GR De Beer, BA, B.Sc

Contributions to the Study of the Development of the Head in Heterodontus.

By G. R. de Beer, B.A., B.Sc, F.L.S., Fellow of Merton College, Demonstrator in Zoology and Comparative Anatomy, University Museum, Oxford.

With 21 Text-figures.

CONTENTS.

PAGK

INTRODUCTION ...... 39

THE SKULL .40

RELATIONS OF THE SKULL TO THE ARTERIES .... 50

RELATIONS OF THE SKULL TO THE NERVES . . . .53

THE VERTEBRAL COLUMN ...... 57

THE TERMINAL AND OLFACTORY NERVES ..... 62

SUMMARY ...... 64

INTRODUCTION. THROUGH the kindness and generosity of Mr. 1\ D. F. Murray, I have been able to study some embryos of Heterodontus philippi at stages varying from 11 mm. to 70 mm. in length I have concerned myself principally with the head, and in this paper I propose to record my observations on the development of the skull and its relations to nerves and blood-vessels, and to add a few remarks concerning certain nerves. The development of the prootic somites into the eye-muscles, and the placodes in connexion with the profundus nerve, I have dealt with elsewhere.1 The work was done in the Department of Zoology and Comparative Anatomy at Oxford. 1 Page 17 of this volume, and a forthcoming paper. 40 G. R. DE BEER

THE SKULL. The works of Gaupp (6), Goodrich (7), Parker (11), Sewert- zoff (14), and Van Wyhe (17 and 18) have acquainted us with the development of the skull in Scyllium, Pristiurus, and Squalus, but as far as I can gather there has as yet been no work done on the Heterodonti except for the earliest stages of development (Haswell, 7 b). The earliest stage at which rudiments of the skull appear is in embryos 37 mm. long. Text-fig. 1 is a reconstruction of such an embryo seen from the left side.

TEXT-FIG. 1.

Reconstruction of an embryo of Heterodontus philippi, 37 mm. long, seen from the left side. EXPLANATION OF LETTERING. a., small artery between the two branches of the olfactory nerve ; ah., abducens nerve ; aba., afferent branchial artery ; acr., arteria centralis retinae ; aha., afferent hyoidean artery ; ale, anterolateral cartilage of the auditory capsule ; anteba., anterior efferent branchial artery; apa., afferent pseudobranchial artery; bal., first branchial arch ; bd., basidorsal cartilage ; buc, ramus buccalis facialis ; bv., basiventral cartilage ; cb., ceratobranchial; fc, cross commissure between efferent branchial arteries; eg., ciliary ganglion; ch., ceratohyal; da., dorsal aorta; dr., dorsal HEAD OF HETERODONTUS 41 The first structures to appear are the parachordal plateB which flank the notochord from about the level of the facial

root of a spinal nerve ; eb., epibranchial; ee., elastica externa ; ep., epithelium; epa., efferent pseudobranchial artery; exb., extrabranchial cartilage; exh., extrahyal cartilage; exo., external branch of the olfactory nerve ; exr., external rectus ; /., facial nerve ; fb., fore-brain ; fepa., foramen for efferent pseudobranchial artery; fgl., foramen for glossopharyngeal nerve; fhv., foramen for hypophysial vein ; fnam., foramen for nerve to adductor branehialis muscle; foe, foramen for oculomotor nerve; fpa., foramen for pathetic nerve; fvr., foramen for ventral root of spinal nerve; gl., glossopharyngeal nerve; h., hyoid arch ; lia., hypobranchial artery ; hb., hypobranchial; hf., ramus hyoideus facialis ; km., hyomandibular ; hmf., ramus hyomandibularis facialis ; hv., hypophysial vein; ic, internal carotid artery ; id., interdorsal cartilage (intercalary) ; ig,, isolated ganglion not connecting with a ventral root; infr., inferior rectus; ino., internal branch of the olfactory nerve; inob., inferior oblique ; inr., internal rectus; isn., invaded sheath of the notochord ; Ic., layer of cartilage connecting two basiventrals ; in., mandible; ma., mandibular artery; max., ramus maxillaris trigemini; md., ramus niandibularis trigemini; mdin., inferior branch of niandibularis trigemini; mds., superior branch of niandibularis trigemini; mexf., ramus niandibularis externus facialis ; minf., ramus niandibularis interims facialis; mxnid., maxillo-mandibulary branch of trigeniinal ; mxn. 1, first mixed spinal nerve ; n, notochord ; oa., occipital arch ; oc, oculomotor nerve ; ol., olfactory sac ; on., optic nerve ; ophma., ophthalmica magna artery ; orba.y orbital artery ; j?., parachordal cartilage; pa., pathetic nerve; pb., pharyngobranchial; pc.t posterior cartilage of the auditory capsule; pee., procartila- ginous rudiment of the ethmoid ; phgl., ramus pharyngeus glossopharyngei; phf., ramus palatinus facialis; ppb., palatobasal process; pq., pterygoquadrate; prf., ramus pretrematicus facialis; prgl., ramus pretrematicus glossopharyngei; prv., ramus pretrematicus vagi; planlgl., ramus post-trematicus anterior glossopharyngei; pteba., posterior efferent branchial artery; ptptgl., ramus post-trematicus posterior glossopharyngei; r., rostrum ; rale, rudiment of the antero-lateral cartilage of the auditory capsule ; ro., rudiment of the olfactory nerve ; rop., ramus ophthahnicus profundus ; ros., combined rami ophthalmici auperiores ; ros VII, ramus ophthalmicus superior facialis; rt., rudiment of nervus terminalis ; s., muscle segment; sa., segmental artery ; sg. 1, first spinal ganglion; sp., spheno-lateral cartilage ; apt., septum; stgl., ramus supratemporalis glossopharyngei; suob., superior oblique ; sur., superior rectus ; sv., small vein running between trigeminal and facial nerves ; t., nervus terminalis ; tc, trabecula cranii; tp., trabecular plate; tr., trigeminal nerve ; ts., tectum synoticum ; tsp., tectum formed by spheno-laterals ; v., vagus nerve ; va., ventral aorta ; vr., ventral root of spinal nerve ; vr. 1,2, 3 ; first, second, and third ventral roots. 42 G. B. DE BEEB nerve and pass back into the region where the procartilage rudiments of the vertebral column are to be found. These plates are quite flat and show no sign of segmentation. Ventral to the anterior extremity of the parachordal cartilages and almost touching them are the trabecular cartilages. These lie fairly wide apart, on either side of the hypophysis and ventral to the floor of the fore-brain to the form of which they are moulded. It will be seen from Text-fig. 1 that the trabeculae make a certain angle with the parachordals, but this angle is smaller than that described for Squalus by Sewertzoff (14),

£c ' i i, bal pq m en Reconstruction of the skull of an embryo 40 mm. long. yet larger than that obtaining in Scyllium according to Goodrich (7). A separate ' Polknorpel', such as Van Wijhe (17) describes, I have not been able to find, probably because the requisite stage of development is not in my possession. It is possible, however, that the ' Polknorpel', though possessing a distinct centre of chondrification of its own, may be connected with the trabeculae by procartilage. In Text-fig. 1 I have indicated the position of the efferent pseudobranchial artery. This is of some importance, as the relations of this artery to the trabeculae differ in Selachians from the conditions obtaining in all other craniates. From HEAD OF HETEHODONTUS 43 the earliest appearance of the trabeculae the artery runs dorsally over them to join the internal carotid, instead of under them as in other forms.1 The mandibular arch contains a cartilage just forming from the procartilage condition, and which represents the palato-

TBXT-FIG. 3.

Reconstruction of the skull of an embiyo 45 mm. long (from a wax model). pterygoquadrate bar and Meckel's cartilage, as yet unseparated. The pterygoquadrate is attached from the first to the trabeculae by dense connective tissue. This connexion represents the processus palatobasalis (or possibly the processus ethmoidalis, since it is at present impossible to diagnose them with certainty), and it is of interest to observe that there is no trace or suggestion of an otic process. The early fossil Heterodonti were amphistylic, and.Heterodontus in its development might have shown traces of an amphistylic condition. That this is 1 This matter is the subject of a shortly forthcoming paper. 44 G. R. DE BEER not the case need not be surprising, for unless the fossil amphistylic (and presumed) ancestral Heterodonti were amphistylic in their embryonic stages there is no reason to believe that Heterodontus philippi would show this character. In later stages the palatoquadrate of Hetero- dontus is larger than that of any other non-amphistylic Selachian, without however possessing a true otic process. A conclusion which might be drawn from this would suggest that the otic process of amphistylic forms was not present in the embryonic stages of such forms but developed late. There are reasons for believing the otic process to represent a modified gill-ray (Allis), and this would fit in with the suggestion that it is a structure of secondary nature and developed late. In the hyoid arch the hyomandibular and ceratohyal cartilages arise in conjunction. The hyomandibular is in contact with the parachordal ventral to the auditory sac. There is no trace of any pharyngohyal element (see Sewertzoff, 15). Anterior to and separate from the parachordals, but extend- ing along the same axis, are the cartilages variously described as alisphenoid (Sewertzoff, 14), pleurosphenoid (Van Wijhe, 18), or sphenolateral (Gaupp, 6). They lie dorsal to the oculomotor and behind the patheticus at this stage. Between the sphenolaterals and the parachordals runs the hypophysial vein. This vein will at later stages lie anterior to the junction between sphenolaterals and parachordals. This junction, the pila antotioa, has not yet appeared, and though present at later stages I am unable to say whether it has a separate origin or not. Behind the hyoid the remaining branchial arches have pro- cartilaginous rudiments only. These rudiments are pierced by the adductor branchialis muscle, which splits off mesially from the main muscles of the visceral arch, as described by Sewertzoff (15). By piercing the visceral arches the adductor branchialis muscles come to lie median to the visceral arches, while the main band of muscles (dorsal, medial, and ventral constrictors and the interbranchials) lies outside them. At the next stage (39 mm., Text-fig. 2) the most noteworthy changes are that in the mandibular and hyoid arches the HEAD OF HETERODONTUS 45 skeletal elements have separated into dorsal and ventral portions, and that by the pila antotica the sphenolaterals are connected with the parachordals. The trabeculae are connected with the parachordals. Dorsolateral to the trabeculae the procartilage rudiments of the ethmoid cartilages appear. Anterior to the pila antotica the hypophysial vein enters the cavity of the skull, and at this stage it has already been surrounded by cartilage forming the interorbital canal. Ventral to it is the efferent pseudobranchial artery which runs over the trabecula to join the internal carotid, and which has likewise become enclosed by the developing cartilage. In the broad gap between the trabeculae and the sphenolaterals the optic and oculomotor nerves pass freely out. The pathetic lies anterior to the sphenolaterals, the trigeminal facial and abducens nerves emerge behind the pila antotica. The parachordals, dorsal to the hyomandibular (which is separated from the ceratohyal), show rudiments of two processes which will be the anterolateral and posterior cartilages of the auditory capsule. They have no separate origin of their own, but develop as processes of the parachordals. The palatoquadrate has separated from Meckel's cartilage ; the former, in close connexion with the trabeculae, begins to show an anteriorly directed palatine process. The branchial arches at this stage are simple hoops of procartilage, as yet undifferentiated into their component elements, though the dorsal half is marked off from the ventral half. Daniel (3) states that in Heterodontus francisci there are traces of a sixth branchial arch. This is not apparent in ray preparations ; possibly it develops at a stage later than any represented in my series. The adductor muscles are still connected with the main muscles dorsally and ventrally, and pass through foramina in the dorsal and ventral halves of the visceral arches. At 45 mm. (Text-fig. 3) the existing elements of the proceed- ing stages have enlarged. The sphenolaterals have extended forwards and enclose the pathetic nerve in a foramen. The space between the sphenolaterals and the trabeculae is closing 46 G. R. DE BEER up from behind forwards, and the oculomotor is enclosed in a foramen. In connexion with the trabeculae the ethmoid cartilages or lamina orbito-nasalis have arisen, stretching forwards lateral to the trabeculae with Avhich they are fused. The trabeculae of each side are joined together anteriorly by a trabecular plate Avhich extends back as far as the hypophysis. This shows that the skull of Heterodontus is platibasic, or ' platitrabic ', to use Van Wijhe's (18) term. The antero-lateral cartilage of the auditory capsule has grown up and reaches as high as the sphenolaterals, towards which it extends anteriorly. The trigeminal, facial, and abducens nerves thus lie between the sphenolaterals and the anterolateral otic cartilages. The posterior cartilage of the auditory capsule is developing and encloses the glossopharyngeal nerve in a foramen. Behind this is the rudiment of the occipital arch between which and the posterior otic cartilage the vagus passes freely out. The rest of the otic capsule is foreshadowed by procartilage, the extent of which is indicated by the dotted line. In the branchial arches the subdivision into four elements, pharyngo-, epi-, cerato-, and hypo- is discernible. The epi- branchials and ceratobranchials are pierced no longer by the adductor branchialis muscle which has lost all connexion with the other muscles, but by the nerves, branches of the ramus post-trematicus branchialis, which supply the adductor branchialis muscles. Other than the presence of these nerves there is no trace of the fact that the adductor branchialis muscles have split off from the constrictors and pierced the visceral arches. A reconstruction showing the relations of the nerves to the cartilage at this stage is given in Text-fig. 4. At 70 mm. the skull has taken on more or less the form of the adult (Text-figs. 5 and 6). The sphenolaterals, ethmoids, and trabeculae have fused up on each side, so that the lateral walls of the cranium are complete and the optic nerve is enclosed in a foramen. The sphenolaterals have fused dorsally with the anterolateral cartilage of the auditory capsule, this enclosing the trigeminal, HEAD OF HETEEODONTUS 47 facial, and abducens nerves in the foramen prooticum, which is then divided into two. The auditory capsule itself is complete, the antero-lateral and posterior cartilages having fused completely. The latter has also joined with the occipital arch with the result that the vagus is enclosed. The trabecular plate has extended backwards and underlies the hypophysis, whose connexion with the exterior has dis-

max & due'

Reconstruction of the skull of an embryo, 45 mm. long, showing the relations of the cartilages to the nerves. Posteriorly the trabecular plate stops short just anterior to the parachordals, and through this foramen the internal carotid enters the skull. Anteriorly the trabecular plate is prolonged into the rostrum, which passes forward between the two olfactory sacs. Laterally these sacs are surrounded by thin cartilages which connect posteriorly with the ethmoid region of the wall of the cranium. At this stage the roof of the skull is forming. The roofing starts in two places : anteriorly in connexion with the sphenolaterals, and posteriorly with the auditory capsule. The latter G. R. DB BEER is the second to form, and at this stage the processes from each side have not yet met in the middle line. The tectum synoticum, therefore (for this posterior roofing process represents this structure), arises later than the anterior roofing, as in Amia

anteba eha '

Reconstruction of the skull of an embryo, 70 mm. long, showing the relations of the cartilages to the arteries (from a wax model). (Pehrson, 12), but this is the opposite of what occurs in Pristiurus according to Sewertzoff (14). The notochord is persistent in the skull and extends a short distance in front of the parachordals. The palatine process of the pterygoquadrate is now well developed. In the visceral arches the conditions do not differ much from those obtaining at the previous stage, except that the gill-rays and extrabranchial cartilages have appeared (Text- HEAD OF HETERODONTUS fig. 7). Gill-raj's are carried by the ceratohyal and the epi- and ceratobranchials. These gill-rays lie posterior to the anterior efferent and afferent arteries and to the post-trematie

TEXT-FIG. 0.

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Reconstruction of the skull of an embryo, 70 ram. long, seen from the dorsal surface. branchial nerves. They lie anterior to the posterior efferent arteries and to the pretrematic branchial nerves. The extrahyal of the hyoid arch is represented only by the ventral element, the dorsal one being absent in the adult Heterodontus (Daniel, 4. Fiirbringer, 5, however, claimed to find a rudiment of it). The extrabranchials at this stage only have the dorsal element. Since the adult has ventral extra- NO. 269 E 50 G. H. DB BEER branchials also, these must appear at a later stage. The visceral arch lies median to the cross commissural vessels between the anterior and posterior efferent arteries ; the extrabranchial arch Iie3 lateral to this vessel (see Krivetski, 8). The labial and prespiracular cartilages have not yet appeared. Posteriorly the skull articulates with the vertebral column, the development of which will be described below.

RELATIONS OP THE SKULL TO THE ARTERIES. The internal carotid, which is the anterior prolongation of the dorsal aorta, enters the skull through the foramen between the parachordals and the trabecular plate. Just before doing so it gives off a small artery dorsally, which pierces the cartilage and emerges at the side of the skull by a foramen situated just beneath that of the facial nerve. This is the orbital artery (external carotid of some authors) which runs down antero- ventrally on the outer margin of the pterygoquadrate (Text-fig. 5). Further forward the internal carotid receives the efferent pseudobranchial artery, which passes up from the pseudobranch median to the orbital artery and enters the skull through a foramen dorsal to the trabecula. The relations of this artery to the trabecula in Selachians have already been commented upon.1 Just before entering the skull the efferent pseudobranchial artery gives off the ophthalmica magna, which enters the eyeball, passing dorsal to the orbital artery. Opposite the optic foramen the internal carotid gives off an artery which accompanies the optic nerve through the optic foramen into the eyeball. This is the arteria centralis retinae. Further forward the internal carotid gives off two small arteries to the olfactory region and then curves upwards and backwards along the under-surfaee of the brain. The two arteries, one on each side, fuse to form the arteria basilaris, which continues backwards beneath the spinal cord. Behind the place where the internal carotid gives off the

1 See previous foot-note. HEAD OP HETERODONTUS 51 orbital artery it passes median to the hyomandibular and receives the efferent hyoidean artery. Prom here it becomes the lateral dorsal aorta, and just in front of and lateral to the pharyngobranchial of the first branchial arch it receives the first efferent branchial artery. The lateral dorsal aorta then

f eha hm

The hyoid and first branchial arches of an embryo, 70 mm. long, showing the relations of cartilages, arteries, and nerves. curves over the pharyngobranchial and passes backwards over its mesial face. The branchial vessels are shown in greater detail in Text- fig. 7. The efferent hyoidean artery represents the posterior efferent artery*of the hyoid arch (the anterior element not being formed), for it lies behind the hyal rays. Ventrally it connects by a cross commissure with the pseudobranch, i.e. by the afferent pseudobranchial artery. 52 G. R. DE BEER This commissure passes on the inside of the ramus mandibularis internus facialis, and therefore cannot represent the original afferent mandibular vessel. The latter vessel is probably represented by a small artery which is given off by the lateral hypobranchial artery running forward from the ventral extremity of the efferent hyoidean artery. The hypobranchial artery runs forward inside the afferent hyoid artery and on the under-surface of the ceratohyal. The true mandi-

TEXT-FIG. 8.

The ophthalmic, oculomotor, pathetic, and abducens nerves and their relations to the eye-muscles. 70 mm. bular artery which it gives off runs up the outer side of the mandible between the two branches of the ramus mandibularis facialis. At earlier stages it connects with the commissure running to the pseudobranch, but at 70 mm. this connexion is lost (cf. Allis, 1). The efferent hyoidian artery is connected with the anterior efferent branchial artery of the first arch beneath the first gill-slit. The afferent hyoidean and branchial arteries lie external to the hyoid and visceral arches respectively, and also external to the cross commissural vessels between the efferent arteries. The lateral hypobranchial arteries connecting the ventral ends of all the efferent arteries are still incomplete. HEAD OF HETERODONTUS 53

KELATIONS OF THE SKULL TO THE NERVES. Of the olfactory nerves little need be said ; they reach the olfactory epithelium through the wide openings in the olfactory capsules. The optic nerve penetrates the optic foramen. The oculomotor pierces the lateral wall of the skull and emerges by a foramen slightly above and behind the optic foramen. It divides into two branches, the more dorsal of which passing over the ramus ophthalrnicus prof undus innervates the superior and internal recti, the ventral branch passing beneath the profundus innervates the inferior rectus and inferior oblique (Text-fig. 8). The pathetic nerve traverses a foramen far forward and dorsal in the lateral wall of the skull, and passes below the rami ophthalmici superficiales trigemini and facialis and above the profundus to the superior oblique. The abducens passes forwards within the skull median to the facial and trigeminal and emerges through the trigeminal foramen anterior to the Gasserian ganglion. It arises by four or five roots. The trigemino-facialis complex (Text-fig. 9) is interesting as differing considerably from that described for S q u a 1 u s by Norris and Hughes (10), and from other forms. The trigeminal ganglion is large compared Avith the facial, and is not over-lain by it as in S q u a 1 u s . The ophthalmic ganglion of the facial emerges through the same foramen as the trigeminal, and the combined superficial ophthalmic nerves run forward along the side of the skull on the dorsal border of the orbit. The trigeminal and facial ganglia are separated by a small vein from the vena capitis lateralis and representing part of the original vena capitis medialis. The hypophysial vein represents a branch of the latter. The profundus leaves the root of the superficial ophthalmic ganglion and runs over the external rectus, and the inferior branch of the oculomotor, under the superior and internal recti, over the optic nerve, under the superior oblique, and re-enters the skull through a small foramen on the anterior G. R. DB BEER edge of the orbit accompanied by a small vein. It continues its forward course on the inside of the lateral wall of th» skull. The rnaxillo-rnandibular branch of the trigeminal passes behind and beneath the external rectus and runs to the outer edge of the pterygoquadrate and divides (Text-fig. 10). The

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epa -*:••••• mxmd8cbuc- h'mf prr The trigemiual and facial ganglia and their relations to the neighbouring arteries and veins. anterior division, ramus maxillaris, runs forward along the outer edge of the palatoquadrate in conjunction with the ramus buccalis of the facial. The posterior or mandibular division divides into two, both branches run down on the outside of the pterygoquadrate, one supplying the adductor mandibulae, the other sending branches to the skin. The facial emerges through the foramen close behind the trigeminal. Ventrally it gives off a branch which divides into two; one portion goes forward beneath the skull as the ramus HEAD OF HETBBODONTUS palatinus, the other passes to the anterior -wall of the spiracle and is th.e ramus pretrematicus. The ramus hyomandibularis passes back dorsal to the hyomandibular and then vertically down along the posterior face of the latter. It divides into

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The relations of the cartilages and nerves in a 70 mm. embryo. three. One branch, the ramus mandibularis externus facialis, runs forward in the lower jaw external to Meckel's cartilage. The ramus mandibularis internus facialis passes in to the median side of Meckel's cartilage, between the latter and the hyomandibular. Before doing so, however, it lies external to the vessel connecting the pseudobrancli with the efferent hyoidean 56 G. E. DE BEER artery. The rarnus hyoideus continues downwards in the hyoid arch behind the ceratohyal. The ramus oticus facialis of Norris and Hughes's description of Squalus I have not been able to find. The glossopharyngeal, which emerges from the skull beneath the auditory capsule, gives off a ramus supratemporalis, a ramus pharyngeus, and pre- and post-trematic branchial nerves. The pretrematic passes lateral to the lateral dorsal

TEXT-FIG. 11. sSfl bd id i

The occipital region of a 70 mm. embryo. aorta but median to the efferent hyoidean artery, and lies behind the hyal rays. The post-trematic branch divides into two : an anterior -which lies inside the lateral dorsal aorta and anterior to the anterior efferent branchial vessel; and a posterior which runs outside the aorta, behind the anterior efferent vessel, median to the afferent vessel, lateral to the cross commissural vessel, and anterior to the branchial rays. It gives off branches which pierce the epibranchial and ceratobranchial to innervate the adductor branchialis muscle. The pretrematic branch of the first vagus root lies behind the branchial rays and median to the cross commissural vessel and the posterior efferent artery. HEAD OF HETERODONTUS 57 In the occipital region (Text-fig. 11) three ventral roots pierce the skull, and each is distributed to a myotome. Corre- sponding to the last of these roots, there is within the skull a dorsal ganglion which is isolated from the ventral root and does not communicate with it. From a notch in the posterior face of the occipital arch there emerges a ventral root which joins with the next dorsal root to form the first mixed spinal nerve. Thereafter the ventral roots pierce the basidorsals and the dorsal roots the interdorsals. I have not yet a sufficient series of early stages to be able to say to which segment the first permanent postotic myotome belongs and how many segments are included in the skull.

THE VERTEBRAL COLUMN. The sheath of the notochord is invaded as in all Selachians by cells from the sclerotome. The invasion takes place not by total disappearance of the elastica externa at four points, but the elastica is perforated at these points and the invading cells pass singly through the perforations. The consequence is that in sections more than 10p thick the elastica externa appears to be intact and does not show any interruptions at the points where invasion is occurring. The centra are formed of basidorsal and basiventrals which arise in the septa between the segments (Text-fig. 12). The basidorsal and basiventral of one centrum arise in one and the same septum. Van Wijhe (18) states that in Squalus along the notochord there are four continuous bands of cartilage Avhich become separated up into the basidorsals and basiventrals of each side. This is contrary to the views of Schauinsland (13), who says : ' Es muss iibrigens noch hervorgehoben werden, dass die Anlagen der einzelnen Bogen sowie auch der Interkalar- stiicke offenbar von "vornherein von einander getrennt sind, obgleich sie in dem scheinbar einheitliehen Gewebe der oben beschriebenen Langsleisten ihren Ursprung nehmen.' The ' Langsleisten ' are the bands running along the notochord which Van Wijhe describes as cartilaginous, and 58 G. R. DE BEER therefore according to him the elements of the vertebrae are at first continuous. I have observed these bands and there seems to be no doubt about their existence; but the question is, are they true cartilage ? In order to test this point I have prepared sections

TEXT-FIG. 12.

6K sa dr 45 mm. embryo. Relations of dorsal and ventral roots to segmental vessels, septa, muscle segments, and vertebral elements.

TEXT-FIG. 13.

70 mm. embryo. Vertical longitudinal section in trunk region, showing the layer of cartilage connecting two basiventrals. from different stages of the trunk, stained in thionin. This stain is metachromic, staining cartilage red, and all other tissues including procartilage blue. At the early stages (40 mm.) the longitudinal bands appear to be homogeneous, but in the regions of the basidorsals and basiventrals thionin shows that chondrification is much more active than in the intervening regions. But there is cartilage in the intervening regions (Text-fig. 13). I therefore agree with Van Wijhe that the elements of the vertebrae are connected from a very early HEAD OF HETERODONTUS 59 but my impression is that the vertebral elements do not arise as subdivisions of a primitive continuous band, but that the band arises by the intervening regions between the vertebral elements becoming ' infected ' by the process of chondrinea- tion. The cartilage in the intervening regions is closely pressed against the elastica externa of the notochord, and is three or four cells thick. In the mid-ventral, mid-dorsal, and mid- lateral lines this cartilage does not exist, so that there are veritably four bands.

TEXT-FIGS. 14 AND 15.

id bd

Relations of nerves and vertebral elements in the tail and trunk regions of Pristiurus. It is difficult to conceive of any other explanation for these bands, for the sclerotomic material is segmented and the continuity must be secondary. In the adult Heterodontus the basidorsal is pierced by the ventral root, and the intercalary immediately behind it is pierced by the dorsal root of any one mixed spinal nerve (see Daniel, 3, for description of the cartilage of adult H. francisci). As Van Wijhe points out, there are two distinct types in the Selachians as regards the relations of the nerve-roots to the elements of the vertebral column. In one type the ventral root pierces the basidorsal and the dorsal root pierces the succeeding intercalary. In the other type the ventral root 60 G. R. DE BEER emerges between the basidorsal and the next posterior intercalary, and the dorsal root between that intercalary and the following basidorsal. To the latter type belong the Scyl- lioidei, to the former the Notidani, Heterodonti, Squaliformes, and Batoidea. It becomes interesting to inquire as to how this variation is brought about. Text-figs. 14 and 15 are reconstructions of the relations in the tail and trunk respectively of a Pristiurus embryo, a form which is of the Scyllioid type. It is plain that the nerve- roots emerge between the skeletal elements and do not pierce them. Primitively, a ventral root running straight to its myotome is intrasegmental, and the corresponding dorsal root is behind it and intersegmental, running in the septum between the myotome supplied by the ventral root and the next posterior myotome. The mixed nerve runs in the septum immediately behind the segment which it supplies (Goodrich, 7). This is plain from Text-figs. 14 and 15. Now in the case of the other type, to which Heterodontus belongs, haA'e the nerve-roots moved forwards or backwards to become enclosed in the cartilages? Text-fig. 16, which is a reconstruction of a 45 mm. embryo, shows that as in the previous case the roots run out in the septum behind the segment which they supply. Text-fig. 17, which represents a 70 mm. embryo, shows the segmental blood-vessel. (One only of the two, artery and vein, for clearness.) The blood- vessel indicates the position of the septum, and if there were a migration of the roots backwards, the nerve would cross the blood-vessel. This does not occur, suggesting that the migration was not backwards but must be forwards. That this is so is proved by the fact that in early stages the ventral root is only just enclosed by the posterior face of the basidorsal in front of it, and that it becomes more enclosed as development proceeds. The dorsal roots come to lie immediately over instead of behind the intercalaries, and these being notched present a heart-shaped appearance before fully enclosing the dorsal root. A complete reconstruction of the relations in a HEAD OF HETERODONTUS 61 70 mm. embryo is given in Text-fig. 18, and it shows that the ventral root pierces the basidorsal anterior to the one corre-

TEXT-FTG. 16.

Heterodontus, 70 ram. longitudinal vertical section. The nerves run in the posterior septum of their segment.

TEXT-FIO. 17.

fvr

The foramina through which the nerves pass.

TEXT-FIG. 18.

Complete diagrammatic reconstruction of the trunk region in a 70 mm. embryo. sponding to the septum in which the mixed nerve runs. This confirms Goodrich (7 a), who states on p. 136 that ' the ventral 62 G. E. DE BEER root . . . comes out behind or through the neural arch ', thus indicating the forward nature of the displacement of the nerves when they pierce the skeletal elements.

THE TERMINAL AND OLFACTORY NERVES. Locy's (9) nervus terminalis is represented in 22 mm. embryos of Heterodontus by a string of stellate cells, close to the fore-brain on its anterior surface, and in the region where the neural crest is disappearing (Text-fig. 19). Locy is

TEXT-FIG. 19.

•Fb

Transverse section through anterior region of 22 mm. stage, showing the rudiment of the terminalis. also of opinion that the neural crest contributes to the terminalis, and the demonstration by Brookover (2) that the terminalis contains elements of a sympathetic nature derived from a placode, leads to the conclusion that the terminalis contains elements of mixed nature. At 45 mm. (Text-fig. 20, reconstructed as seen from in front) the terminalis is connected with the brain, and the point of connexion is not ATentral as in Ami a and Squalus but on the anterior surface as in Raja. It runs straight to the placode formed by the olfactory organ and which gives rise to the olfactory nerve. At 70 mm. (Text-fig. 21) the olfactory nerve is double, consisting of a lateral and a median element. At this time the HEAD OF HETERODONTUS 68

TEXT-FIG. 20.

0/ f Reconstruction of the fore-brain from in front. 45 mm.

TEXT-FIG. 21.

Reconstruction of the fore-brain from in front, 70 mm., showing the relations of the terminalis to the olfactory nerves. 64 G. K. DE BEER second set of Schneiderian folds (Sund, 16) has appeared ; but as Norris and Hughes (10) find for S q u a 1 u s, so in H e t e r o - dontus the subdivisions of the olfactory nerve and of the olfactory sac do not correspond. The terminalis runs outwards dorsally to the median section of the olfactory nerve, and between it and the lateral section swells out into a ganglion. It is interesting that there should be much variation in connexion with these ganglia, for in Squalus Locy describes two, in Must el us none; as far as I can make out Heterodontus has one. This ganglion lies dorsal to a small artery which, running forwards under the brain from the internal carotid, passes between the two divisions of the olfactory nerve. From the ganglion the terminalis fibres run in olose association with those of the lateral olfactory nerve to the olfactory epithelium, but without appropriate neurological methods I have not been able to follow their distribution further.

SUMMARY. 1. The development of the skull of Heterodontus is described. 2. There is no trace of an otic process. 3. The tectum synoticum forms after the more anterior roofing. 4. The efferent pseudobranchial artery is from the first dorsal to the trabecula. 5. The afferent pseudobranchial artery is not the mandibular artery. 6. Longitudinal bands of cartilage exist along the notochord but are probably not primary. 7. The dorsal and ventral roots have moved forwards so as to be enclosed in cartilages. 8. The olfactory nerve is subdivided into two, which subdivisions do not correspond with the subdivisions of the olfactory sac. 9. The neural crest contributes to the terminalis, which shows one ganglion. HEAD OF HETERODONTUS 65

LIST OP LITERATURE CITED. 1. AUis, E. P.—" The so-called mandibular artery, &c", ' Journ. Morph.', 27, 1916. 2. Brookover, C.—" The olfactory nerve and nervus terrainalis in Amia, calva ", ' Journ. Comp. Neur.', 20, 1910. 3. Daniel, J. F.—" The anatomy of Heterodontus francisci", ' Journ. Morph.', 26, 1915. 4. • ' The Elasmobranch fishes', Univ. California Press, 1922. 5. Ftirbringer, K.—" Beitrage zur Kenntnis des Visceralskeletts der Selachier ", ' Morph. Jahrb.', 31, 1903. 6. Gaupp, E.—" Die Entwickelung des Kopfskelettes", Hertwig's ' Handbuch der Etitwickelungslehre '. 7. Goodrich, E. S.—" On the development of the segments of the head in Scyllium ", ' Quart. Journ. Micr. Sci.', 63, 1918. 7 a. ' Vertebrata Oraniata. Cyclostomes and Fishes.' London, 1909. 76. Haswell, W. A.—" On the development of Heterodontus philippi ", ' Proc. Linn. Soc. New South Wales', 22, 1898. 8. Krivetski, A.—" Sur la morphologie des elements de l'arc hyoide chez les Selaciens ", ' Rev. Zool. Rus.', 2, 1917. 9. Locy, W. A.—" A new cranial nerve in Selachians ", ' Mark. Anniv. Vol.', 1903. 10. Norris, H. W., and Hughes, S. R.—"Nerves of Squalus acanthias", ' Journ. Comp. Neiir.', 25, 1920. 11. Parker, W. K.—" On the structure and development of the skull in Sharks and Skates ", ' Trans. Zool. Soc.', 10, 1878. 12. Pehrson, T.—" Some points in the cranial development of Teleo- stomian fishes ", ' Acta Zoologica ', 3, 1922. 13. Schauinsland, H.—" Die Entwickelung der Wirbelsiiule ", Hertwig's ' Handbuch der Entwickelungslehre '. 14. Sewertzoff, A. N.—' Die Entwickelung des Selachierschadels,' Festschr. v. K. von Kupffer. 15. " Die Morphologie des Visceralapparates der Elasmobranchier ", ' Anat. Anz.', 56, 1923. 16. Sund, O.—" Die Entwickelung des Geruchsorgans bei Spinax ", ' Zool. Jahrb., Abt. Anat.', 22, 1906. 17. Wijhe, J. van.—" Die Entwickelung des Kopfskelettes bei Selachiern ", ' Comptes rendus 6° Cong. Int. Zool. Berne ', 1904. 18. " Friihe Entvvickelungsstadien des Kopf- und Rumpfskelettes von Acanthias vulgaris ", ' Bijdragen tot de Dierkunde, Amsterdam ', 22, 1922.

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