Scyllium in longitudinal and horizontal section.The skeleton of the fin was still in the condition of embryonic cartilage.b.p.basipterygium (eventual metapterygium);fr.fin rays;p.g.pectoral girdle in transverse section;f.foramen in pectoral girdle;pc.wall of peritoneal cavity.
The primitive skeleton of the pectoral fin is shewn in longitudinal section infig. 344, and that of the pelvic fin at a slightly later stage infig. 345.
A transverse section shewing the basipterygium (mpt) of the pectoral fin, and the plate passing from it into the fin, is shewn infig. 346.
Before proceeding to describe the later history of the two fins it may be well to point out that their embryonic structure completely supports the view which has been arrived at from the consideration of the soft parts of the fin.
My observations shew that the embryonic skeleton of the paired fin consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft part of the fin which has the form of a longitudinal ridge, and are continuous at their base with a longitudinal bar, which may very probablybe due to secondary development. As pointed out by Mivart, a longitudinal bar is also occasionally formed to support the cartilaginous rays of unpaired fins. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of primitively independent rays, of which they believe the fin to have been originally composed. This view is probable enough in itself, but there is no trace in the embryo of the bar in question being formed by the coalescence of rays, though the fact of its being perfectly continuous with the bases of the rays is somewhat in favour of this view[215].
Illustration: Figure 345Fig. 345. Pelvic fin of a very young female embryo of Scyllium stellare.bb.basipterygium;pu.pubic process of pelvic girdle;il.iliac process of pelvic girdle.
Fig. 345. Pelvic fin of a very young female embryo of Scyllium stellare.bb.basipterygium;pu.pubic process of pelvic girdle;il.iliac process of pelvic girdle.
A point may be noticed here which may perhaps appear to be a difficulty,viz.that to a considerable extent in the pectoral, and to some extent in the pelvic fin the embryonic cartilage from which the fin-rays are developed is at first a continuous lamina, which subsequently segments into rays. I am however inclined to regard this merely as a result of the mode of conversion of the indifferent mesoblast into cartilage; and in any case no conclusion adverse to the above view can be drawn from it, since I find that the rays of the unpaired fin are similarly segmented from a continuous lamina. In all cases the segmentation of the rays is to a large extent completed before the tissue in question is sufficiently differentiated to be called cartilage by an histologist.
Thacker and Mivart both hold that the pectoral and pelvic girdles have been evolved by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.
There is, so far as I see, no theoretical objection to be taken to this view, and the fact of the pectoral and pelvic girdles originating continuously, and long remaining united with thelongitudinal bars of their respective fins is in favour of rather than against this view. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.
The later development of the skeleton of the two fins is more conveniently treated separately.
Illustration: Figure 346Fig. 346. Transverse section through the pectoral fin of a young embryo of Scyllium stellare.mpt.basipterygial bar (metapterygium);fr.fin ray;m.muscles;hf.horny fibres.
Fig. 346. Transverse section through the pectoral fin of a young embryo of Scyllium stellare.mpt.basipterygial bar (metapterygium);fr.fin ray;m.muscles;hf.horny fibres.
The pelvic fin. The changes in the pelvic fin are comparatively slight. The fin remains through life as a nearly horizontal lateral projection of the body, and the longitudinal bar—the basipterygium—at its base always remains as such. It is for a considerable period attached to the pelvic girdle, but eventually becomes segmented from it. Of the fin rays the anterior remains directly articulated with the pelvic girdle on the separation of the basipterygium (fig. 347), and the remaining rays finally become segmented from the basipterygium, though they remain articulated with it. They also become to some extent transversely segmented. The posterior end of the basipterygial bar also becomes segmented off as the terminal ray.
The pelvic fin thus retains in all essential points its primitive arrangement.
The pectoral fin. The earliest stage of the pectoral fin differs from that of the pelvic fin only in minor points. There is the same longitudinal or basipterygial bar to which the fin-rays are attached, whose position at the base of the fin is clearly seen in the transverse section (fig. 346,mpt). In front the bar is continuous with the pectoral girdle (figs.344and348).