The Works of Francis Balfour 3-19

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata

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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London. Modern Notes:

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Vol. III. A Treatise on Comparative Embryology 2 (1885)

CHAPTER XIX. THE SKULL

THREE distinct sets of elements may enter into the composition of the skull. These are (i) 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 described p. 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 Duges 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 vertebrae ; 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 number

THE SKULL.

565

of 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.

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 the parachordals (pa. ck}. These plates together with the notochord (nc) enclosed between them form a floor for the hind- and midbrain. The continuous plate, formed by them and the notochord, is known as the basil ar plate.

(2) A pair of bars forming the floor for the fore-brain,

iff

Cl?

pa.ch.

CIA

FIG. 323. HEAD OK EMBRYO DOGFISH, SECOND STAGE ; BASAL VIEW OF CRANIUM FROM ABOVE, THE CONTENTS HAVING BEEN REMOVED. (From

Parker.)

ol. olfactory sacs ; an. auditory capsule; nc. notochord; py. pituitary body ; pa.ch. parachordal cartilage ; tr. trabecula ; inf. infundibulum ; C.ir. cornua trabeculse ; pn. prenasal element ; sp. spiracular cleft ; br. external branchiae; Cl. 2, 4. visceral clefts.

known as the trabeculae (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 way

566

THE PARACHORDALS AND NOTOCHORD.

as 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 (ait) 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 (vide p. 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.

It will be convenient to say a few words 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 in fig. 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).

Kolliker has shewn that in the Rabbit 1 , 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.

here with reference to the nib

FIG. 324. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO.

cer. commencement of the cerebral hemisphere; pn. pineal gland ; ///.infundibulum ; //.ingrowth from mouth to form the pituitary body ; nib. mid-brain ; cb. cerebellum ; ch, notochord; al. alimentary tract; laa. artery of mandibular arch.

" Embryologische Mittheilungen." Festschrift d. Nattirfor. G^//., Halle, 1879.

THE SKULL.

567

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 Kolliker 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 (i) 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 that the 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 that it 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

The trabeculae. The trabeculae, 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 trabeculae would probably seem to indicate, as

568

THE TRABECUL^i.

pointed out by Gegenbaur, that these parts, in spite of their similarity to the parachordals, have not the same morphological significance.

C V 1

FlG. 325. VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL/E 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.

cv i. cerebral vesicle (sliced oft") ; e. eye ; nc. notochord ; iv. investing mass ; 9. foramen for the exit of the ninth nerve ; d. cochlea ; hsc. horizontal semicircular canal; q. quadrate; 5. notch for the passage of the fifth nerve; Ig. expanded anterior end of the investing mass ; pts. pituitary space ; tr. trabeculse. The reference line tr. has been accidentally made to end a little short of the cartilage.

The nature of the trabeculae 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 trabeculas must . I think be admitted, I see no reason against supposing that the trabecuke 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 trabeculse 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 pituitary

THE SKULL.

569

space is an argument, not without force, which has been appealed to in support of this view.

In the majority of the lower forms the trabeculys arise quite independently of the parachordals, though the two sets of elements soon unite ; while in Birds (fig. 325) and Mammals the parachordals and trabeculae are formed as a continuous whole. The junction between the trabeculae and parachordals becomes marked by a cartilaginous ridge known as the posterior clinoid.

The trabeculae are usually somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts (figs. 323 and 325). Into this space there

so

f

bbr

cbr

FJG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THK

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. basi-hyal ; cbr. and ebr. cerato-branchial ; bbr. basibranchial.

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 trabeculae soon unite together both in front and behind and form a complete plate underneath the fore-brain, and extending into the nasal region 1 . A special

1 In Man (Kolliker) the trabeculce form from the first a continuous plate in front of the pituitary space, and the latter very early acquires a cartilaginous floor.

5/0 THE TKAHECUL/E.

vertical 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. an alisphenoidal region (fig. 326, as) behind, growing out from what is known as the basisphenoidal region of the primitive trabeculae, and an orbitosphenoidal region in front growing out from the presphenoidal region of the trabecula,\ 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 the lateral 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 trabeculae 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 trabeculse have occasionally been regarded as rudimentary praeoral 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 with

THE SKULL.

the 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, No) a capsule open

FIG. 327. SKULL OF ADULT DOGFISH, SIDE VIEW. (From Parker.) O. C, occipital condyle ; Au. periotic capsule; Pt.O. pterotic ridge ; Sp. 0. sphenotic process ; S. Or. supraorbital ridge ; Na. nasal capsule ; P.N. prenasal cartilage; 77. optic foramen ; V. trigeminal foramen ; PL TV., 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. mandibulo-hyoid ligament; Ph.Br. pharyngobranchial ; E.Br. epibranchial ; C.br. ceratobranchial ; H.Br. hypobranchial ; B.Br. basibranchial ; Ex.Br. extrabranchial ; l\ 2 , 3 , 4 , 5 . labial cartilages ; the dotted lines within Mck. indicate the basihyal.

below, and continuous with the ethmoid region of the trabeculse. In most types however it becomes more closely united with the ethmoid region and the accessory parts belonging to it.

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.

572 THE BRANCHIAL BARS.

Branchial skeleton.

The most primitive type of branchial skeleton in any existing form would appear to be that of the Petromyzonidae, 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 in fig. 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 extrabranchial 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 (//). 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 hyo-mandibular cleft, and a second arch, known as the hyoid arch, placed in front of the hyo-branchial 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 early

THE SKULL.

573

Fir

HORIZONTAL

stage of development is shewn in the dog-fish (Scyllium) in fig. 329.

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

J SECTION THROUGH THE PEN such forms no longer exist. The first ULTIMATE VISCERAL ARCH arch has in all cases changed its F RUS AN EMHRYO < function and has become converted ^ epiblast; vc. pouch of

into a supporting skeleton for the hypoblast which will form the , , ,11 -1 1 ., i i i. walls of a visceral cleft ; pp.

mouth ; the hyoid arch, though retain- segme nt of body-cavity in vis ing in Some forms its branchial func- ceral arch ;aa. aortic arch.

tion, 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

c.a

Bnl

ffm

LrJt

Sn.f

FIG. 329. HEAD OF EMBRYO DOGFISH, n LINES LONG. (From Parker.) TV. trabecula ; Pl.Pt. pterygo-quadrate ; M.Pt. metapterygoid region; Mn. mandibular cartilage ; Hy. hyoid arch; Br. i. first branchial arch; Sp. mandilmlohyoid cleft; C/ 1 . hyo-branchial cleft; Lch. groove below the eye; Net. olfactory rudiment; E. eyeball; An. auditory mass; C i, 2, 3. cerebral vesicles; Hm. hemispheres; f.n.p. nasofrontal process.

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.

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 at

574

THE BRANCHIAL BARS.

a 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 lamellae.

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.

According to Parker the posterior process of this plate in the adult is a remnant of the fourth branchial bar ; the next one is the third branchial bar, while the anterior lamina behind the hyoid is stated by him (though this is somewhat doubtful) to be a remnant of the first two bars.

In the Amniota, the branchial arches become still more

Pmx

FIG. 330. YOUNG FROG, WITH TAIL JUST ABSORBED ; SIDE VIEW OF SKULL. (From Parker.)

An. auditory capsule; in front of it is the cranial side wall ; A.N. external nostril ; St. stapes; Mck. Meckelian cartilage; B.Hy. basihyobranchial plate; St.Hy. stylohyal or ceratohyal; Br.i. first branchial arch.

Bones: E-0. exoccipital; Pr.O. prootic ; Pa. parietal ; Fr. frontal ; Na. nasal ; Pmx. premaxillary ; MX. maxillary; Pt. pterygoid; Sq. squamosal; Qn-J't. quadra tojugal; Art. articular; D. dentary.

THE SKULL. 575

degenerated, in correlation with the total disappearance of a branchial respiration at all periods of life. Their remnants become more or less important parts of the hyoid bone, and are solely employed in support of the tongue. Their basal portions are best preserved, forming parts of the body of the hyoid. The posterior (thyroid) cornua of the hyoid are remnants of the true arches. Of these there are two in the Chelonia and Lacertilia, and one in the Aves and Mammalia. In Aves the cornu formed from the first branchial arch (fig. 331, cbr) is always larger than that of the true hyoid arch (cJi).

Mandibular and Hyoid arches. The adaptations of both the mandibular and hyoid bars, to functions entirely distinct from

FlG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL

OF A FOWL ON THE FOURTH DAY OF INCUBATION. (After Parker.) cv i. cerebral vesicles ; e. eye ; fn. frontonasal process; n. nasal pit; tr. trabeculre ; pts. pituitary space ; mr. superior maxillary process ; pg. pterygoid ; pa. palatine ; q. quadrate; mk. Meckel's cartilage; ch. cerato-hyal ; bh. basihyal ; cbr. ceratobranchial ; ebr. proximal portion of the cartilage in the third visceral (first branchial) arch; bbr. basibranchial ; i. first visceral cleft; 2. second visceral cleft; 3. third visceral arch.

those which they primitively served, are most remarkable ; and the adaptations of the two bars are in many cases so intimately bound together, that it is not possible to treat them separately.

The most important change of function is undoubtedly that of the mandibular arch, which becomes entirely converted into a skeleton for the jaws. It may be noted as a peculiarity of the

576 MAND1BULAR AND HYOID BARS.

mandibular arch that it is never provided with an unpaired basal element.

The simplest forms of metamorphosis are those undergone by Elasmobranchii, of which the Dog-fish (Scyllium) and Skate (Raja) have been studied (Parker, No. 456). In some of these forms, e.g. the Skate, part of the mandibular bar is still related to the hyo-mandibular cleft (the spiracle).

Elasmobranchii. In Scyllium the hyoid and mandibular arches are at first very similar to those which follow. Soon however each of them sends an anteriorly directed dorsal process (fig. 329). The regions which may be distinguished owing to the growth of these processes have received names from ossifications in them which are found in other types. The anterior process of the mandibular arch is known as the pterygo-quadrate bar (Pl.Pt) ; the dorsal end of the primitive bar from which it starts (M.Pt] is known as the metapterygoid process; while the ventral end of the bar forms the Meckelian cartilage. The upper end of the hyoid arch is known as the hyomandibular.

In a somewhat later stage changes take place which cause these parts practically to assume the adult form (fig. 327). The mandibular arch becomes segmented at its bend into (i) a pterygo-quadrate bar (Pl.Pf) which grows forwards in front of the mouth, and forms an upper jaw, and (2) a Meckelian cartilage (Mck} which is placed behind the mouth, and forms a lower jaw. The two jaws are articulated together, and the cartilages of the two sides composing them meet each other distally.

At the articulation of the Meckelian cartilage with the quadrate part of the pterygo-quadrate is situated a ligament (M.Pf), which takes the place of the metapterygoid process of the previous stage, and passes up on the anterior side of the spiracle, to be attached to the cranium in the front part of the auditory region. This ligament, which is supplemented by a second ligament, the ethmopalatine ligament, passing from the pterygo-quadrate bar to the antorbital region of the skull, is not the most important support of the jaw. The main support is, on the contrary, given by the hyoid arch ; the hyomandibular segment of which (H.M) as well as the adjoining segment (ceratohyoid C.Hy) are firmly attached by ligament to the mandibular

THE SKULL.

577

arch. The hyomandibular is articulated with the cranium beneath the pterotic ridge (Pt.O),

In the type just described, the hyoid and mandibular arches undergo less modification than in almost any other case. The hyoid arch has altered its form, but retains its respiratory function. It has however acquired the secondary function of supporting the mandibular arch. The mandibular arch is divided into two elements, which form respectively the upper and lower jaws. It is not directly articulated with the skull, and its mode of support by the hyoid arch has been called by Huxley (No. 445) hyostylic.

The development of the hyoid and mandibular arches in the Skate is characterised by a few important features (fig. 333). The anterior element of the hyoid

arch, which forms the hyo- \ ^ Sp

mandibular (H.M], becomes entirely separate from the posterior part of the arch, and only serves to support the jaws. The posterior part of the arch (Hy} carries on the respiratory functions of the hyoid, and is closely connected with the first branchial arch. The upper or metapterygoidelementof the mandibular arch (M.Pt} has a considerable development,

FIG. 333. HEAD OF EMBRYO SKATE, i\ IN. LONG. (From Parker.)

Tr. trabecula ; Pl.Pt. pterygo- quadrate bar ; Mn. mandibular bar ; M.Pt. metapterygoid cartilage ; H.M. hyomandibular ; Hy. remainder of hyoid arch ; Br. \. first branchial arch ; Sp. mandibulo-hyoid cleft or spiracle ; Pn. pineal gland ; Au. au ditory vesicle ; C. i, C. 2, and C. 3. vesicles of the brain.

and, becoming separated from the remainder of the arch, forms a mass of cartilage with one or two branchial rays, in the front wall of the spiracle, and constitutes a section of the mandibular arch still retaining traces of its primitive function in supporting the wall of a branchial pouch.

Although the development of other Elasmobranch types is not known, it is necessary to call attention to the mode of support of the mandibular arch in certain forms, notably Notidanus, Hexanchus and Cestracion, where the pterygo-quadrate region of the mandibular arch is directly articulated to the B. in. 37

578 MANDIKULAR AND HYOID BARS.

cranium between the optic and trigeminal foramina. In the two former genera the metapterygoid region of the arch is moreover continuous with the pterygo-quadrate, and articulates with the post-orbital process of the auditory region of the skull. In spite of these attachments the mandibular arch continues to be partially supported by the hyomandibular. The skulls in which the mandibular arch has this double form of support have been called by Huxley amphistylic.

Considering the in many respects primitive characters of the forms with amphistylic skulls it seems not improbable that they

SOr

l } a.ch.

Gtly

FIG. 334. CRANIAL SKELETON OF A SALMON FRY, SECOND WEEK AFTER HATCHING; MEMBRANE BONES, EYEBALLS, AND NASAL SACS REMOVED. (From Parker.)

T.Cr. tegrnen cranii; 'S. Or. supraorbital band; Fo. superior fontanelle; Au. auditory capsule ; Pa.ch. parachordal cartilage; Ch. notochord; 7>. trabecula; above the trabecula, the interorbital septum is seen, passing into the cranial wall above and reaching the supraorbital band; //. optic foramen; V. trigeminal foramen; /', I". labial cartilages ; PI. Ft. palatopterygoid bar ; M. Pt. metapterygoid tract ; Qu. quadrate region; Mck. Meckelian cartilage; H.M. hyomandibular cartilage; Sy. symplectic tract; I.Hy. interhyal; C.Hy. ceratohyal; II. fly. hypohyal; G.ffy. glossohyal; Br.\. first branchial arch.

preserve the original mode of support of the mandibular arch ; from which differentiations in two directions have taken place, viz. differentiations in the direction of a complete support of the mandibular arch by the hyoid, which is characteristic of most Elasmobranchii and, as will be shewn below, of Ganoidei and Tclcostei ; and differentiations towards a direct articulation or attachment of the mandibular arch to the cranium, without the

THE SKULL. 579

intervention of the hyoid. The latter mode of attachment is called by Huxley autostylic. It is found in Holocephala, Dipnoi, Amphibia and the Amniota.

Teleostei. In addition to that of Elasmobranchii, the skull of the Salmon is the only hyostylic skull in which, by the admirable investigation of Parker (No. 451), the ontogeny of the hyoid and mandibular bars has been satisfactorily worked out. Apart from the presence of a series of membrane bones, the development of these bars agrees on the whole with the types already described.

The hyoid arch, though largely ossified, undergoes a process of development very similar to that in Raja. It is formed as a simple cartilaginous bar, which soon becomes segmented longi ft 1.3 Sp.

FIG. 335. YOUNG SALMON OF THE FIRST SUMMER, AKOUT 2 INCHES LONG;

SIDE VIEW OF SKULL, EXCLUDING BRANCHIAL ARCHES. (From Parker.)

The palato-mandibular and hyoid tracts are detached from their proper situations,

a line indicating the position where the hyomandibular is articulated beneath the

pterotic ridge.

oL olfactory fossa; c.tr. trabecular cornu; /*. /. upper labial cartilages ; p.s.

presphenoid tract ; t.cr. tegmen cranii ; s.o.b. supraorbital band; fo. superior fonta nelle; n.c. notochord; b.o. basilar cartilage; //'. trabecula; p.c. condyle for palatine

cartilage; 5. trigeminal foramen ; fa. facial foramen; 8. foramen for glossopharyngeal

and vagus nerves; mk. Meckelian cartilage; op.c. opercular condyle.

Bones: e.o. exoccipital; s.o. supraoccipital; e.p. epiotic; pt.o. pterotic; sp.o.

sphenotic ; op. opisthotic; pro. prootic; I'.s. basisphenoid ; al.s. alisphenoid; o.s.

orbitosphenoid ; I.e. ectethmoid or lateral ethmoid ; pa. palatine ; pg. pterygoid ;

m.pg. mesopterygoid ; mt.pg. metapterygoid ; qu. quadrate; ar. articular; h.m.

hyomandibular; sy. symplectic ; i.h. interhyal ; ep.h. epiceratohyal ; c.h. ceratohyal ;

h.h. hypohyal; g.h. glosso- or basihyal.

372

580 MANDIBULAR AND HYOID BARS.

tudinally into an anterior and a posterior part (fig. 334). The former constitutes the hyomandibular (H.M], while the latter, becoming more and more separated from the hyomandibular, constitutes the hyoid arch proper ; owing to the disappearance of the hyobranchial cleft, it loses its primitive function, and serves on the one hand to support the operculum covering the gills, and on the other to support the tongue. It becomes segmented into a series of parts which are ossified (fig. 335) as the epiceratohyal (ep./t) above, then a large ceratohyal (c/t), followed by a hypohyal (JiJi), while the median ventral element forms the basi- or glossohyal (gJi).

The hyomandibular itself is articulated with the skull below the pterotic process (fig. 334, H.M}. Its upper element ossifies as the hyomandibular (fig. 335, fun.}, while its lower part (fig. 334, Sy), which is firmly connected with the mandibular arch, ossifies as the symplectic (fig. 335, sy). A connecting element between the two parts of the hyoid bar forms an interhyal (i/i).

There are more important differences in the development of the mandibular arch in Elasmobranchii and the Salmon than in that of the hyoid arch, in that, instead of the whole arcade of the upper jaw being formed from the mandibular arch, a fresh element, in the form of an independently developed bar of cartilage, completes the upper arcade in front ; but even with this bar the two halves of the upper branch of the arch do not meet anteriorly, but are separated by the ends of the trabeculae.

The anterior bar of the upper arcade is known as the palatine ; but it appears to me as yet uncertain how far it is to be regarded as an element, primitively belonging to the upper arcade of the mandibular arch, which has become secondarily independent in its development ; or as an entirely distinct structure which has no counterpart in the Elasmobranch upper jaw. The latter view is adopted by Parker and Bridge, and a cartilage attached to the hinder wall of the nasal capsule of many Elasmobranchii is identified by them with the palatine rod of the Teleostei.

The arch itself is at first very similar to the succeeding arches ; its dorsal extremity soon however becomes broadened, and provided with an anteriorly directed process. This part (fig. 334, M.Pt and Qii] is then segmented from the lower region,

THE SKULL. 581

and forms what may be called the pterygo-quadrate cartilage, though not completely homologous with the similarly named cartilage in Elasmobranchs ; while the lower region forms the Meckelian cartilage (Mck], which has already grown inwards, so as to meet its fellow ventrally below the mouth. The whole arch becomes at the same time widely separated from the axial parts of the skull.

Nearly simultaneously with the first differentiation of the mandibular arch, a bar of cartilage the palatine bar already spoken of is formed on each side, below the eye, in front of the mouth. The dilated anterior extremity of this bar soon comes in contact with an anterior process of the trabeculse, known as the ethmopalatine process.

In a later stage the pterygoid end of the pterygo-quadrate cartilage unites with the distal end of the palatine bar (fig. 334, Pl.Pt], and there is then formed a continuous cartilaginous arcade for the upper jaw, which is strikingly similar to the cartilaginous upper jaw of Elasmobranchii.

A large dorsal process of the primitive pterygo-quadrate now forms a large metapterygoid tract (M.Pt] ; while the whole arch becomes firmly bound to the hyomandibular (H.M}.

In the later stages the parts formed in cartilage become ossified (fig. 335). The palatine is first ossified, the pterygoid region of the pterygo-quadrate is next ossified as a dorsal mesopterygoid (m.pg] and a ventral pterygoid proper (pg). The quadrate region, articulating with the Meckelian cartilage, becomes ossified as a distinct quadrate (qu\ while the dorsal region becomes also ossified as a metapterygoid (int.pg).

In the Meckelian cartilage a superficial ossification of the ventral edge and inner surface forms an articulare (ar) ; but the greater part of the cartilage persists through life.

Some of the above ossifications, at any rate those of the palatine and pterygoid, seem to be started by dental osseous plates adjoining the cartilage. They will be spoken of further in the section dealing with the membrane bones.

Amphibia. The development of the autostylic piscine skulls has unfortunately not yet been studied ; and the most primitive autostylic types whose development we are acquainted with are

582 MANDIBULAR AND HYOID BARS.

those of the Amphibia ; on which a large amount of light has been shed by the researches of Huxley and Parker.

The modifications of the hyoid arch are comparatively simple and uniform. It forms a rod of cartilage, which soon articulates in front with the quadrate element of the mandibular arch, and is subsequently attached by ligaments both to the quadrate and to the cranium. In those Amphibia in which external gills and gill clefts are lost, it fuses with the basal element of the hyoid (fig. 330), which, together with the basal portions of the following arches, forms a continuous cartilaginous plate. On the completion of these changes the paired parts of the hyoid arch have the form of two elongated rods, known as the anterior cornua of the hyoid, which attach the basihyal plate to the cranium behind the auditory capsule.

It is still uncertain whether there is any distinct element corresponding to the hyomandibular of fishes.

Parker holds that the columella auris of the Anura is the homologue of the hyomandibular. The columella develops comparatively late and independently of the remainder of the hyoid arch, but the similarity between its relations to the nerves and those of the hyomandibular is put forward by Parker as an argument in favour of his view. The early ligamentous connection between the quadrate and the upper end of the primitive hyoid is however an argument in favour of regarding the upper end of the primitive hyoid as the hyomandibular element, not separated from the remainder of the arch.

The history of the mandibular arch is more complicated than that of the hyoid. The part of it which corresponds with the upper jaw of Elasmobranchii exhibits most striking variations in development ; so striking indeed as to suggest that the secondary modifications it has undergone are sufficiently considerable to render great caution necessary in drawing morphological conclusions from the processes which are in some instances observable. A more satisfactory judgment on this point will be . possible after the publication of a memoir with which Parker is now engaged on the skulls of the different Anura.

The membrane bones applying themselves to the sides of the mandibular arch are relatively far more important than in the lower types. This is especially the case with the upper jaw where the maxillary and premaxillary bones functionally replace the primitive cartilaginous jaw ; while membranous pterygoids

THE SKULL.

583

and palatines apply themselves to, and largely take the place of, the cartilaginous palatine and pterygoid bars.

Two types worked out by Parker, viz. the Axolotl and the common Frog, may be selected to illustrate the development of the mandibular arch.

In the Axolotl, which may be taken as the type for the Urodela, the mandibular arch is constituted at a very early stage of (i) an enlarged dorsal element, corresponding with the pterygo-quadrate of the lower types, but usually known as the quadrate ; and (2) a ventral or Meckelian element. The Meckelian bar very early acquires its investing bones, while the dorsal part of the quadrate becomes divided into two characteristic

FIG. 336. YOUNG AXOLOTL, i\ INCHES LONG ; UNDER VIEW OF SKULL,

DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED.

(From Parker.)

nc. notochord ; oc.c. occipital condyle; f.o. fenestra ovalis; si. stapes; tr. trabecular cartilage; i.n. internal nares; c.tr. cornu trabeculse; pd. pedicle of quadrate; (/. quadrate; pg. outline of pterygoid cartilage; 5'. orbito-nasal nerve; 7. facial nerve.

BonCS I pa.s. parasphenoid ; e.o. exoccipital ; v. vomer; px. premaxillary ; mx. maxillary; pa. palatine; pg. pterygoid.

processes, viz. an anterior dorsal process which grows towards and soon permanently fuses with the trabecular crest, and a posterior process known as the otic process, which applies itself to the outer side of the auditory region. The anterior of these processes, as pointed out by Huxley, is probably homologous with the anterior process of the pterygo-quadrate bar in Notidanus, which articulates with the trabecular region of the cranium, while the otic process is homologous with the meta

584 MANDIBULAR AND HYOID BARS.

pterygoid process. Hardly any trace is present of an anterior process to form a pterygoid bar, but dentigerous plates forming a dermal palato-pterygoid bar have already appeared.

At a somewhat later stage a fresh process, called by Huxley the pedicle, grows out from the quadrate, and articulates with the ventral side of the auditory region (fig. 336, pd). Shortly afterwards a rod of cartilage grows forward from the quadrate under the membranous pterygoid (pg), which corresponds with the cartilaginous pterygoid bar of other types (fig. 336), and an independent palatine bar, arising even before the pterygoid process, is formed immediately dorsal to the dentigerous palatine plate (pa\ and is attached to the trabecula. These two bars eventually meet, but never become firmly united to the more important membrane bones placed superficially to them.

The mandibular arch in the Frog stands, so far as development is concerned, in striking contrast to the mandibular arch of the Axolotl, in spite of the obvious similarity in the arrangement of the adult parts in the two types. FlG . 33? . EMBRYO FROG, JUST BE In the earliest stage it FORE HATCHING ; SIDE VIEW OF HEAD,

WITH SKIN REMOVED. (From Parker.) forms a simple bar in the ,, lf , , - . , .. ,

Na. olfactory sack; E. involution for

membranous mandibular arch, eyeball; Ati. auditory sack; 7>. trabe11 i , .1 cula; Mn. mandibular : Hy. hyoid ; Br.I.

parallel to and very similar to first branchial arch . ' th / gili.buds are

are

the hyoid bar behind (figf 337, seen on the first two branchial arches; /. M \ T u, * u labial cartilages.

Mn). In the next stage ob served, that is to say in Tadpoles of four, five, to six lines long, an astonishing transformation has taken place. The mandibular arch (fig. 338) is turned directly forwards parallel to the trabecula, to which it is attached in front (p.pg) and behind (pd}. The proximal part of the arch thus forms a subocular bar, and the space between it and the trabecula a subocular fenestra. In front of the anterior attachment it is continued forwards for a short distance, and to the free end of this projecting part is articulated a small Meckelian cartilage directed upwards (mk}. The Meckelian cartilage is at this stage placed in front of the nasal sacks, in the lower lip of the suctorial

THE SKULL.

585

mouth. The greater part of the arch, parallel with the trabeculae, is equivalent to what has been called in the Axolotl the

mJr

FIG. 338. TADPOLE OF COMMON TOAD, ONE-THIRD OF AN INCH LONG ; CRANIAL AND MANDIBULAR CARTILAGES SEEN FROM ABOVE ; THE PARACHORDAL

CARTILAGES ARE NOT YET DEFINITE. (From Parker.)

nc. notochord; ms. muscular segments; au. auditory capsule; py. region of pituitary body; tr. trabecula; c.tr, cornu trabeculae ; p-pg. palatopterygoid bar ; pd. pedicle; q. quadrate condyle; mk. Meckelian piece of mandibular arch; s.o.f. subocular fenestra ; u.l. upper labial cartilage. The dotted circle within the quadrate region indicates the position of the internal nostril.

quadrate, while its anterior attachment to the trabeculae is the rudiment of the palato-pterygoid cartilage. The posterior attachment is known as the pedicle.

The condition of the mandibular arch during this and the next stage (fig. 339) is very perplexing. Its structure appears adapted in some way to support the suctorial mouth of the Tadpole.

Reasons have been offered in a previous part of this volume for supposing that the suctorial mouth of the Tadpole is probably not simply a structure secondarily acquired by this larva, but is an organ inherited from an ancestor provided through life with a suctorial mouth.

The question thus arises, is the peculiar modification of the mandibular arch of the Tadpole an inherited or an acquired feature ?

If the first alternative is accepted we should have to admit that the mandibular arch became first of all modified in connection with the suctorial mouth, before it was converted into the jaws of the Gnathostomata ; and that the peculiar history of this arch in the Tadpole is a more or less true record of its phylogenetic development. In favour of this

586

MANDIBULAR AND HYOID BARS.

view is the striking similarity which Huxley has pointed out between the oral skeleton of the Lamprey and that of the Tadpole ; and certain peculiarities of the mandibular arch of Chimaera and the Dipnoi can perhaps best be explained on the supposition that the oral skeleton of these forms has arisen in a manner somewhat similar to that in the Frog ; though with reference to this point further developmental data are much required.

On the other hand the above suppositions would necessitate our admitting that a great abbreviation has occurred in the development of the mandibular arch of the otherwise more primitive Urodela ; and that the simple mode of growth of the jaws in Elasmobranchii, from the primitive mandibular arch, is phylogenetically a much abbreviated and modified process, instead of being, as usually supposed, a true record of ancestral history.

If the view is accepted that the characters of the mandibular arch of the Tadpole are secondary, it will be necessary to admit that the adaptation of the mandibular arch to the suctorial mouth took place after the suctorial mouth had come to be merely a larval organ.

In view of our imperfect knowledge of the development of most Piscine skulls I would refrain from expressing a decided opinion in favour of either of these alternatives.

or.p

eth

FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OF SKULL

WITHOUT THE BRANCHIAL ARCHES. (From Parker.)

n.c. notochord; au. auditory capsule; between it and eth. the low cranial side wall is seen; eth. ethmoidal region; st. stapes; 5. trigeminal foramen; 2. optic foramen; ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu trabeculae; ./. upper labial, in outline; su. suspensorium (quadrate); pd. its pedicle; ot.fr. its otic process; or.p. its orbitar process; t.m. temporal muscle, indicated by dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; ;;//. Meckelian cartilage; /./. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The upper outline of the head is shewn by dotted lines.

As the tail of the Tadpole gradually disappears, and the metamorphosis into the Frog becomes accomplished, the mandibular arch undergoes important changes (fig. 339): the

THE SKULL.

palato-pterygoid attachment (pa.pg) of the quadrate subocular bar becomes gradually elongated ; and, as it is so, the front end of the subocular bar (su) rotates outwards and backwards, and soon forms a very considerable angle with the trabeculae. The Meckelian cartilage (ink) at its free end becomes at the same time considerably elongated. These processes of growth continue till (fig. 330) the palato-pterygoid bar (Pf) forms a subocular bar, and is considerably longer than the original subocular region of the quadrate ; while the Meckelian cartilage (Mck] has assumed its permanent position on the hinder border of the no longer suctorial mouth, and has grown forwards so as nearly to meet its fellow in the median line.

The metapterygoid region of the quadrate gives rise to a posterior and dorsal process (fig. 339, ot.pr), the end of which is constricted off as the tympanic annulus (fig. 340, a.f) ; while

pmx

FIG. 340. YOUNG FROG, NEAR END OF FIRST SUMMER ; UPPER VIEW OF SKULL, WITH LEFT MANDIBLE REMOVED, AND THE RIGHT EXTENDED OUTWARDS. (From Parker.)

b.o. basioccipital tract; s.o. supraoccipital tract; fo. frontal fontanelle; e.n, external nostril; internal to it, internasal plate; a.t. tympanic annulus.

Bones : e.o. exoccipital; pr.o. prootic, partly overlapped by/, parietal; f. frontal ; eth. rudiment of sphenethmoid ; na. nasal ; pmx. premaxillary ; mx. maxillary; /-. pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal ; s<j. squamosal; ar. articular; d. dentary; m.mk. mento-Meckelian.

the proximal part of the process remains as the otic (metapterygoid) process, articulating with the auditory cartilage.

The pedicle (pd} retains its original attachment to the skull.

588 MAND1BULAR AND IIYOID BARS.

The palato-pterygoid soon becomes segmented into a transversely placed palatine, and a longitudinally placed pterygoid (fig. 340). With the exception of a few ossifications, which present no features of special interest, the parts of the mandibular arch have now reached their final condition, which is not very different from that in the Axolotl.

Sauropsida. In the Sauropsida the modifications of the hyoid and mandibular arches are fairly uniform.

The lower part of the hyoid arch, including the basihyoid, unites with the remnants of the arches behind to form the hyoid bone, to which it contributes the anterior cornu and anterior part of the body.

The columella is believed by Huxley and Parker to represent, as in the Anura, the independently developed dorsal (hyomandibular) element of the hyoid, together with the stapes with which it has become united 1 .

The membranous mandibular arch gives off in the embryos of all the Sauropsida an obvious bud to form the superior maxillary process, and the formation of this bud appears to represent the growth forwards of the pterygoid process in Elasmobranchii, which is indeed accompanied by the formation of a similar bud ; but the skeletal rod, which appears in the axis of this bud, is as a rule independent of that in the true arch (fig- SS 1 ./^. PS}- The former is the pterygo-palatine bar; the latter the Meckelian and quadrate cartilages.

The pterygo-palatine bar is usually if not always ossified directly, without the intervention of cartilage.

Born has recently shewn that Parker was mistaken in supposing that the palato-pterygoid bone is cartilaginous in Birds. In the Turtle a short cartilaginous pterygoid process of the quadrate would seem to be present (Parker, No. 458).

The quadrate and Meckelian cartilages are either from the first separate, or very early become so.

1 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 (vide Huxley, 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 Huxley vide Peters, "Ueb. d. Gehorknochelchen u. ihr Verhaltniss zu. Zungenbeinbogen b. Sphenodon." Berlin MoHOtsbtnekU, 1874.

THE SKULL.

589

The quadrate cartilage ossifies as the quadrate bone, and supplies the permanent articulation for the lower jaw. Its upper end exhibits a tendency to divide into two processes, corresponding with the pedicle and otic processes of the Amphibia. The Meckelian cartilage becomes soon covered by investing bones, and its proximal end ossifies as the articulare. The remainder of the cartilage usually disappears.

Mammalia. The most extraordinary metamorphosis of the hyoid and mandibular arches occurs in the Mammalia, and has been in part known since the publication of the memoir of Reichert (No. 461).

Both the hyoid and mandibular arches develop at first more completely than in any of the other types above Fishes; and are

pn.ch nc

FIG. 341. EMBRYO PIG, TWO-THIRDS OF AN INCH LONG ; ELEMENTS OF THE

SKULL SEEN SOMEWHAT DIAGRAMMATICALLY FROM BELOW. (From Parker.) pa.ch. parachordal cartilage; nc. notochorcl; au. auditory capsule; py. pituitary body; tr. trabeculse; c.lr. trabecular cornu; pn. prenasal cartilage; e.n. external nasal opening; ol. nasal capsule; p-pg- palatopterygoid tract enclosed in the maxillopalatine process; mn. mandibular arch ; hy. hyoid arch; th.h. first branchial arch; ja. facial nerve; 8a. glossopharyngeal ; 86. vagus; 9. hypoglossal.

articulated to each other above, while the pterygo-palatine bar is quite distinct. The main features of the subsequent development are undisputed, with the exception of that of the upper end of the hyoid, which is still controverted. The following is Parker's (No. 452) account for the Pig, which confirms in the main the view originally put forward by Huxley (No. 445).

The mandibular and hyoid arches are at first very similar

5QO MANDIBULAR AND HYOID BARS.

(fig. 341 mn and hy), their dorsal ends being somewhat incurved, and articulating together.

In a somewhat later stage (fig. 342) the upper end of the mandibular bar (mb\ without becoming segmented from the ventral part, becomes distinctly swollen, and clearly corresponds to the quadrate region of other types. The ventral part of the bar constitutes the Meckelian cartilage (mk).

The hyoid arch has in the meantime become segmented into two parts, an upper part (z), which eventually becomes one of

FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS DISPLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From Parker.)

tg. tongue; ink. Meckelian cartilage; ml. body of malleus; mb. manubrium or handle of the malleus; t.ty. tegmen tympani; i. incus; st. stapes; i.hy, interhyal ligament; st.h. stylohyal cartilage; h.h. hypohyal ; ^.//.basibranchial; th.h. rudiment of first branchial arch; -ja. facial nerve.

the small bones of the ear the incus and a lower part which remains permanently as the anterior cornu of the hyoid (st./i). The two parts continue to be connected by a ligament.

The incus is articulated with the quadrate end of the mandibular arch, and its rounded head comes in contact with the stapes (fig. 342, st) which is segmented from the fenestra ovalis. The main arch of the hyoid becomes divided into a hypohyal (h.h) below and a stylohyal (st. h] above, and also becomes articulated with the basal element of the arch behind (b/i).

In the course of further development the Meckelian part of the mandibular arch becomes enveloped in a superficial ossification forming the dentary. Its upper end, adjoining the quadrate region, becomes calcified and then absorbed, and its lower, with the exception of the extreme point, is ossified and subsequently incorporated in the dentary.

The quadrate region remains relatively stationary in growth

TIIK SKULL. 591

as compared with the adjacent parts of the skull, and finally ossifies to form the malleus bone of the ear. The processus gracilis of the malleus is the primitive continuation into Meckel's cartilage.

The malleus and incus are at first embedded in the connective tissue adjoining the tympanic cavity (hyomandibular cleft, vide p. 528) ; and externally to them a bone known as the tympanic bone becomes developed so that they become placed between the tympanic bone and the periotic capsule. In late fcetal life they become transported completely within the tympanic cavity, though covered by a reflection of the tympanic mucous membrane.

The dorsal end of the part of the hyoid separated from the incus becomes ossified as the tympano-hyal, and is anchylosed with the adjacent parts of the periotic capsule. The middle part of the bar just outside the skull forms the stylo-hyal (styloid process in Man) which is attached by ligament to the anterior cornu of the hyoid (cerato-hyal).

While the account of the formation of the malleus, incus, and stapes just given is that usually accepted in this country, a somewhat different view of the development of these parts has as a rule been adopted in Germany. Reichert (No. 461) held that both the malleus and the incus were derived from the mandibular bar ; and this view has been confirmed by Giinther, Kolliker and other observers, and has recently been adopted by Salensky (No. 462) after a careful research especially directed towards this point. Reichert also held that the stapes was derived from the hyoid bar ; but, though his observations on this point have been very widely accepted, they have not met with such universal recognition as his views on the origin of the malleus and incus. Salensky has recently arrived at a view, which is in accord with that of Parker, in so far as the independence of the stapes of both the hyoid and mandibular arches is concerned. Salensky however holds that it is formed from a mass of mesoblast surrounding the artery of the mandibular arch, and that the form of the stapes is due to its perforation by the mandibular artery. A product of this artery permanently perforates the stapes in a few Mammalia, though in the majority it atrophies.

In view of the different accounts of the origin of the incus the exact nature of this bone must still be considered as an open question, but should Reichert's view be confirmed the identification of the incus with the columella of the Amphibia and Sauropsida must be abandoned.

592 MEMBRANE BONES.

Membrane bones and ossifications of the cranium.

The membrane bones of the skull may be divided into two classes, viz. (i) those derived from dermal osseous plates, which as explained above (p. 542) are primitively formed by the coalescence of the osseous plates of scales ; and (2) those formed by the coalescence of the osseous plates of teeth lining the oral cavity. Some of the bones sheathing the edge of the mouth have been formed partly by the one process and partly by the other.

In the Fishes there are found all grades of transition between simple dermal scutes, and true subdermal osseous plates forming an integral part of the internal skeleton. Dermal scutes are best represented in Acipenser and some Siluroid Fishes.

Where the membrane bones still retain the character of dermal plates, those on the dorsal surface of the cranium are usually arranged in a series of longitudinal rows, continuing in the region of the head the rows of dermal scutes of the trunk ; while the remaining cranial scutes are connected with the visceral arches. The dermal bones on the dorsal surface of the head are very different in number, size, and arrangement in different types of Fishes ; but owing to their linear disposition it is usually possible to find a certain number both of the paired and unpaired bones which have a similar situation in the different forms. These usually receive the same names, but both from general considerations as to their origin, as well as from a comparison of different species, it appears to me probable that there is no real homology between these bones in different species, but only a kind of general correspondence 1 .

It is not in fact till we get to the types above the Fishes that we can find a series of homologous dorsal membrane bones covering the roof of the skull. In these types three paired sets of such bones are usually present, viz, from behind forwards the parietals, frontals and nasals, the latter bounding the posterior surface of the external nasal opening. Even in the higher

1 For some interesting remarks on the arrangement of these bones in Fishes, vide Bridge, "On the Osteology of Polyodon folium." Phil. Trans., 1878.

THE SKULL. 593

types these bones are liable to vary very greatly from the usual arrangement.

Besides these bones there is usually present in the higher forms a lacrymal bone on the anterior margin of the orbit derived from one of a series of periorbital membrane bones frequently found in Fishes. Various supraorbital and postorbital bones, etc. are also frequently found in Lacertilia, etc. which are not impossibly phylogenetically independent of the membrane bones inherited from Fishes; and may have been evolved as bony scutes in the subdermal tissue of the papillae of the sauropsidan scales.

The visceral arches of Fishes, especially of the Teleostei, are usually provided with a series of membrane bones. In the true branchial arches these take the form of dentigerous plates ; but no such plates are found in the Amphibia or Amniota.

The opercular flap attached to the hyoid arch is usually supported by a series of membrane bones, which attain their highest development in the Teleostei. One of these bones, the praeopercular, is very constant and is primitively attached along the outer edge of the hyomandibular. It seems to be retained in Amphibia as a membrane bone, overlapping the attachment of the quadrate and known as the squamosal ; though it is not impossible that this bone may be derived from a superficial membrane bone, widely distributed in Teleostei and Ganoids, which is known as the supra-temporal. In Dipnoi the bone which appears to be clearly homologous with the squamosal would seem from its position to belong to the series of dorsal plates, and therefore to be the supra-temporal ; but it is regarded by Huxley (No. 446) as the praeopercular 1 .

In the Amniota the squamosal forms an integral part, of the osseous roof of the skull ; but in the Sauropsida it continues, as in Amphibia, to be closely related to the quadrate.

A larger series of persistent membrane bones are related to the mandibular, and its palato-quadrate process.

Overlying the palato-quadrate process are two rows of bones,

1 It is not impossible that the solution of the difficulty about the praeopercular is to be found by supposing that the praeopercular 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.

B. III. 3 8

594 MEMBRANE BONES.

one row lying at the edge of the mouth, on the outer side of the pterygo-palatine process, and the other set on the roof of the mouth superficial to the pterygo-palatine process.

The outer row is formed of the praemaxilla, maxilla, jugal, and very often quadrato-jugal. Of these bones the maxilla and prsemaxilla, as is more especially demonstrated by their ontogeny in the Urodela, are partly derived from dentigerous plates and partly from membrane plates outside the mouth; while the jugal, and quadrato-jugal when present, are entirely extra-oral. In the Amphibia and Amniota the praemaxillae and maxillae are the most important bones in the facial region, and are quite independent of any cartilaginous substratum.

The second row of bones is clearly constituted in the Dipnoi and Amphibia by the vomer in front, then the palatine, and finally the pterygoid behind. Of these bones the vomer is never related to a cartilaginous tract below, while the palatines and pterygoids usually are so. The position and growth of the three bones in many Urodela (Axolotl) are especially striking (Hertwig. No. 442). In the Axolotl they form a continuous series, the vomer and palatine being covered by teeth, but the pterygoid being without teeth. The vomer and palatine originate from the united osseous plates of the bases of the teeth, while the pterygoid is in the first instance continuous with the palatine.

In Teleostei, Amia, etc., there are dentigerous plates forming a palatine and pterygoid, which in position, at any rate, closely correspond with the similarly named bones in Amphibia ; and there is also a dentigerous vomer which may fairly be considered as equivalent to that in Amphibia.

In the Amniota the three bones found in Amphibia are always present, but with a few exceptions amongst the Lacertilia and Ophidia, are no longer dentigerous. The cartilaginous bars, which in the lower types are placed below the palatine and pterygoid membrane bones, are usually imperfectly or not at all developed.

On Meckel's cartilage important membrane bones are almost always grafted. On the outside and distal part of the cartilage a dentary is usually developed, which may envelope and replace the cartilage to a larger or smaller extent. Its oral edge

THE SKULL. 595

is usually dentigerous. The splenial membrane bone is the most important bone on the inner side of Meckel's cartilage, but other elements known as the coronoid and angular may also be added. In Mammalia the dentary is the only element present (vide p. 590).

On the roof of the mouth a median bone, the parasphenoid, is very widely present in the Amphibia and Fishes, except the Elasmobranchii and Cyclostomata, and has no doubt the same phylogenetic origin as the vomer and membranous palatines and pterygoids.

It is less important in the Sauropsida, and becomes indistinguishably fused with the sphenoid in the adult, while in Mammalia it is no longer found.

Ossification of the Cartilaginous Cranium. In certain Fishes the cartilaginous cranium remains quite unossified, while completely enveloped in dermal bones. Such for instance is its condition in the Selachioid Ganoids. In most instances, however, the investment of the cartilaginous cranium by membrane bones is accompanied by a more or less complete ossification of the cartilage itself.

In the Dipnoi this occurs to the smallest extent, the only ossifications occurring in the lateral parts of the occipital region, and forming the exoccipitals.

In Teleostei and bony Ganoids, a considerably greater number of ossifications occur in the cartilage.

In the region of the occipital cartilaginous ring there appears a basioccipital and supraoccipital and two exoccipitals. The basioccipital is the only bone on the floor of the skull ossifying that part into which the notochord is primitively continued 1 .

In the region of the periotic cartilage a large number of bones may appear. In front there is the prootic, which often meets the exoccipital behind ; behind there is above and in close connection with the supraoccipital the epiotic, and below in close connection with the exoccipital the opisthotic. On the dorsal side of the cartilage there is a projecting ridge composed mainly of a bone known as the pterotic, sometimes erroneously

1 The notochord appears also to enter into the posterior part of the region which ossifies as the basisphenoid.

383

59 6 OSSIFICATIONS OF THE CARTILAGINOUS CRANIUM.

called the squamosal, and continued in front by the sphenotic. The pterotic, or the cartilaginous region corresponding to it, always supplies the articular surface for the hyomandibular.

In the floor of the skull, in the region of the pituitary body, there is formed a basisphenoid; while in the lateral parts of the wall of this part of the cranium, there is a bone known as the alisphenoid.

In front, parts of the lateral walls of the cranium ossify as the orbitosphenoids.

In view of the very imperfect ossification of the cartilaginous cranium of the Dipnoi, and of the fact that there is certainly no direct genetic connection between the Teleostei on the one hand, and the Amphibia and Amniota on the other, it is very difficult to believe that most of the ossifications of the cranium in the Amphibia and Amniota have more than a general correspondence with those in the Teleostei.

In the Amphibia the ossifications in the cartilage are comparatively few. In the occipital region there is a lateral ossification on each side of the exoccipital. the basioccipital region being unossified, and the supraoccipital at the utmost indurated by a calcareous deposit.

The periotic capsule is ossified by a prootic centre, which meets the exoccipital behind.

The front part of the cartilaginous cranium is ossified by a complete ring of bone the sphenethmoid bone which embraces part of the ethmoid region, and of the orbitosphenoid and presphenoid regions.

In the Amphibia the cartilaginous cranium, with its centres of ossification, is easily separable from the membranous investing bones.

In the Amniota the cartilaginous cranium, whose development in the embryo has already been described, becomes in the adult much more largely ossified, and the bones which replace the primitive cartilage unite with the membrane bones to form a continuous bony cranium.

The centres of ossification become again much more numerous. In the occipital segment analogous centres to those of Teleostei are again found ; and it is probable that the exoccipitals are homologous throughout the series, the supraoccipital and basioc

THE SKULL. 597

cipital bones of the higher types being merely identical in position with the similarly named bones in Fishes.

In the periotic there are usually three centres of ossification, first recognised by Huxley. These are the prootic, the epiotic and opisthotic, the situations of which have already been defined. Of these the prootic is the most constant.

In Reptiles, the prootic and opisthotic frequently remain distinct even in the adult.

In Birds, the epiotic and opisthotic are early united with the supra- and exoccipital ; and at a later period the prootic is also indistinguishably fused with the adjacent parts.

In Mammals the three ossifications fuse into a continuous whole the periotic bone which may be partially united with the adjacent parts.

In the pituitary region of the base of the cranium a pair of osseous centres or in the higher types a single centre (Parker 1 ) gives rise to the basisphenoid bone, and in front of this another basal or pair of basal ossifications forms the presphenoid, while laterally to these two centres there are formed centres of ossification in the alisphenoid and orbitosphenoid regions, which may be extremely reduced in various Sauropsida, leaving the side walls of the skull almost entirely formed of membrane or cartilage.

In the ethmoid region there may arise a median ossification forming the mesethmoid and lateral ossifications forming the lateral ethmoids or prefrontals ; which may assist in forming the front wall of the brain-case, or be situated quite externally to the brain-case and be only related to the olfactory capsules.

The labial cartilages. In most Fishes a series of skeletal structures, known as the labial cartilages, are developed at the front and sides of the mouth, and in connection with the olfactory capsules ; and these cartilages still persist in connection with the olfactory capsules, though in a reduced form, in the higher types. They are more developed in the Cyclostomata than in any other Vertebrate type.

The meaning of these cartilages is very obscure ; but, from their being in part employed to support the lips and horny teeth of the Cyclostomata and the Tadpole, I should be inclined to regard them as remnants of a primitive skeleton supporting the suctorial mouth, with which, on the grounds already stated (p. 317), I believe the ancestors of the present Vertebrata to have been provided.

1 According to Kblliker there are two centres in Man in both the basisphenoid and presphenoid.

598 BIBLIOGRAPHY.

BIBLIOGRAPHY.

(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a leur differents ages." Paris, Mem. savans etrang. 1835, and An. Set. A 7 af. Vol. I. 1834.

(440) C. Gegenbaur. Untersuchwigen z. vergleich. Anat. d. Wirbelthiere, III. Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.

(441) Giinther. Beob. iib. die Entwick. d. Gehororgans. Leipzig, 1842.

(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr. Anat., Vol. xi. 1874, suppl.

(443) T.H.Huxley. " On the theory of the vertebrate skull." Proc. Royal Soc., Vol. ix. 1858.

(444) T. H. Huxley. The Elements of Comparative Anatomy. London, 1869.

(445) T.H.Huxley. "On the Malleus and Incus." Proc. Zool. Soc., 1869.

(446) T.H.Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.

(447) T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon." Journ. of Anat. and Phys., Vol. X. 1876.

(448) T.H.Huxley. The Anatomy of Vertebrated Animals. London, 1871.

(449) W. K. Parker. "On the structure and development of the skull of the Common Fowl (Callus Domesticus)." Phil. Trans., 1869.

(450) W. K. Parker. "On the structure and development of the skull of the Common Frog (Rana temporaria)." Phil. Trans., 1871.

(451) W. K. Parker. "On the structure and development of the skull in the Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.

(452) W. K. Parker. "On the structure and development of the skull in the Pig (Sus scrofa). " Phil. Trans., 1874.

(453) W. K. Parker. "On the structure and development of the skull in the Batrachia." Part n. Phil. Trans., 1876.

(454) W. K. Parker. "On the structure and development of the skull in the Urodelous Amphibia." Part in. Phil. Trans., 1877.

(455) W. K. Parker. "On the structure and development of the skull in the Common Snake (Tropidonotus natrix)." Phil. Trans., 1878.

(456) W. K. Parker. " On the structure and development of the skull in Sharks and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.

(457) W. K. Parker. "On the structure and development of the skull in the Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans., 1879.

(458) W. K. Parker. "The development of the Green Turtle." The Zoology of the Voyage of H. M.S. Challenger. Vol. I. pt. V.

(459) W. K. Parker. "The structure and development of the skull in the Batrachia." Pt. in. Phil. Trans., 1880.

(460) W. K. Parker and G. T. Belt any. The Morphology of the Skull. London, 1877.

(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1839.

(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Miiller's Archiv, 1837.

(462) W. Saleusky. "Beitragez. Entwick. d. knorpeligen Gehorknochelchen." Morphol. Jahrbuch, Vol. VI. 1880.

Vide also Kolliker (No. 298), especially for the human and mammalian skull; Gotte (No. 296).