Illustration: Figure 322Fig. 322. Longitudinal section through the intervertebral ligament and adjacent parts of two vertebræ from the thoracic region of an advanced embryo of a Sheep.(From Kölliker.)la.ligamentum longitudinale anterius;lp.ligamentum long. posterius;li.ligamentum intervertebrale;k,k´. epiphysis of vertebra;w.andw´.anterior and posterior vertebræ;c.intervertebral dilatation of notochord;c´.andc´´.vertebral dilatation of notochord.
Fig. 322. Longitudinal section through the intervertebral ligament and adjacent parts of two vertebræ from the thoracic region of an advanced embryo of a Sheep.(From Kölliker.)la.ligamentum longitudinale anterius;lp.ligamentum long. posterius;li.ligamentum intervertebrale;k,k´. epiphysis of vertebra;w.andw´.anterior and posterior vertebræ;c.intervertebral dilatation of notochord;c´.andc´´.vertebral dilatation of notochord.
The fate of the notochord is in important respects different from that in Birds. It is first constricted in thecentre of the vertebræ(figs.320E and321) and disappears there shortly after the ossification; while in the intervertebral regions it remains relatively unconstricted (figs.320E,321and322c) and after undergoing certain histological changesremains through life as part of the nucleus pulposus in the axis of the invertebral ligaments[201]. There is also a slight swelling of the notochord near the two extremities of each vertebra (fig. 322c´andc´´). In the persistent vertebral constriction of the notochord Mammals retain a more primitive and piscine mode of formation of the vertebral column than the majority either of the Reptilia or Amphibia.
Bibliographyof Notochord and Vertebral column.
(415)Cartier. “Beiträge zur Entwicklungsgeschichte der Wirbelsäule.”Zeitschrift für wiss. Zool.,Bd.XXV. Suppl. 1875.(416)C. Gegenbaur.Untersuchungen zur vergleichenden Anatomie der Wirbelsäule der Amphibien und Reptilien.Leipzig, 1862.(417)C. Gegenbaur. “Ueber die Entwickelung der Wirbelsäule des Lepidosteus mit vergleichend anatomischen Bemerkungen.”Jenaische Zeitschrift,Bd.III. 1863.(418)C. Gegenbaur. “Ueb. d. Skeletgewebe d. Cyclostomen.”Jenaische Zeitschrift,Vol.V. 1870.(419)Al. Götte. “Beiträge zur vergleich. Morphol. des Skeletsystems d. Wirbelthiere.”II.“Die Wirbelsäule u. ihre Anhänge.”Archiv f. mikr. Anat.,Vol.XV.1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), andVol.XVI.1879 (Teleostier).(420)Hasse und Schwarck. “Studien zur vergleichenden Anatomie der Wirbelsäule u. s. w.” Hasse,Anatomische Studien, 1872.(421)C. Hasse.Das natürliche System d. Elasmobranchier auf Grundlage d. Bau. u. d. Entwick. ihrer Wirbelsäule.Jena, 1879.(422)A. Kölliker. “Ueber die Beziehungen der Chorda dorsalis zur Bildung der Wirbel der Selachier und einiger anderen Fische.”Verhandlungen der physical. medicin. Gesellschaft in Würzburg,Bd.X.(423)A. Kölliker. “Weitere Beobachtungen über die Wirbel der Selachier insbesondere über die Wirbel der Lamnoidei.”Abhandlungen der senkenbergischen naturforschenden Gesellschaft in Frankfurt,Bd.V.(424)H. Leboucq. “Recherches s. l. mode de disparition de la corde dorsale chez les vertébrés supérieurs.”Archives de Biologie,Vol.I. 1880.(425)Fr. Leydig.Anatomisch-histologische Untersuchungen über Fische und Reptilien.Berlin, 1853.(426)Aug. Müller. “Beobachtungen zur vergleichenden Anatomie der Wirbelsäule.” Müller’sArchiv.1853.(427)J. Müller. “Vergleichende Anatomie der Myxinoiden u. der Cyklostomen mit durchbohrtem Gaumen, I. Osteologie und Myologie.”Abhandlungen der königlichen Akademie der Wissenschaften zu Berlin.1834.(428)W. Müller. “Beobachtungen des pathologischen Instituts zu Jena, I. Ueber den Bau der Chorda dorsalis.”Jenaische Zeitschrift,Bd.VI.1871.(429)A. Schneider.Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.Berlin, 1879.
Ribs and Sternum.
Ribs. Embryological evidence on the development of the ribs, though somewhat inadequate, indicates that they arise as cartilaginous bars in the connective tissue of the intermuscular septa, and that they are placed, in Elasmobranchii andAmphibia, on the level of division between the dorso-lateral and ventro-lateral divisions of the muscle-plates. This does not appear to hold true for either Ganoidei or Teleostei. In Teleostei they are entirely below the muscles along the lines of the intermuscular septa, and this is partially true for Ganoidei, though not wholly so in Lepidosteus. They may be attached either to the hæmal (Pisces) or neural (Amphibia and Amniota) arches. The connective tissue from which they are formed is continuous with the processes of the vertebræ to which they are attached; but the conversion of the tissue into cartilage takes place more or less independently of that of the arches, although in many cases the cartilage of the two becomes continuous, the separation of the ribs being then effected by a subsequent process of segmentation (Fick,No.431). It is possible that the ribs of Pisces may not be homologous with those of Amphibia and the Amniota, but till the reverse can be proved it is more convenient to assume that the ribs are homologous structures throughout the vertebrate series.
In Elasmobranchii the ribs are relatively of less importance in the adult than in the embryo. By a careful examination of their early development, I have satisfied myself that the differentiation of the ribs isindependentof that of the hæmal processes to which they are attached, although the differentiation proceeds in such a manner that, when both are converted into cartilage, they are quite continuous. Subsequently the ribs become segmented off from the hæmal processes. At the junction of the tail and trunk, where the hæmal processes commence to be ventrally prolonged, eventually to unite in the region of the tail below the caudal vein, the ribs are attached to short processes which spring from the sides of the hæmal arches (fig. 317). The ventral hæmal arches of these fishes are therefore clearly in no part formed by the ribs.
In Ganoidei and Teleostei there is very great difficulty in determining the homologies of the ribs.
In the cartilaginous Ganoidei there are well developed rib-like structures, which might be regarded as homologous with Elasmobranch ribs, and indeed probably are so; but at the same time their relations are in some respects very different from those of Elasmobranch ribs in the caudal region. In Ganoids the ribs, in approaching the tail, become shorter and then fuse with the ends of the hæmal processes, and finally in the caudal region form together with the hæmal arches a closed hæmal canal which superficially resembles that in Elasmobranchii.
In Lepidosteus and Amia, especially the former, the same phenomenon is still more marked; and in Lepidosteus it is easy, in passing backwards,to trace the ribs bending ventralwards, and uniting ventrally in the caudal region to form, with the hæmal processes, a complete hæmal canal.
It might have been anticipated that the Teleostean Ganoids would resemble the Teleostei, but, from an examination of adult Teleostei, it would seem to be clear that the relations of the parts are the same as in Elasmobranchii,i.e.that the ribs have no share in forming the hæmal canal in the tail. Aug. Müller and Götte have however brought embryological evidence (though not of a conclusive character), to shew that in the embryo the ribs really fuse with the hæmal processes in the tail, and so assist, as in the Ganoids, in forming the hæmal canal. Götte moreover holds that the ribs in Elasmobranchii are not homologous with those of Teleostei and Ganoids; but that the hæmal arches in the tail are homologous in the three groups.
Without necessarily following Götte in these views it is worth pointing out that the undoubtedly close affinity between the bony Ganoids and the Teleostei is in favour of the view on the hæmal arches of Teleostei at which he has arrived on embryological grounds.
In Amphibia the formation of the ribs from the connective tissue of the intermuscular septa, their secondary attachment to the transverse processes of the neural arches, and their subsequent separation was first clearly established by Fick (No.431), whose statements have since been confirmed by Hasse, Born,&c., and in part by Götte, who holds however that, though converted into cartilage independently of the transverse processes, they are formed in membrane as outgrowths of these processes.
In the Amniota the ribs are also independently established (Hasse and Born), though they subsequently become united to the transverse processes and to the bodies of the vertebræ, or to the transverse processes only. This junction is however stated by the majority of authorities, never to be effected by the fusion of the cartilage of the two parts, but always by fibrous tissue; though Hoffmann (No.435) takes a different view on this subject, holding that the ribs are at first continuous with the intervertebral regions of the primitive cartilaginous tube surrounding the notochord.
Sternum. In dealing with the development of the sternum it will be convenient to leave out of consideration the interclavicle or episternum which is, properly speaking, only part of the shoulder-girdle and to confine my statements to the sternum proper.
This structure is found in all the Amniota except the Ophidia, Chelonia, and some of the Amphisbænæ.
From the older researches of Rathke, and from the newer ones of Götte, etc., it appears that the sternum is always formed from the fusion of the ventral extremities of a certain number of ribs. The extremities of the ribs unite with each other frombefore backwards, and thus give rise to two cartilaginous bands. These bands become segmented off from the ribs with which they are at first continuous, and subsequently fuse in the median ventral line to form an unpaired sternum. The Mammalian presternum (manubrium sterni) and xiphosternum have the same origin as the main body of the sternum (Ruge,No.438).
In the Amphibia there is no structure which admits from its mode of development of a complete comparison with the sternum of the Amniota; and it must for this reason be considered doubtful whether the median structure placed behind the coracoids in the Anura, which is usually known as the sternum, is really homologous with the sternum of the Amniota[202].
The remaining Ichthyopsida are undoubtedly not provided with a sternum.
Bibliographyof Ribs and Sternum.
(430)C. Claus. “Beiträge z. vergleich. Osteol. d. Vertebraten. I. Rippen u. unteres Bogensystem.”Sitz. d. kaiserl. Akad. Wiss. Wien,Vol.LXXIV.1876.(431)A. E. Fick. “Zur Entwicklungsgeschichte d. Rippen und Querfortsätze.”Archiv f. Anat. und Physiol.1879.(432)C. Gegenbaur. “Zur Entwick. d. Wirbelsäule des Lepidosteus mit vergleich. anat. Bemerk.”Jenaische Zeit.,Vol.III. 1867.(433)A. Götte. “Beiträge z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergürtel.”Archiv f. mikr. Anat.,Vol.XIV.1877.(434)C. Hasseu.G. Born. “Bemerkungen üb. d. Morphologie d. Rippen.”Zoologischer Anzeiger, 1879.(435)C. K. Hoffmann. “Beiträge z. vergl. Anat. d. Wirbelthiere.”Niederländ. Archiv Zool.,Vol.IV. 1878.(436)W. K. Parker. “A monograph on the structure and development of the shoulder-girdle and sternum.”Ray Soc.1867.(437)H. Rathke.Ueb. d. Bau u. d. Entwicklung d. Brustbeins d. Saurier.1853.(438)G. Ruge. “Untersuch. üb. Entwick. am Brustbeine d. Menschen.”Morphol. Jahrbuch.,Vol.VI. 1880.
[199]This tube consists of a peculiar form of fibrous tissue rather than true cartilage, though part of it subsequently becomes hyaline cartilage.[200]The presence of intercalated pieces in the neural arch system of Elasmobranchii, Chimæra, etc. is probably not the indication of an highly differentiated type of neural arch, but of a transitional type between an imperfect investment of the spinal cord by isolated cartilaginous bars, and a complete system of neural arches like that in the higher Vertebrata.[201]This view was first put forward by Lushka, and his surmises have been confirmed by Kölliker and other embryologists. Leboucq (No.424) however holds that the cells of the notochord in the intervertebral regions fuse with those of the adjoining tissue; and Dursy and others deny that the nucleus pulposus is derived from the notochord.[202]The so-called sternum of the Amphibia develops in proximity with certain rudimentary abdominal ribs, and Ruge has with some force urged (against Götte) that it may be for this reason a rudimentary structure of the same nature as the sternum of the higher types.
[199]This tube consists of a peculiar form of fibrous tissue rather than true cartilage, though part of it subsequently becomes hyaline cartilage.
[200]The presence of intercalated pieces in the neural arch system of Elasmobranchii, Chimæra, etc. is probably not the indication of an highly differentiated type of neural arch, but of a transitional type between an imperfect investment of the spinal cord by isolated cartilaginous bars, and a complete system of neural arches like that in the higher Vertebrata.
[201]This view was first put forward by Lushka, and his surmises have been confirmed by Kölliker and other embryologists. Leboucq (No.424) however holds that the cells of the notochord in the intervertebral regions fuse with those of the adjoining tissue; and Dursy and others deny that the nucleus pulposus is derived from the notochord.
[202]The so-called sternum of the Amphibia develops in proximity with certain rudimentary abdominal ribs, and Ruge has with some force urged (against Götte) that it may be for this reason a rudimentary structure of the same nature as the sternum of the higher types.
Three distinct sets of elements may enter into the composition of the skull. These are (1) the cranium proper, composed of true endoskeletal elements originally formed in cartilage, to which are usually added exoskeletal osseous elements, formed in the manner already describedp.542, and known in the higher types as membrane bones. (2) The visceral arches formed primitively as cartilaginous bars, but in the higher types largely supplemented or even replaced by exoskeletal elements. (3) The labial cartilages.
These parts present themselves in the most various forms, and their study constitutes one of the most important departments of vertebrate morphology, and one which has always been a favourite subject of study with anatomists. At the end of the last century and during the first half of the present century the morphology of the skull was handled from the point of view of the adult anatomy by Goethe, Oken, Cuvier, Owen, and many other anatomists, while Dugés and, nearer to our own time, Rathke, laid the foundation of an embryological study of its morphology. A new era in the study of the skull was inaugurated by Huxley in his Croonian lecture in 1858, and in his lectures on Comparative Anatomy subsequently delivered before the Royal College of Surgeons. In these lectures Huxley disproved the then widely accepted view that the skull was composed of four vertebræ; and laid the foundation of a more satisfactory method of dealing with the homologies of its constituent parts. Since then the knowledge of the development of the skull has made great progress. In this country a numberof very interesting memoirs have been published on the subject by Parker, which together constitute a most striking contribution to our knowledge of the ontogeny of the skull in a series of types; and in Germany Gegenbaur’s monograph on the cephalic skeleton of Elasmobranchii has greatly promoted a scientific appreciation of the nature of the skull.
In the present chapter only the most important features in the development of the skull will be touched on.
It will be convenient to describe, in the first instance, the development of the cartilaginous elements of the skull.
Illustration: Figure 323Fig. 323. head of embryo Dogfish, second stage; basal view of cranium from above, the contents having been removed.(From Parker.)ol.olfactory sacs;au.auditory capsule;nc.notochord;py.pituitary body;pa.ch.parachordal cartilage;tr.trabecula;inf.infundibulum;C.tr.cornua trabeculæ;pn.prenasal element;sp.spiracular cleft;br.external branchiæ;Cl.2, 4. visceral clefts.
Fig. 323. head of embryo Dogfish, second stage; basal view of cranium from above, the contents having been removed.(From Parker.)ol.olfactory sacs;au.auditory capsule;nc.notochord;py.pituitary body;pa.ch.parachordal cartilage;tr.trabecula;inf.infundibulum;C.tr.cornua trabeculæ;pn.prenasal element;sp.spiracular cleft;br.external branchiæ;Cl.2, 4. visceral clefts.
The Cranium. The brain is at first enveloped in a continuous layer of mesoblast known as the membranous cranium, into the base of which the anterior part of the notochord is prolonged for some distance. The primitive cartilaginous cranium is formed by a differentiation within the membranous cranium, and is always composed of the following parts (fig. 323):
(1) A pair of cartilaginous plates on each side of the cephalic section of the notochord, known as theparachordals(pa.ch). These plates together with the notochord (nc) enclosed between them form a floor for the hind- and mid-brain. The continuous plate, formed by them and the notochord, is known as thebasilar plate.
(2) A pair of bars forming the floor for the fore-brain, known as thetrabeculæ(tr). These bars are continued forward from the parachordals. They meet behind and embrace the front end of the notochord; and after separating for some distance bend in again in such a wayas to enclose a space—the pituitary space. In front of this space they remain in contact and generally unite. They extend forwards into the nasal region (pn).
(3) The cartilaginous capsules of the sense organs. Of these the auditory (au) and olfactory capsules (ol) unite more or less intimately with the cranial walls; while the optic capsules, forming the usually cartilaginous sclerotics, remain distinct.
The parachordals and notochord. The first of these sets of elements,viz.the parachordals and notochord, forming together the basilar plate, is always an unsegmented continuation of the axial tissue of the vertebral column. It forms the floor for that section of the brain which belongs to the primitive postoral part of the head (videp. 314), and its extension is roughly that of the basioccipital of the adult skull. Its mode of development is almost identical with that of the vertebral column, except that the notochord, even in many forms where it persists in the vertebral column, disappears in the basilar plate; though in a certain number of cases remnants of it are found in the adult state.
Illustration: Figure 324Fig. 324. Longitudinal section through the brain of a young Pristiurus embryo.cer.commencement of the cerebral hemisphere;pn.pineal gland;In.infundibulum;pt.ingrowth from mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
Fig. 324. Longitudinal section through the brain of a young Pristiurus embryo.cer.commencement of the cerebral hemisphere;pn.pineal gland;In.infundibulum;pt.ingrowth from mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
It will be convenient to say a few words here with reference to the notochord in the head. It always extends along the floor of the mid- and hind-brains, but ends immediately behind the infundibulum. The limits of its anterior extension are clearly shewn infig. 43. The front end of the notochord often becomes more or less ventrally flexed in correspondence with the cranial flexure; its anterior end being in some instances (Elasmobranchii) almost bent backwards (fig. 324).
Kölliker has shewn that in the Rabbit[203], and I believe that a more or less similar phenomenon may also be observed in Birds, the anterior end of the notochord is united to the hypoblast of the throat in immediate contiguity with the opening of the pituitary body; but it is not clear whether this is to be looked upon as the remnant of a primitive attachment of the notochord to the hypoblast, or as a secondary attachment.
Before the parachordals are formed the anterior end of the notochord has usually undergone a partial atrophy; and its front end often becomes somewhat dorsally flexed. Within the basilar plate it often exhibits two or more dilatations, which have been regarded by Parker and Kölliker as indicative of a segmentation of this plate; but they hardly appear to me to be capable of this interpretation.
In Elasmobranchs where, as shewn above, a very primitive type of development of the vertebral column is retained, we find that the basilar plate is at first formed of (1) the notochord invested by its cartilaginous sheath, and (2) of lateral masses of cartilage, the parachordals, homologous with the arch tissue of the vertebral column. This development probably indicates thatthe basilar plate contains in itself the same elements as those from which the neural arches and the centra of the vertebral column are formed; but thatit never passes beyond the unsegmented stage at first characteristic of the vertebral column. The hinder end of each parachordal forms a condyle articulating with the first vertebra; so that in the cartilaginous skull there are always two occipital condyles. The basilar plate always grows up behind (fig. 326,so), and gives rise to a complete cartilaginous ring enveloping the medulla oblongata, in the same manner that the neural arches envelope the spinal cord. This ring forms an occipital cartilaginous ring; in front of it the basilar plate becomes laterally continuous with the periotic cartilaginous capsules, and the occipital ring above usually spreads forward to form a roof for the part of the brain between these capsules. In the higher Vertebrates the periotic cartilages may be developed continuously with the basilar plate (fig. 325).
The trabeculæ. The trabeculæ, so far as their mere anatomical relations are concerned, play the same part in forming the floor for the front cerebral vesicle as the parachordals for the mid- and hind-brains. They differ however from the parachordals in one important feature,viz.that, except at their hinder end (fig. 323), they do not embrace between them the notochord.
The notochord constitutes, as we have seen, the primitive axial skeleton of the body, and its absence in the greater part of the region of the trabeculæ would probably seem to indicate, aspointed out by Gegenbaur, that these parts, in spite of their similarity to the parachordals, have not the same morphological significance.
Illustration: Figure 325Fig. 325. View From above of the investing mass and of the trabeculæ of a chick on the fourth day of incubation.(After Parker.)In order to shew this, the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading.cv1. cerebral vesicle (sliced off);e.eye;nc.notochord;iv.investing mass; 9. foramen for the exit of the ninth nerve;cl.cochlea;hsc.horizontal semicircular canal;q.quadrate; 5. notch for the passage of the fifth nerve;lg.expanded anterior end of the investing mass;pts.pituitary space;tr.trabeculæ. The reference linetr.has been accidentally made to end a little short of the cartilage.
Fig. 325. View From above of the investing mass and of the trabeculæ of a chick on the fourth day of incubation.(After Parker.)In order to shew this, the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading.cv1. cerebral vesicle (sliced off);e.eye;nc.notochord;iv.investing mass; 9. foramen for the exit of the ninth nerve;cl.cochlea;hsc.horizontal semicircular canal;q.quadrate; 5. notch for the passage of the fifth nerve;lg.expanded anterior end of the investing mass;pts.pituitary space;tr.trabeculæ. The reference linetr.has been accidentally made to end a little short of the cartilage.
The nature of the trabeculæ has been much disputed by morphologists. The view that they cannot be regarded as the anterior section of the vertebral axis is supported by the consideration that the forward limit of the primitive skeletal axis, as marked by the notochord, coincides exactly with the distinction we have found it necessary to recognise, on entirely independent grounds, between the fore-brain, and the remainder of the nervous axis. But while this distinction between the parachordals and the trabeculæ must I think be admitted, I see no reason against supposing that the trabeculæ may be plates developed to support the floor of the fore-brain, for the same physiological reasons that the parachordals have become formed at the sides of the notochord to support the floor of the hind-brain. By some anatomists the trabeculæ have been held to be a pair of branchial bars; but this view has now been generally given up. They have also been regarded as equivalent to a complete pair of neural arches enveloping the front end of the brain. The primitive extension of the base of the fore-brain through the pituitaryspace is an argument, not without force, which has been appealed to in support of this view.
In the majority of the lower forms the trabeculæ arise quite independently of the parachordals, though the two sets of elements soon unite; while in Birds (fig. 325) and Mammals the parachordals and trabeculæ are formed as a continuous whole. The junction between the trabeculæ and parachordals becomes marked by a cartilaginous ridge known as the posterior clinoid.
Illustration: Figure 326Fig. 326. Side view of the cartilaginous cranium of a Fowl on the seventh day of incubation.(After Parker.)pn.prenasal cartilage;aln.alinasal cartilage;ale.aliethmoid; immediately below this is the aliseptal cartilage.eth.ethmoid;pp.pars plana;ps.presphenoid or interorbital;pa.palatine;pg.pterygoid;z.optic nerve;as.alisphenoid;q.quadrate;st.stapes;fr.fenestra rotunda;hso.horizontal semicircular canal;psc.posterior vertical semicircular canal: both the anterior and the posterior semicircular canals are seen shining through the cartilage.so.supraoccipital;eo.exoccipital;oc.occipital condyle;nc.notochord;mk.Meckel’s cartilage;ch.ceratohyal;bh.basihyal;cbr.andebr.ceratobranchial;bbr.basibranchial.
Fig. 326. Side view of the cartilaginous cranium of a Fowl on the seventh day of incubation.(After Parker.)pn.prenasal cartilage;aln.alinasal cartilage;ale.aliethmoid; immediately below this is the aliseptal cartilage.eth.ethmoid;pp.pars plana;ps.presphenoid or interorbital;pa.palatine;pg.pterygoid;z.optic nerve;as.alisphenoid;q.quadrate;st.stapes;fr.fenestra rotunda;hso.horizontal semicircular canal;psc.posterior vertical semicircular canal: both the anterior and the posterior semicircular canals are seen shining through the cartilage.so.supraoccipital;eo.exoccipital;oc.occipital condyle;nc.notochord;mk.Meckel’s cartilage;ch.ceratohyal;bh.basihyal;cbr.andebr.ceratobranchial;bbr.basibranchial.
The trabeculæ are usually somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts (figs.323and325). Into this space there primitively projects the whole base of the fore-brain, but the space itself gradually becomes narrowed, till it usually contains only the pituitary body. The carotid arteries always pass through it in the embryo; but in the higher forms it ceases to be perforated in the adult. The trabeculæ soon unite together both in front and behind and form a complete plate underneath the fore-brain, and extending into the nasal region[204]. A specialvertical growth of this plate in the region of the orbit forms the interorbital plate of Teleostei, Lacertilia and Aves (fig. 326,ps), on the upper surface of which the front part of the brain rests. The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which in the higher types two regions may be distinguished,viz.analisphenoidal region(fig. 326,as) behind, growing out from what is known as thebasisphenoidal regionof the primitive trabeculæ, and anorbitosphenoidal regionin front growing out from thepresphenoidal regionof the trabeculæ. These plates form at first a continuous lateral wall of the cranium. At the front end of the brain they are continued inwards, and more or less completely separate the true cranial cavity from the nasal region in front. The region of the cartilage forming the anterior boundary of the cranial cavity is known as thelateral ethmoid region, and it is always perforated for the passage of the olfactory nerves.
The cartilaginous walls which grow up from the trabecular floor of the cranium generally extend upwards so as to form a roof, though almost always an imperfect roof, for the cranial cavity. In the higher types, in Mammals more especially, this roof can hardly be said to be formed at all. The region of the trabeculæ in front of the brain is the ethmoid region. The basal part of this region forms an internasal plate, from which an internasal septum may grow up (fig. 326). To its sides the olfactory capsules are attached, and there are usually lateral outgrowths in front forming the trabecular cornua, while from the posterior part of the ethmoidal plate, forming the anterior boundary of the cranial cavity, there often grows out a prefrontal or lateral ethmoidal process.
These and other processes growing out from the trabeculæ have occasionally been regarded as rudimentary præoral branchial arches. I have already stated it as my view that the existence of branchial arches in this region is highly improbable, and I may add that the development of these structures as outgrowths of the skull is in itself to my mind a nearly conclusive argument against their being branchial arches, in that true branchial arches hardly ever or perhaps never arise in this way.
The sense capsules. The most important of these is the auditory capsule, which, as we have seen, fuses intimately withthe lateral walls of the skull. In front there is usually a cleft separating it from the alisphenoid region of the skull, through which the third division of the fifth nerve passes out. This cleft becomes narrowed to a small foramen (fig. 327,V). The sclerotic cartilage is always free, but profoundly modifies the region of the cranium near which it is placed. The nasal investment forms in Elasmobranchs (fig. 327,Na) a capsule open below, and continuous with the ethmoid region of the trabeculæ. In most types however it becomes more closely united with the ethmoid region and the accessory parts belonging to it.
Illustration: Figure 327Fig. 327. Skull of adult Dogfish, side view.(From Parker.)O.C.occipital condyle;Au.periotic capsule;Pt.O.pterotic ridge;Sp.O.sphenotic process;S.Or.supraorbital ridge;Na.nasal capsule;P.N.prenasal cartilage;II.optic foramen;V.trigeminal foramen;Pl.Pt., Qu.pterygo-quadrate arcade;M.Pt.metapterygoid ligament (including a small cartilage);Pl.Tr.ethmo-palatine or palato-trabecular ligament;Mck.lower jaw;Sp.spiracle;H.M.hyomandibular;C.Hy.ceratohyal;m.h.l.mandibulohyoid ligament;Ph.Br.pharyngobranchial;E.Br.epibranchial;C.br.ceratobranchial;H.Br.hypobranchial;B.Br.basibranchial;Ex.Br.extra-branchial;l1,2,3,4,5. labial cartilages; the dotted lines withinMck.indicate the basihyal.
Fig. 327. Skull of adult Dogfish, side view.(From Parker.)O.C.occipital condyle;Au.periotic capsule;Pt.O.pterotic ridge;Sp.O.sphenotic process;S.Or.supraorbital ridge;Na.nasal capsule;P.N.prenasal cartilage;II.optic foramen;V.trigeminal foramen;Pl.Pt., Qu.pterygo-quadrate arcade;M.Pt.metapterygoid ligament (including a small cartilage);Pl.Tr.ethmo-palatine or palato-trabecular ligament;Mck.lower jaw;Sp.spiracle;H.M.hyomandibular;C.Hy.ceratohyal;m.h.l.mandibulohyoid ligament;Ph.Br.pharyngobranchial;E.Br.epibranchial;C.br.ceratobranchial;H.Br.hypobranchial;B.Br.basibranchial;Ex.Br.extra-branchial;l1,2,3,4,5. labial cartilages; the dotted lines withinMck.indicate the basihyal.
The cartilaginous cranium, the development of which has been thus briefly traced, persists in the adult without even the addition of membrane bones in the Cyclostomata, Elasmobranchii (fig. 327) and Holocephali. In the Selachioid Ganoids it is also found in the adult, but is covered over by membrane bones. In all other types it is invariably present in the embryo, but becomes in the adult more or less replaced by osseous tissue.
Branchial skeleton.
The most primitive type of branchial skeleton in any existing form would appear to be that of the Petromyzonidæ, which is developed in a superficial subdermal tissue, and consists of a series of bars united by transverse pieces, so as to form a basketwork. It is known as an extra-branchial system, and an early stage of its development in the Lamprey is shewn infig. 47. In the higher forms this system is replaced by a series of bars, known as the branchial bars, so situated as to afford support to the successive branchial pouches. Outside these bars there may be present in some primitive forms (Elasmobranchii) cartilaginous elements, which are supposed to be remnants of the extra-branchial system (fig. 327,Ex.Br); while a series of membrane bones is also usually added to them, which will be dealt with in a separate section. The branchial bars are developed as simple cartilaginous rods in the deeper parts of the mesoblast which constitutes the primitive branchial arches.
The position of the branchial bars in relation to the somatopleure and splanchnopleure can be determined from their relation to the so-called head cavities. These cavities atrophy before the formation of the cartilaginous branchial bars, but it will be observed (fig. 328), that the artery of each arch (aa) is placed on the inner side of the head cavity (pp). The cartilaginous bar arises at a later period on the inner side of the artery, and therefore on the inner side of the section of the body cavity primitively present in the arches.
An anterior arch, known as the mandibular arch, placed in front of the hyomandibular cleft, and a second arch, known as the hyoid arch, placed in front of the hyobranchial cleft, are developed in all types. The succeeding arches are known as the true branchial arches, and are only fully developed in the Ichthyopsida.
In some Sharks (Notidani) seven branchial arches may be present (not including the hyoid and mandibular). In other Ichthyopsida five are usually present, in the embryo at any rate, while in the Amniota there are usually two or three post-hyoid membranous arches, in the interior of which a cartilaginous bar is usually formed. The general form of these bars at an earlystage of development is shewn in the dogfish (Scyllium) infig. 329.
Illustration: Figure 328Fig. 328. Horizontal section through the penultimate visceral arch of an embryo of Pristiurus.ep.epiblast;vc.pouch of hypoblast which will form the walls of a visceral cleft;pp.segment of body-cavity in visceral arch;aa.aortic arch.
Fig. 328. Horizontal section through the penultimate visceral arch of an embryo of Pristiurus.ep.epiblast;vc.pouch of hypoblast which will form the walls of a visceral cleft;pp.segment of body-cavity in visceral arch;aa.aortic arch.
The simple condition of these bars in the embryo renders it highly probable that forms existed at one time with a simple branchial skeleton of this kind: at the present day however such forms no longer exist. The first arch has in all cases changed its function and has become converted into a supporting skeleton for the mouth; the hyoid arch, though retaining in some forms its branchial function, has in most acquired additional functions and has undergone in consequence various peculiar modifications. The true branchial arches retain their branchial functions in Pisces and some Amphibia, but are secondarily modified and largely aborted in the abranchiate forms. Since the changes undergone by the true branchial bars are far less complicated than those of the hyoid and mandibular bars it will be convenient to treat of them in the first instance.
Illustration: Figure 329Fig. 329. Head of embryo Dogfish, 11 lines long.(From Parker.)Tr.trabecula;Pl.Pt.pterygo-quadrate;M.Pt.metapterygoid region;Mn.mandibular cartilage;Hy.hyoid arch;Br.1. first branchial arch;Sp.mandibulohyoid cleft;Cl1. hyobranchial cleft;Lch.groove below the eye;Na.olfactory rudiment;E.eyeball;Au.auditory mass;C1, 2, 3. cerebral vesicles;Hm.hemispheres;f.n.p.nasofrontal process.
Fig. 329. Head of embryo Dogfish, 11 lines long.(From Parker.)Tr.trabecula;Pl.Pt.pterygo-quadrate;M.Pt.metapterygoid region;Mn.mandibular cartilage;Hy.hyoid arch;Br.1. first branchial arch;Sp.mandibulohyoid cleft;Cl1. hyobranchial cleft;Lch.groove below the eye;Na.olfactory rudiment;E.eyeball;Au.auditory mass;C1, 2, 3. cerebral vesicles;Hm.hemispheres;f.n.p.nasofrontal process.
These bars are, as already mentioned, most numerous in certain very primitive forms (seven in Notidanus), while as we ascend the series there is a gradual tendency for the posterior of them to disappear. This tendency is the result of a gradual atrophy of the posterior branchial pouches, which commenced ata stage in the evolution of the Chordata long prior to the appearance of cartilaginous or osseous branchial bars, and reaches its climax in the Amniota.
In a fully developed branchial bar the primitively simple rod of cartilage becomes divided into a series of segments, usually four, articulated so as to be more or less mobile: and either remaining cartilaginous or becoming partially or wholly ossified. Each bar (fig. 327) forms a somewhat curved structure, embracing the pharynx. The dorsal and somewhat horizontally placed segment is known as the pharyngobranchial (Ph.Br), the next two as the epibranchial (E.Br) and ceratobranchial (C.Br), and the ventral segment as the hypobranchial (H.Br). There is also typically present a basal unpaired segment, uniting the bars of the two sides, known as the basibranchial (B.Br). The arches often bear cartilaginous rays which support the gill lamellæ.
In Teleostei dental plates are usually developed as an exoskeletal covering on parts of the branchial arches.
In the Amphibia four or three branchial arches are present in the embryo. These parts are more or less completely retained in the Perennibranchiata and Caducibranchiata, but in the Myctodera and Anura they become largely reduced, and entirely connected with the hyoid.
In the Anura they never reach any considerable development, and are soon reduced to a plate (fig. 330)—the coalesced basihyal and basibranchial plate—the posterior processes of which represent the remnants of the branchial arches.