On the Nature of the Trabecula Cranii.By

G. R. de Beer, M.A., B.Sc.,Fellow of Merton College, Jenkinson Lecturer in Embryology, and

Demonstrator in Zoology in the University of Oxford.

With Plates 40-6 and 3 Text-figures.

CONTENTS.PAGE

1. I N T R O D U C T I O N T O T H E P R O B L E M . . . . . . 7 0 1

2. O B S E R V A T I O N S 7 1 0

(i) S e l a o h i i 7 1 0

(i i ) T e l e o s t e i 7 1 4

( i i i ) A n u r a 7 1 7

( iv) U r o d e l a 7 1 8

3 . D I S C U S S I O N . . . . . . . . . . 7 1 9

4 . S U M M A R Y 7 2 8

5 . L I S T O F L I T E R A T U R E C I T E D 7 2 9

6. E X P L A N A T I O N O P P L A T E S 7 3 1

E X P L A N A T I O N OF L E T T E R I N G . . . . . . 7 0 7

1. INTRODUCTION TO THE PROBLEM.

THE trabeculae of the chondrocranium, discovered first byRathke (1839) in the grass-snake, were recognized by W. K.Parker as the essential and fundamental pair of elements of theanterior region of the cartilaginous skull, just as the cartilageswhich Huxley (1874) termed parachordals were the basis fromwhich the posterior region of the skull arose. But whereas thenature of the parachordal cartilages as parts of the axialskeleton, flanking the notochord, and developed from thesclerotomes of the mesodermal somites of the head, has neverbeen in doubt, the nature of the trabeculae is still a matter fordiscussion.

Huxley (1874 and 1875) regarded the trabeculae as part of thesplanchnocranium, and as belonging to the same class of struc-ture as the visceral arches. Parker (1878) was first of theopinion that only the facial or anterior part of the trabeculae

702 G. R. DB BEER

was of visceral origin; then he accepted Huxley's view that thewhole of the trabecular rods was visceral; and finally he returnedto his first opinion. Platt (1898, p. 422) thought that 'thetraheculae were primarily bars of a prae-oral visceral arch', and,more recently, Allis (1923,1924, and 1931), has upheld the viewthat the trabeculae are visceral. While he identifies the posteriorportion or polar cartilage of van Wijhe (1904) as the pharyngealelement of the mandibular visceral arch, he considers that theanterior portion or trabecula sensu s t r i c t o represents thedorsal half of the premandibular arch.

Against these opinions, it must be mentioned that van Wijhe(1922) and Goodrich (1930) are disinclined to accept the viewthat the trabeculae are visceral, and consider them as structuressui g e n e r i s , developed in relation to the forebrain.

The problem may therefore be stated in the following terms:(1) Are the trabeculae cranii portions of the chordal skeleton,formed from the sclerotomes of mesodermal somites, and of anature similar to that of the parachordal cartilages? And ifnot: (2) Are the trabeculae cranii visceral structures, formedfrom the mesenchyme of the visceral arches, and of a naturesimilar to that of the splanchnocranium ?

The answer to the first of these questions necessitates anexcursion into a number of fields of zoological inquiry, andforemost among these is the problem of the localization of themorphological anterior extremity of the head, and its relationto the foremost point of the axial skeleton.

It has been held for some time that the true morphologicalanterior extremity of the head of a chordate animal is to befound in the vicinity of that region in which the hypophysis,the hinder point of closure of the neuropore, the pre-oral gut(Seessel's pocket), the tip of the notochord, and the transversecommissure between the two premandibular somites are to befound. This region is, therefore, to be regarded as derived fromthe animal pole of the egg: a view which is supported byexperimental results. The observations of Stockard (1910) onthe effects of certain toxic substances to which very early stagesof development of the fish Pundulus are subjected show thatfish so treated are deficient in that they lack the optic chiasma

TRABECTJLAE CRAN1I 708

and the region between the eyes, and present the appearanceof c y c 1 o p i a. It is to be noted that the missing region corre-sponds very closely to that which is held on morphologicalgrounds to represent the anterior end of the head. These resultshave been confirmed in the frog by Bellamy (1919). Treatmentwith toxic substances is known to exert a differential effect onthe tissues of the animal pole of the egg and, since this treatmenthas resulted in the absence of certain structures, there is experi-mental evidence for the view that the missing region (opticchiasma and its neighbourhood) represents the morphologicalanterior extremity of the head.

Attention may now be turned to the question as to how farforward the axial skeleton extends. Since it is derived frorn thesclerotomes of the mesodermal somites, its anterior end must belooked for in the region of the first somite, which is situated inthe first or premandibular segment. It is to be rememberedthat there is firm evidence for homologizing the premandibularsomites of Craniates with the anterior head-cavities of Am-p h i o x u s (Goodrich, 1917), and therefore there is no groundfor the view that there ever were any segments in front of thatwhich is recognized in Craniates as the premandibular segment.

The premandibular somites are interconnected with oneanother across the middle line by a commissure which is formedby being pulled off from the roof of the pre-oral gut by the tip ofthe notoehord (de Beer, 1926). This commissure is thereforesituated precisely in the region regarded as the anterior end ofthe head, and it has been observed to coincide closely with thestructure known as the acrochordal, and even to give rise to it.The acrochordal does not chondrify in Selachii (van Wijhe,1922), but it does in birds (Sonies, 1907, and Jager, 1924). Now theacrochordal forms the dorsum sellae turcicae or ridge overhang-ing the pituitary fossa from behind, and it is situated at theextreme front end of the basal or parachordal plate. Further,the acrochordal is situated in line with the pilae antoticae andis continuous with the base of these structures, which, them-selves, are attached to the foremost edge of the basal plate oneach side. There is, therefore, definite evidence that the mostanterior part of the chordal skeleton is formed by the acrochor-

704 G. R. DE BEER

dal, at the front of the parachordal or basal plate. It follows,therefore, that since the trabeculae are in front of the acro-chordal, whatever they may be, on the existing evidence theyare probably not part of the chordal or sclerotomic skeleton.

Attention may now be turned to the second question for-mulated above. If the trabeculae are of visceral origin, theymust form part of a visceral arch in front of the mandibulararch. That such a premandibular arch did exist in earlyChordates would seem to follow from Stensie's descriptions ofcertain Ostracoderma. Now, if a pair of premandibular archeswere present in the embryo of a Craniate, it is difficult to seewhat region they would occupy other than that which is actuallytaken up by the trabeculae. It might be said that the suitabilityof the position of the trabeculae in Selachian embryos forregarding them as visceral arches is assisted by the pronouncedcranial flexure, as a result of which the trabeculae lie almost atright angles to the parachordals and parallel with the recognizedvisceral arches. But in the Teleostei, Anu r a, and U r o d e 1 a,where the trabeculae arise almost in line with the parachordals,their position is no less suitable for regarding them as a pair ofpremandibular visceral arches, and, curiously enough, in theseforms the recognized visceral arches slope forwards and down-wards in such a way that they are not far off parallel with thetrabeculae. These relations are indicated in Text-fig. 1.

Allis's (1923, 1924, and 1931) claim that the trabeculae arevisceral is based on morphological considerations. Arguing fromthe fact that the position of the primitive wall of the skull asa brain-case is indicated by the thick nieningeal membrane ordura mater, he has shown that a space—the subpituitary space—through which the pituitary vein passes, is external andventral to the dura mater, although it is enclosed above thetrabeculae in that cavity, which, neglecting the dura mater, isusually and loosely known in the dry skull as the cranial cavity.The trabeculae are attached to the primitive wall of the skull—the parachordals—behind. In front of this point the trabeculaeenclose the subpituitary space between themselves and the duramater. In front of this, again, the trabeculae bend upwards andcome into contact with the dura mater. These relations (which

TRABECULAE CEANII 705

are indicated in the text-figures illustrating the writer's paper onthe Development of the skull of S c y 11 i u m) are impossible tounderstand if the trabeculae are regarded as elements formingpart of the original cranial wall, whereas they become easy tointerpret if the trabeculae are visceral arches which have becomeattached by their dorsal and posterior end to the front of thechordal skeleton—the parachordals—and have entered intorelations with the forebrain.

A large part of the forebrain, including all structures in frontof the recessus preopticus, must be regarded as the result of aforward expansion of dorsal regions of the neural tube whichoriginally lay behind the morphological anterior extremity ofthe head. In forms in which the forebrain is but slightlydeveloped, the cartilages of the chordal skeleton—the para-chordals—would be sufficient to form a foundation for theneurocranium. As will be shown later, there is reason to believethat the C y c l o s t o m a t a are in this primitive condition. Butwith the expansion and enlargement of the forebrain which ischaracteristic of G n a t h o s t o m a t a , the concourse of thetrabeculae would become necessary for the enclosure of the brain.It may be urged as an objection to this view that it is difficultto understand how or why a visceral structure could enter sointimately into the formation of the brain-case as does thetrabecula. To this may be answered, first, that an analogouscase is provided by the processus ascendens, ala temporalis,and Mammalian alisphenoid, structures of undisputed visceralorigin which have nevertheless come to play an important partin the limitation of the brain-case; and, second, that the trabe-culae invariably arise as elements separate and distinct fromthe parachordals: a fact which is so commonplace as to beoverlooked, and which indicates that the intimacy of the rela-tions into which the trabeculae eventually get with the para-chordals must not be exaggerated.

Thus far Allis's case is strong and his theory falls into linewith a number of other views. But he goes farther, and wishesto identify the polar cartilage with the pharyngomandibular,the trabecula sensu s t r i c t o with the dorsal half of thepremandibular arch, and the palatine process of the pterygo-

b a

cb

PIMe' ch

TEXT-FIG. 1.

Diagram prepared from camera luoida drawings of victoria bluepreparations of the chondrocranium showing the relation of thetrabeculae to other structures in A, Scyllium;JS, Salmo; andC. Tr i ton .

TRABBCULAE CRANII 707

EXPLANATION OF LETTERING.

as, auditory sac; c, remnant of transverse commissure between pre-mandibular somites; ca, cerebral artery; cb, ceratobranchial cartilage;ch, ceratohyal cartilage; cs, connecting strand between trabecula andmesenchymatous plaque; e, eye; eb, epibranchial cartilage; er, externalrectus muscle; fb, forebrain; /</, foregut; fn, facial nerve; gn, glossopharyn-geal nerve; gsl-5, gill-slit 1-5; h, hypophysis; ha, hyoid arch; lib, hind-brain ; hs, hypophysial stalk; hy, hyomandibular cartilage; i, infundibulum;io, inferior oblique muscle; ir, inferior rectus muscle; itr, internal rectusmuscle; Ij, lower jaw; m, mesenchyme proliferated beneath hypophysisand forebrain by maxillary process; ma, mandibular arch; mb, midbrain;Me, Meckel's cartilage; mm, visceral muscle-plate of mandibular arch;mp, maxillary process; ms, mandibular visceral cleft; mv, mandibularblood-vessel; n, notochord; ns, nasal sac; nt, neural tube; o, orbitalcartilage; on, optic nerve; p, parachordal cartilage; pb, pharyngobranchialcartilage; pc, polar cartilage; ph, pharynx; pi, mesenchymatous plaque;pma, premandibular arch; pn, profundus nerve; ppg, peripharyngealgroove; pq, pterygo-quadrate cartilage; rh, remnant of hyoid pouch;rMc, rudiment of Meckel's cartilage; roc, rudiment of orbital cartilage;rpc, rudiment of polar cartilage; rpq, rudiment of pterygoquadrate cartilage;rr, rudiment of rostral cartilage; rt, rudiment of trabecula cranii; si, firstor premandibular somite; s2, second or mandibular somite; sh, side-wallof hypophysial sac; sp, spiracular slit; spp, spiracular pouch; sr, supra-rostral cartilage; st, stomodaeum; stp, lateral pouches of stomodaeum;su, ventral sucker; t, trabecula cranii; tc, transverse commissure betweenpremandibular somites; til, trigeminal nerve; ul, upper lip; v, velum;vn, vagus nerve.

quadrate cartilage with the ventral half of this arch. Here heis on much more uncertain ground; for one thing, there isanother element which is a claimant for the position of pharyngo-mandibular (see Sewertzoff and Disler,' 1924), and, for another,there seems to be no reason for regarding the palatine processof the pterygo-quadrate as anything but a secondary develop-ment of that cartilage. It seems wiser, therefore, not to burdenthe hypothesis of the visceral nature of the trabeciilae with thesespeculations of more doubtful value.

Stone (1926) has performed experiments which, althoughdifficult to interpret in one sense, nevertheless have an im-portant bearing on the present problem. He removed the neuralcrest from the head of embryos of A m b l y s t o m a , and foundthat not only did such embryos show a deficiency in the carti-lages of the visceral arches on the operated side, but that the

708 G. R. DE BEER

trabeculae were also deficient, and did not extend farther for-ward than the optic nerve. Prom the present point of view,it is unnecessary to consider the controversial question as towhether visceral cartilage is derived from the cells of the neuralcrest; the essential part of Stone's results is the fact that theoperation has produced the same effect on the trabeculae ason the visceral arches, and no effect on undoubted sclerotomicor chordal cartilage. It may be inferred, therefore, that thetrabeculae are of a nature different from that of the chordalcartilages and similar to that of the visceral cartilages.

From the considerations dealt with above, it will be apparentthat there is a considerable body of inference on which to founda p r i m a facie case for the view that the trabeculae arevisceral structures. Attention may now be turned to the actualobservations which have been made on the origin of the trabe-culae. Such evidence is astonishingly meagre. Starting withthe fish, Sewertzoff (1916, p. 17) states that in A c i p e n s e rthe trabeculae do not arise from the premandibular somites,and (1917) that the sclerotomes of the foremost mesodermalsomites in A c i p e n s e r give rise to the parachordals.

According to Haller (1923) the mandibular arch of theSelachian embryo (Squalus) is to be regarded as composedof four portions which are externally distinguishable. Theseare the Kieferaugenspaltstuck, Oberkiefersttick, Zwischenstiick,and Unterkieferstiick. The latter three portions contain materialwhich will give rise to the skeleton of the mandibular arch(pterygo-quadrate and Meckel's cartilage). But the first or mostdorsal portion is said to contain procartilage which gives riseto the posterior region of the trabecular bar, i.e. to the polarcartilage.

A pair of maxillary processes then make their appearance,' growing inwards and forwards from the top of the mandibulararch, pushing their way between the skia and the floor of thebrain. In this manner the floor of the brain comes to lie at somedistance beneath the superficial epidermis, and Rathke's pocketor the hypophysial invagination becomes relatively deepened,as Woerdeman (1914), Haller, and Allis (1923) stated. Theanterior portion of the trabecular bars (trabecula sensu

TEABBCULAE CEANII 709

s t r i c to ) arises from mesenchyme in these maxillary processes,and must therefore he held to be of visceral origin.

That the origin of the trabecula in the Teleost is curiousemerges from Lundborg's (1899) attempt to show that inS a 1 m o it arose from the ectodermal lining of the roof of thestomodaeum! Further, he attributed the same method of originto the trabecula in E a n a. Concerning the latter animal, thereis a careful study by Spemann (1898) which is, however, notconcerned with the details of the origin of the mesenchymalcondensations under consideration in the present study.

As regards U r o d e l a , Sewertzoff (1916) states that hisresearches on these forms (as also Filatoff s on Eeptilia) ledhim to the same conclusion as those on A c i p e n s e r , viz. thatthe trabeculae do not arise from the premandibular somites.Platt (1893, 1898) has devoted a considerable amount of studyto the problem of the origin of the trabeculae and visceralskeleton of Necturus. She believes that the anterior part ofthe trabeculae and the cartilages of the visceral arches arisefrom mesenchyme, and not from segmental sclerotomes. It istrue that she is also concerned with the origin of this mesen-chyme from the neural crest, which is another question, withwhich the present paper has nothing to do. But, from the presentpoint of view, the essential feature of Platt's results is thataccording to her the trabeculae (or rather the anterior part ofthem) arise from mesenchyme, and her results are confirmedin a remarkable way by Stone's (1926) experiments. Platt alsodraws attention to the fact that a histological distinction existsbetween the rudiments formed from the sclerotomes, which con-tain yolk-granules, and those which form the anterior part ofthe trabeculae and visceral arches, which contain no yolk-granules. Further, the nuclei in the cartilage derived fromsclerotomes are farther apart than in the other. A similarhistological distinction between the anterior part of the trabe-culae and the cartilage of the basal plate was reported by Stohrin T r i t o n (1880). The structures which Platt calls 'theposterior part of the trabecular bars', and Stohr the 'Balken-platte', would seem to be nothing but the anterior horns of theparachordals.

710 G. R. DE BEER

In view of the difference of opinion and uncertainty whichexists in regard to the nature of the trabeculae, it seemedadvisable to reinvestigate the matter, and I have accordinglystudied their origin in four types, viz. Scy l l i um c a n i c u l a ,Salmo f a r io , E a n a t e m p o r a r i a , and A m b l y s t o m at i g r i n u m. This work was done in the Department of Zoologyand Comparative Anatomy of the Oxford University Museum,where I had the privilege of using the admirable series of develop-mental stages belonging to the late Dr. J. W. Jenkinson and toProfessor E. S. Goodrich, F.B.S., as well as my own. Forpurposes of illustration I have had recourse to photomicro-grams, for in a study of this nature, which consists largely ina hunt for mesenchymal condensations, drawings by hand areboth more laborious and less satisfactory. I wish to express mygratitude to my colleague Mr. J. Z. Young, of Magdalen College,for the assistance which he has kindly and willingly affordedme in the preparation of these photomicrograms, and toProfessor Goodrich I would like to acknowledge my indebted-ness for his helpful criticism and encouragement.

2. OBSERVATIONS.

(i) S e l a c h i i .

Since the trabeculae are said to arise in connexion with themaxillary processes, it seemed advisable at the outset to in-vestigate the origin of these maxillary processes, and to obtaina clear picture of the geography of the region in which thetrabeculae will make their appearance.

Fig. 1, PI. 40, is of a median longitudinal section throughan embryo of Scy l l ium at stage J, for comparison withfig. 2 (on the same plate), which is a similar section of anembryo 26 mm. in length, corresponding roughly to stage 0.The chief point to notice is that in the latter the hypophysialcavity is much longer and deeper, and there is a considerablequantity of mesenchyme ventroposterior to it. Just in frontof the opening of the hypophysial sac there is also more mesen-chyme than at the previous stage, with the result that the fore-brain in this region is farther removed from the superficialepidermis. The question is, where has this mesenchymatous

TRABBCULAE CRANII 711

tissue come from? This matter is not easy to determine withcertainty, but there is little doubt that it is contributed largelyfrom each side. Fig. 3, PI. 40, is of a transverse section, which,owing to the cranial flexure, cuts the ventral surface of the headtangentially. This section and that shown in fig. 4, PL 40, arefrom embryos at stage M. The sides of the hypophysial sac areindicated by a thickening of the epidermis, and they are stillrelatively far apart. Immediately lateral to them can be seena heaping-up of mesenchyme which gives the appearance ofextending towards the middle line, which would, of course, havethe effect of narrowing the hypophysial sac and carrying itsaperture farther forward. This can be seen in fig. 4, PI. 40,which is of a section horizontal to the body, and thereforetransverse to the head. From both figs. 3 and 4, PI. 40, it isobvious that the mesenchyme which grows in towards the middleline in this manner is related to the mandibular visceral arch,which latter is identified among other things by the plate ofvisceral muscle and the mandibular blood-vessel which willeventually become the efferent pseudobranchial artery. At thesame time, it is clear that no proliferations of mesenchyme fromthe premandibular or any other somite take part in the forma-tion of the maxillary processes.

Eventually the two maxillary processes meet beneath theforebrain, and, in front of the aperture of the hypophysial sac,a median raphe is discernible in the mesenchyme. Behind theaperture of the hypophysial sac no such raphe is visible, whichmeans that there has been a growth forwards of mesenchymebeneath the hypophysial sac, accompanying the ingrowth ofthe maxillary processes.

The result of this preliminary investigation is, therefore, toconfirm the accounts given of the origin of the maxillary processand deepening of the hypophysial sac by overgrowth given byWoerdeman (1914) and Haller (1923), and to justify to a certainextent Allis's (1923) contention that the hypophysial pit is 'theresult of the folding or rolling together of two ectodermal sur-faces and their fusion with each other excepting along the medianline'. Allis contends that the hypophysial pit arises by noimagination at all, but by overgrowth. In a matter of this kind

712 G. R. DE BEER

and where there are no immutable fixed points it is alwaysdifficult to decide how much of a pit is due to invagination andhow much to overgrowth. That overgrowth plays a large partin the formation of the hypophysial sac in Selachii is clear, butit seems hard to deny to invagination any part of its formation,especially as in other forms such as the Teleostei and Amphibiathere is no doubt of its development as a definite ingrowth,albeit solid. This matter has, however, no direct bearing on theproblem at issue, and the conclusion so far arrived at is thatmesenchyme derived from the mandibular visceral arch findsits way to a position on each side of the hypophysial sac,between the floor of the forebrain and the epidermis of theventral side of the head.

Fig. 5, PI. 41, is of a transverse section passing tangentiallythrough the ventral side of the head in an embryo 23 mm. inlength, just cutting the floor of the forebrain and the opticnerve. On each side of the forebrain can be seen a mesenchyma-tous condensation in the maxillary process, and close examina-tion reveals the fact that it is a double condensation, consistingof the rudiment of the palatine process of the pterygo-quadrateexternally, and of the rudiment of the trabecula internally. Atan earlier stage (19 mm.) these two rudiments are indistin-guishable, and at 23 mm. they are still intimately connectedby mesenchyme a little less dense. It is important to noticethat these rudiments, of the palatine process of the pterygo-quadrate as well as of the trabecula, are condensations in s i t uof the mesenchyme of the maxillary process; their cells arecontinuous by gradual transition with the cells of the remainderof the mesenchyme, and there is no evidence of proliferationfrom any other source. Fig. 6, PI. 41, showing a section trans-verse to the head, is from an embryo 23 mm. long but slightlymore developed than the previous one, and gives the relationsof the trabeciilar rudiment to the optic nerve. The rudiment ofthe inferior oblique muscle derived from the premandibularsomite is present, but it enters into no relation with the trabe-cular rudiment.

Fig. 7, PL 41, is of a paramedian longitudinal section throughan embryo 25 mm. long, showing the relations of the trabecular

TRABECULAE CRANII 713

rudiment to the optic nerve and other structures in the head.The connexion between the trabecula and the palatine processof the pterygo-quadrate is still evident, and it is also clear thatif the side of the mouth (as cut in the section) be regardedas occupying the position of a mandibular visceral cleft, thetrabecular rudiment occupies a position serially comparablewith that of the skeletal elements of the mandibular and pos-terior visceral arches.

Fig. 8, PL 41, is also from a 25 mm. embryo, and it shows thatthe trabecular rudiment and the palatine process of the pterygo-quadrate are now more distinct. The rudiment of the polarcartilage is also present. This element has been shown (thisvolume, p. 591) to chondrify in contact with the foremost part ofthe parachordal, and the present investigation indicates thatits procartilaginous rudiment also arises in that position. Ihave not been able, however, to satisfy myself from the materialat my disposal that this polar cartilage rudiment arises fromthe mandibular arch, probable though it may be.

Fig. 9, PL 42 (26 mm. stage) is not much more advanced thanthe previous one; the trabeculae extend farther backwards(upwards in appearance, owing to the cranial flexure) withoutconnecting with the polar cartilage rudiments, and the para-chordals are now definitely cartilaginous.

Fig. 10, PL 42, shows the relations of the trabeculae in asection transverse to the head of an embryo 26 mm. long, cutslightly obliquely. The trabeculae are some distance beneaththe dura mater, and are connected to one another by a thinstrand of mesenchyme which includes the disappearing duct ofthe hypophysial sac. Figs. 11 and 12, PI. 42, are of transversesections of an older embryo, 29 mm. long, in which the trabe-culae are now definitely cartilaginous, continuous with the polarcartilages and parachordals, and forming the floor of the pre-chordal part of the brain-case. Except in the region of the sub-pituitary space, the trabeculae have now approximated moreclosely to the dura mater, although still lying outside it.

Summing up, therefore, the evidence obtained from a studyof the Selachii, the following points lend support to the viewthat the trabeculae are visceral structures:

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714 G. R. DE BEER

The origin of the maxillary processes from the visceralmesenchyme associated with the mandibular arch;

The origin of the trabeculae as condensations in s i t u in themaxillary processes;

The community of origin of the mesenchymatous condensa-tions of the rudiments of the trabeculae and of the palatineprocesses of the pterygo-quadrate;

The absence of any evidence of proliferation from the pre-mandibular somite.

(ii) T e 1 e o s t e i.

In S a 1 m o f a r i o the conditions of the origin of the maxillaryprocesses are more difficult to determine; for one thing thehypophysis arises as a solid ingrowth, and it is, therefore, notpossible to estimate the relative importance of invagination andovergrowth as in the Selachii; for another, the notochord doesnot extend so far forward as in other forms, and its anteriorend appears to be undergoing reduction. This is evident fromText-fig. 2. The position of the transverse commissure connect-ing the premandibular somites of Selachii and of Ami a (deBeer, 1924) is indicated by a strand of mesenchyme continuouswith the rudiments of the eye-muscles and extending inwardsto a point on the hind-wall of the infundibulum. The gapbetween this point and the tip of the notochord at this stageis probably not unconnected with the subsequent formationof the myodome.

On each side of the ventral surface of the head, lateral to thehypophysis and median to the eyes, there is a plaque of thickenedmesenchyme, in close contact with the epidermis. Posteriorlythese plaques are continuous with the mesenchyme of themandibular arch, as may be seen in fig. 14, PI. 43, which isof a transverse section (trout 25-2) cut slightly obliquely, andshowing the angle of the lower jaw on the right side. From theinner side of each of these mesenchymatous plaques, at aboutthe level of the hypophysis, the trabeculae are proliferated in-wards, upwards, and backwards, as shown in fig. 13, PL 43(trout 24-2). Posteriorly, the mesenchyme of the trabecularrudiment becomes less dense, and it comes into contact with

TEABBCULAE CBANII 715

the strand of mesenchyme which, as described above representsthe transverse commissure between the two premandibularhead cavities. It might be supposed that this contact wasevidence for the origin of the trabeculae from the transversecommissure and so from the premandibular somite. But theconnexion of the trabecular rudiment with the mesenchymatousplaque is so dense and intimate, while that with the transverse

TBXT-PIO. 2.

Median longitudinal section through the head of an embryo ofSal mo fario (trout 23) showing the relation of the hypophysis,notochord, and transverse commissure between the preniandibulursomites.

commissure is so ephemeral, that there can be little doubt thatit is from the former and not from the latter that the trabeculaeare formed. There is no connexion whatever between themesenchymatous plaque and the transverse commissure of thepremandibular somites, and at an earlier stage (trout 24-1), inwhich the trabeculae are not yet present, there is no suggestionthat they will arise from the transverse commissure. It isfurther to be noticed that the orientation of the nuclei in the

716 G. R. DB BBBK

transverse commissure and in the trabecular rudiments isdifferent. If the transverse commissure represents the acro-chordal of other forms, there would be nothing surprising inthe fact that the hinder ends of the trabeculae came into contactwith it.

Another point of interest is the distance which separates thetrabecular rudiments from the floor of the brain or infundi-bulum, as is evident from all the figures on PI. 43.

Eig. 15, PL 43, shows the conditions as seen in a paramedianlongitudinal section (trout 23 long.). Here, again, it is clearthat if the angle of the mouth is regarded as occupying theposition of a former mandibular visceral cleft, the mesenchy-matous plaque from which the trabecular rudiments arise is inthe correct position to represent a premandibular visceral arch.

Fig. 16, PI. 43, is of a transverse section through an embryoslightly older than the foregoing (trout 25 A). The trabeculaeare just cartilaginous, and they are still connected by a strandof dense tissue with the mesenchyme of the mandibular arch.This strand is a remnant of the mesenchymatous plaque of theprevious stages, and its interest lies in the fact that in it therudiments of the palatine processes of the pterygo-quadrate willmake their appearance. At the present stage, however, they areabsent, and the only skeletal structures of the palate are thetrabeculae.

Yig. 17, PI. 43, is from a still older embryo (trout 26-1); thetrabeculae are at a considerable distance from the floor of theinfundibulum, and the connecting strand is still evident. Infig. 18, PI. 43 (trout 29 II), which is of a section slightly fartherforward than that of fig. 17, the trabeculae are seen approximat-ing to one another in a manner which will eventually give riseto the trabecula communis, and the palatine process of thepterygo-quadrate has appeared in the connecting strand.

The evidence provided by the Teleostei concerning the originof the trabecula leads, therefore, to the same conclusion as thatprovided by the Selachii, and largely for the same^ reasons.Although the hinder ends of the trabeculae soon come intorelation with the transverse commissure of the premandibularsomites, the evidence points to their origin from a mesenchy-

TRABECULAE CRANII 717

matous plaque, itself related to the mandibular arch and fromwhich the palatine process of the pterygo-quadrate will eventuallyarise. The trabeculae in the trout are even more aloof from thedura mater than they are in Scyl l iu rn .

(iii) A n u r a .

The first appearance of the rudiments of the trabeculae inE a n a t e m p o r a r i a is shown in fig. 19, PI. 44, and they takethe form of slender condensations of mesenchyme parallel withthe pterygo-quadrate and half-way between the latter and thefloor of the forebrain (frog A A 2). These rudiments are nothingbut faint condensations in s i tu of the mesenchyme, and theyenter into no relations with the derivatives of the premandibularsomites. Par forward, in the region of the upper lip, are therudiments of the suprarostrals, and, as Spemann (1898) em-phasized, they are independent of the trabeculae at this stage.

The trabeculae soon enter into relations with the pterygo-quadrate, and fig. 20, PL 44, is of a section through a frog embryo(frog AA 3) passing through the commissura quadratocranialisanterior of Gaupp's nomenclature. The condensations of thetrabecular and pterygo-quadrate rudiments are here seen to becontinuous. Fig. 21, PI. 44, is from the same embryo, at a moreposterior level, and shows the relation of the trabeculae to theoptic nerve and the floor of the forebrain. The same is shown inhorizontal section in fig. 22, PI. 44 (frog II B). Posteriorly, thetrabecular rudiments are now continuous with the 'Balken-platte' of Stohr's (1882) description, which probably representthe foremost parts of the parachordals. Anteriorly, the trabe-cular rudiments connect with those of the suprarostrals. Thisis also shown in fig. 23, PI. 44, which is of a transverse section(frog B T). The suprarostrals were considered by Stohr (1882)to represent the foremost portions of the trabecular bars,but Spemann (1898) contests this view on the grounds thatprocartilaginous continuity between two elements is no proofof their genetic affinity. The question is difficult to decide, andthe suprarostrals seem to be structures s u i gene r i s developedin relation to the conditions of the larval mouth of the tadpole.That the suprarostral elements are visceral structures there can

718 G. R. DB BEER

be no doubt owing to their position and their mode of origin, farin front of the premandibular somites. That they should beconnected with the trabeculae by procartilage may, as Spemannsays, be no proof of their affinity, but it leads to the sus-picion that the trabeculae have visceral affinities. It may besaid that the trabeculae become intimately connected with theparachordals, and that, as the latter are certainly not visceralstructures, the criterion of connexion means nothing. But itmust be remembered that, at early stages of development, thetrabeculae are no t connected with the parachordals byprocartilage.

Fig. 24, PI. 44, is from the same embryo as fig. 23, PL 44, butthe section is taken more posteriorly. The trabeculae are nowdefinitely cartilaginous and are pressed close against the duramater, a position which they did not previously occupy. Oncomparing fig. 24, PI. 44, with figs. 19 or 21, PI. 44, it wouldseem that the connexion of the trabeculae with the dura materis due to the expansion of the brain and its overlying duramater, the trabeculae remaining relatively passive.

The evidence provided by the A n u r a concerning the methodof origin of the trabeculae cannot be regarded as conclusive, forthe conditions do not easily lend themselves to a determinationof the origin of the mesenchyme of the maxillary process. Butthe origin of the trabecular rudiments in s i tu in this mesen-chyme, in a manner identical with that of the pterygo-quadrate,their relations with the pterygo-quadrate in the form of thecommissura quadratocranialis anterior, and their relations withthe suprarostrals, argue in favour of their visceral nature.

(iv) TJrodela.In A m b l y s t o m a the trabecular rudiments are first found

in the form of elongated mesenchymatous condensations runningforward beneath the optic nerves and median to the eyes andnasal sacs. Eig. 25, PI. 45, is of a horizontal section (Axolotl A I)in which the rudiment is just discernible, and all that can besaid of it is that it constitutes a very slightly condensed regionof the general mesenchyme of the head.

Fig. 26, PL 45 (Axolotl B. T. I.), is of an embryo slightly more

TRABECULAE CRANII 719

developed, and the trabeeular rudiments take the form of rodsthe diameter of which is made up of three or four cells. Theinterest of this specimen lies in the fact that the rudiments canbe seen to be continuous with the adjacent mesenchyme, andin many cases it is hard to say whether any given cell formspart of the trabeeular rudiment or not. Here, again, therefore,there is evidence that the trabeeular rudiments arise as con-densations in s i t u , and it is necessary to conclude that thesource from which the trabeculae arise has also produced allthe mesenchyme of this region of the head. Since there is noreason to believe that this mesenchyme has been produced bythe sclerotomes of the mesodermal somites, it must be inferredthat the trabeculae are not formed from the somites either.

Kg. 27, PI. 45 (Axolotl Aoo II), is of a longitudinal para-median section showing the relations of the mesenchyme fromwhich the trabeeular rudiments arise to the optic nerve and tothe mouth and visceral arches. Kg. 28, PI. 45, is from a slightlyolder embryo in which chondrification has just set in (Axolotlh. L), and by studying the section of this embryo it can be seenthat the trabeculae do not lie in a perfectly straight line in frontof the parachordals, but dip downwards as they run forwards,thus lying more or less parallel with the general trend of thevisceral arches.

Kg. 29, PL 45 (Axolotl B. J. K.), is a transverse sectionthrough an older embryo showing how the trabeculae acquirerelations with the dura mater by the infundibulum dippingdown between them, as in the frog. There is in A m b 1 y s t o m ano palatine process of the pterygo-quadrate, and therefore thetrabeculae are the only cartilaginous skeletal structures in the

On the whole, the evidence from U r o d e l a confirms Platt'sview that the trabeculae are not derived from the sclerotomes,but arise from mesenchyme which can hardly be anything otherthan visceral.

3. DISCUSSION.

The evidence derived from a study of the four types describedin this paper provides a convergence of results •which accords

720 G. R. DB BEER

well with the view that the trabeculae are not derived fromsclerotomes, but from visceral mesenchyme, belonging presum-ably to the premandibular arch. It now remains to considersome of the implications of this view.

If the trabeculae (and by this term is meant the trabeculaesensu s t r i c t o , for it has not been possible to come todefinite conclusions regarding the polar cartilages) representpart of the cartilaginous skeleton of the premandibular arch,important changes must have taken place with regard to themouth and the anterior region of the gut in those forms in whichthe trabeculae contribute to the formation of the brain-case.Originally the trabeculae must, on the hypothesis that they arevisceral arches, have surrounded the anterior end of the gut,and the mouth opening must have passed backwards, as it were,between their legs. It may be imagined that there were ventralelements in the premandibular arch, corresponding serially toMeckel's cartilage and the ceratohyal.

Behind the trabeculae or premandibular arch there musthave been a mandibular visceral cleft to separate the premandi-bular from the mandibular arches, and this cleft must have beenrelated to the pre- and post-trematic branches of the nerve ofthe mandibular arch (the maxillary and mandibular branchesof the trigeminal nerve) exactly as the spiracle is related tothe pre- and post-trematic branches of the facial nerve, and thegill-slits are related to the glossopharyngeal and vagus nerves.The profundus nerve would then be the nerve of the premandi-bular arch, which is very appropriate, since its correspondingventral root (the oculomotor) innervates the premandibularsomite, and this profundus nerve would not be expected todivide into pre- and post-trematic branches, for there was novisceral cleft for it to fork over. The dorsal ends of the skeletalelements of the premandibular and following visceral archesmay be regarded as having been more or less firmly attached tothe axial skeleton in the form of the notochord and its sheaths.

An idea of this state of affairs may be obtained from a con-sideration of the conditions which obtain in A m p h i o x u s ,although that animal is considerably modified from what theoriginal chordate type must have been. It is to be noted that

tn Fn gn vn nt r,

'msm4pha"3s'pn LP f

qs5nt

SPP

qs 5

tiTEXT-FIG. 3.

Diagram showing the relations of the mouth, visceral arches,visceral pouches and clefts, and dorsal nerve-roots in A, hypo-thetical ancestral chordate; £, Ammocoete larva of P e t r o -my zon; and C, embryo of Scyl l ium, to illustrate the theorythat the trabeculae are premandibular visceral arches.

722 G. R. DB BEER

the mouth in the adult A m p h i o x u s is a perforation througha vertical membrane, the velum, and that it leads into the fore-most part of the gut; i.e. there is no pre-oral gut. Added to thisit must be noted that the neural tube of A m p h i o x u s doesnot extend farther forward than the position of the neuropore.The extreme anterior extension of the notochord must beassociated with the secondary formation of the anterior fin,since originally the notochord cannot have extended beyondthe morphological anterior end of the body, represented by apoint close to that of the neuropore. The relations of the essentialfeatures of the hypothetical ancestral chordate are shown inText-fig. 3 A.

The origin of the Craniates, and particularly of the Gnatho-stomes, was accompanied by the formation of a large andexpanded anterior region of the neural tube, giving rise to thebrain. This expansion took place in a remarkable way, for theposition of the neuropore remained close to the hypophysis andthe tip of the notochord, and it was the dorsal part of theneural tube, behind the neuropore, which bulged forwards anddownwards, eventually coming to lie in front of the point ofclosure of the neuropore, giving rise to the forebrain. Practi-cally speaking, the neural tube became U-shaped, with theconcavity ventral, forming the plica encephali ventralis.

The formation of the forebrain, and especially of the telen-cephalon, in this manner must have resulted in the displacementdownwards and backwards of the mouth opening. The stomo-daeum, instead of being anterior and opening straight backbetween the premandibular arches, would come to be situated onthe ventral side of the head. It would no longer open into theextreme anterior end of the gut, but slightly farther back andbeneath this foremost portion, which would then become whatis known as the pre-oral gut. The skeletal elements of the pre-mandibular arch would no longer lie astride the mouth opening,for the latter would have been withdrawn from under them;they would lie on the anterior dorsal wall of the stomodaeum,behind and beneath the newly formed telencephalon. Thedorsal portions of the skeletal elements of the premandibulararch would thus become the trabeculae cranii, while the ventral

THABECULAB CRANII 723

elements would presumably disappear. These relations areshown in Text-fig. 3 c.

By this ventral and posterior displacement of the stomodaeumthe hypophysial sac would be rotated so that it pointed upwards,between the trabeculae. Another important result of this dis-placement would have been the approximation of the mouthopening to the mandibular visceral clefts, for the latter wouldno longer be separated from the former by a visceral arch, thestomodaeum having slipped beneath and behind the premandi-bular arch. It might be supposed that the mouth actuallybecame confluent with the mandibular clefts, and there wouldthen be an explanation of the fact that the lateral portions ofthe mouth of G n a t h o s t o m a t a (the 'angles') bear relationsto the trigeminal nerve which suggest, as Johnston says (1907,p. 22), that the maxillary and mandibular branches representpre- and post-trematic rarni of a branchial nerve. But it mustbe remembered that the maxillary and mandibular branches ofthe trigeminal nerve differ from a typical branchial nerve inthat they lack the visceral sensory component: A more satis-factory explanation is that in the G n a t h o s t o m a t a themouth has extended sideways and upwards behind the pre-mandibular arch, obliterated the mandibular visceral pouch andcleft, and occupied its position. With the disappearance of theessential feature of a visceral cleft—the endodermal mandibularpouch—the visceral sensory component of the trigeminal nervewould have disappeared also, leaving the general cutaneous andvisceral motor components, forking as pre- and post-trematicrami round the angles of the mouth, which angles have usurpedthe position of the former mandibular cleft.

It may be mentioned that Dohrn (1882, p. 255) claimed tofind representatives of a pair of visceral clefts in the mouth ofcertain fish. He was concerned with the since-abandonedproblem of finding for the vertebrates a mouth different fromthat of their then-supposed Annelid ancestors. He stated thatin Gobi u s , H i p p o c a m p u s , and B e 1 o n e there was apair of endodermal pouches extending towards the skin fromthe fore-gut, in front of the hyoid or spiracular pouches, and thatthe mouth first became perforated on each side in connexion

724 G. E. DB BEER

with these ' mandibular' pouches, and subsequently becameperforated in the middle line. A similar state of affairs wasreported by Platt (1891) for Batrachus tau. More recently, apair of prespiracular endodermal pouches has been describedin L e p i d o s t e u s by Veit (1924) and van Schrick (1927).But these pouches subsequently become reduced, and absorbedinto the hyoid pouches. The presence of these pouches is ofgreat interest, and their reduction and disappearance supportsthe view that the mouth of the G n a t h o s t o m a t a hasobliterated them, by extending laterally, between the pre-mandibular arch in front and the mandibular arch behind. Itis interesting to note that Platt (1891) considered the conditionsof the formation of the mouth in S q u a l u s as 'indicating apossible reduction of gill-clefts which once belonged to theanterior region of the head'. In this manner it can be understoodhow the mouth became able to bite, by the approximation ofits anterior and posterior borders as upper and lower jaws. Thelower jaw is constituted by the mandibular arch, the upper jawstrictly by the premandibular.

It may now be asked what light a consideration of theC y c l o s t o m a t a throws on the foregoing argument. InP e t r o m y z o n Koltzoff (1901) showed that the sclerotomesof the first three somites of the head contributed to the forma-tion of the structures which he, following Parker (1883), calledthe trabeculae. These observations were confirmed by Filatoff(apparently unpublished, but quoted by Sewertzoff, 1916). ButSewertzoff (1916) has contended that the structures in theAmmocoete larva which Parker and subsequent authors hadcalled t r a b e c u l a e are really the anterior portions of thep a r a c h o r d a l s of G n a t h o s t o m a t a , and that, conse-quently, the brain-case of Pe t rorAyzon is entirely chordaland sclerotomic in composition. He bases this contention onthe relations of the cartilages in question to the notochord, thebrain, and the hypophysis (1917, p. 430 et seq.). An examinationof my material of P e t r o m y z o n has shown me that at a stage4£ mm. in length, in which there is very young cartilage, theso-called trabeculae are perfectly continuous without suture ofany kind with the cartilage which flanks the notochord closely

TRABECULAE CEANII 725

and which is obviously parachordal. Dr. Tribe and Dr. Wyeth,who are investigating the development of the skull of P e t r o -m y z o n , have kindly permitted me to mention that in thematerial which they have studied they have made the sameobservation. I am therefore inclined to accept Sewertzoff'sopinion that the so-called trabeculae of P e t r o m y z o n arereally parachordals, and that the brain-case of this animal,being entirely chordal in composition, contains no trabeculae.This being so, if the trabeculae are the premandibular visceralarches, and the brain-case of P e t r o m y z o n does not containthem, the trabeculae in P e t r o m y z o n should retain theiroriginal relations as premandibular visceral arches. Now thereis in the Ammocoete a pair of bars of muco-cartilage whichcorrespond exactly to the required premandibular arches.Sewertzoff (1916) called them the third prebranchial arches,the first prebranchial being the hyoid and the second prebranchialthe mandibular arches. These arches are shown in figs. 30 and31, PI. 46.

These premandibular arches of P e t r o m y z o n lie astridethe stomodaeal invagination and close to its side-wall and roof;their dorsal ends extend to a point just beneath the front endsof the parachordals (trabeculae of Parker) to Avhich theyultimately become attached (just as in G n a t h o s t o m a t athe trabeculae become attached to the parachordals) formingthe anterior attachment of the subocular bar of the adult; theylie in a position corresponding serially to that of the remainingvisceral arches. Sewertzoff attempted to recognize in the adultP e t r o m y z o n a number of other prebranchial visceral archesin front of that which is here called the premandibular, but hisevidence is insufficient. It can scarcely be without significancethat in the Ammocoete larva the arch which is here called thepremandibular is the most anterior of all: that the nerve relatedto it or its supposed homologue the trabecula (the profundusnerve) is the most anterior dorsal nerve-root; and that theventral nerve-root (the oculomotor) corresponding to the pro-fundus innervates a somite (the premandibular) which isbelieved with good reason to be the most anterior of the wholeseries. In fact, the recognition of a premandibular visceral arch

726 6. E. DB BEER

fits in perfectly with all the other evidence of chordate cephalo-geny, and it would seem that the trabecula is the skeleton ofthat arch. But there is no room on the evidence for anyarches anterior to the premandibular, and the structures whichSewertzoff has called the fourth and fifth prebranchial archesmust be structures associated with the very specialized develop-ment of the upper lip and sucking mouth of the present-dayC y c l o s t o m a t a .

The skeletal elements of P e t r o m y z o n are, then, betterrather than less well understood on the view that the trabeculaeof G n a t h o s t o m a t a represent premandibular visceral arches.The conditions in the Ammocoete are shown in Text-fig. 3 B.Attention may now be turned to the mouth and related struc-tures of P e t r o m y z o n . The Ammocoete larva possesses avelum which, like that of A m p h i o x u s , must be regarded asthe remains of the oral plate perforated by the mouth openingwhen communication is established between the fore-gut andthe stomodaeal invagination. The position of the velum willof course depend on that of the stomodaeum, and if the latteris a deep invagination the velum will be situated farther backthan if the stomodaeum were only a shallow pit. In P e t r o -myzon the stomodaeum is extraordinarily deep, and it ex-tends back into a pair of lateral pouches on each side of thevelum and fore-gut, called by Sewertzoff the second prebranchialpouches. They are indicated in figs. 30 and 31, PI. 46.

Immediately behind the velum is the pair of transientpouches which Dohrn (1886 and 18S7) identified with the hyoidand described as giving rise to the peripharyngeal ciliated bands.But the fact that the first endodermal pouch behind the velumin P e t r o m y z o n is the hyoid is no evidence that the hyoidpouch was the most anterior in the ancestral chordate. A pair ofmandibular pouches may have existed in front of the hyoid andbecome perforated to the exterior to form mandibular clefts.Indeed, Stensio's (1927) reconstructions of Ostracoderrns (whichmay be considered without necessarily accepting his interpre-tation of them) definitely show a pair of open mandibular clefts.In the history of Petromyzon it would seem that the mandi-bular pouches became reduced (as they do in Lep idos t eua ) ,

TRABECULAE CEANII 727

ceased to communicate with the exterior, and had their placetaken by the extensive imagination of the stomodaeum. Thedepth of this imagination would necessarily result in a relativelydepressed position of the velum. The mouth of P e t r o m y z o nwould resemble that of the G n a t h o s t o m a t a if the lateralpouches of the stomodaeum (Sewertzoff's second prebranchialpouches) were perforated to the exterior confluently with thecentral mouth opening, behind the premandibular arch, thusgiving rise to 'angles'.

The position of the velum in the Ostracoderms must of coursebe a matter of conjecture, but since the mandibular clefts ofnecessity presuppose a pair of mandibular endodermal pouches,the velum must have been situated anterior to them and to theclefts, and not posterior to them as suggested by Stensi0 (1927,fig. 37, p. 347). This point has also been stressed by Goodrich(1931A).

The Ammocoete larva possesses peripharyngeal ciliatedgrooves as does A m p h i o x u s , but their comparison is a matterof almost indescribable complexity and confusion. Dohrn(1886 and 18S7) claimed emphatically that the peripharyngealciliated grooves of the Ammocoete larva themselves w ere theremains of the transient hyoid visceral pouch. After examiningmy own fairly extensive material of early stages of developmentof P e t r o m y z o n , I am inclined to regard these grooves asdeveloped in the posterior wall of and therefore b e h i n d thehyoid pouches, the latter being represented by the blind sacsshown in figs. 30 and 31, PI. 46. But be this as it may, the peri-pharyngeal grooves of the Ammocoete either themselves a r e ,or are b e h i n d , the hyoid visceral pouch. In A m p h i o x u s ,on the other hand, the peripharyngeal grooves pass in f ron tof the internal opening of the club-shaped gland and of thefirst (transient) gill-slit of the primary series (Willey, 1891).However the gill-slits be considered—whether the club-shapedgland be considered an antimere to the first primary gill-slitor itself a primary gill-slit (see Goodrich, 1931 B), or whetherthe mouth of A m p h i o x u s be regarded as a gill-slit (see vanWijhe, 1904)—the fact remains that the grooves cannot corre-spond perfectly in A m p h i o x u s and in the Ammocoete

728 G. R. DE BEER

larva. It seems probable that the ciliated groove might ariserunning up any visceral arch, and in the Ammocoete it wouldseem to be on the hyoid arch. Judging by A m p h i o x u s , theprimitive position of the grooves must have been in front ofthe originally foremost visceral cleft (whichever that was), andtherefore the position of these grooves in the Ammocoete pro-vides no evidence that the hyoid visceral pouch was originallythe most anterior of all.

4. SUMMARY.

1. The existing evidence concerning the origin and nature ofthe trabecula cranii is reviewed, and it is shown that it con-stitutes a p r i m a facie case for supporting Huxley's opinionthat it represents a visceral structure.

2. The origin of the trabecula is studied in ScyI l iumc a n i c u l a , Sa lmo fa r io , R a n a t e m p o r a r i a , andA m b l y s t o m a t i g r i n u m , and the results of this investiga-tion support Huxley's opinion.

3. The grounds for adhering to Huxley's view are chieflythat: the trabecular rudiment is a mesenchymatous condensa-tion in the maxillary process; there is no evidence of the trabe-cular rudiment being derived from the somites; the trabecularrudiment is closely associated with that of the pterygo-quadrate;if the trabecular rudiment is of sclerotomic origin, then thepalatine process of the pterygo-quadrate and the mesenchyme ofthe ventral side of the front of the head must also be derivedfrom the sclerotonies: an impossible conclusion.

4. The implications of the recognition of the trabecula asa premandibular arch are considered, and it is concluded that themouth of G n a t h o s t o m a t a represents the original velarperforation of A m p h i o x u s which has extended to the sideand obliterated a pair of mandibular clefts or the dermalpouches corresponding to them.

5. Sewertzoff's view that the brain-case of P e t r o m y z o nis wholly chordal in composition is supported, and the homo-logues of the trabeculae are represented by a pair of premandi-bular visceral arches.

TRABECTJLAE CRANII 7 2 9

5. L I S T OF LITERATURE CITED.

Allis, E. P. (1923).—"Are the polar and trabecular cartilages of vertebrateembryos the pharyngeal elements of the mandibular and preraandibulararches?" 'Journ. Anat.', 58.

(1924).—"In further explanation of my theory of the polar andtrabecular cartilages", ibid., 59.

(1931).—"Concerning the Homologies of the Hypophysial Pit andthe polar and trabecular cartilages", ibid., 65.

Beer, G. R. de (1924).—"The prootic somites of Heterodontus and ofAmia", 'Quart. Journ. Micr. Sci.', 68.

(1926).—"Studies on the vertebrate head. II" , ibid., 70.Bellamy, A. W. (1919).—"Differential susceptibility as a basis for modifica-

tion and control of early development in the frog", ' Biol. Bull.', 37.Dohrn, A. (1882).—"Studien zur Urgeschichte des Wirbeltierkorpers. I.

Der Mund der Knochenfische", 'Mitt. Zool. Stat. Neapel', 3.(1886).—" Studien zur Urgeschichte des Wirbeltierkorpers. VIII. Die

Thyreoidea bei Petromyzon, Amphioxus und den Tunicaten", 'Mitt.Zool. Stat. Neapel', 6.

(1887).—"Studien zur Urgeschichte des Wirbeltierkorpers. XII.Thyreoidea und Hypobranchialrinne, Spritzlochsack und Pseudo-branchialrinne bei Fischen, Ammocoetes und Tunicaten", ibid., 7.

Goodrich, E. S. (1917).—" 'Proboscis-pores' in Craniate vertebrates. Asuggestion concerning the premandibular somites and the hypophysis",'Quart. Journ. Micr. Sci.', 62.

(1930).—'Studies on the structure and development of vertebrates.'London.

(1931A).—"On the Relationship of the Ostracoderms to the Cyclo-stomes", 'Proc. Linn. Soc. London', 142.

(1931 B).—"The development of the Club-shaped gland in Am-phioxus", 'Quart. Journ. Micr. Sci.', 74.

Haller, Graf (1923).—"tlber die Bildung der Hypophyse bei Selachiern",'Morph. Jahrb.', 53.

Huxley, T. H. (1874).—"On the structure of the skull and of the heart ofMenobranchus lateralis", 'Proc. Zool. Soc'

(1875).—"Preliminary note upon the brain and skull of Amphioxuslanceolatus", 'Proc. Roy. Soc.', 23.

Jager, J. (1924).—'Uber die Segmentierung der Hinterhauptregion unddie Beziehung der Cartilago acrochordalis zur Mesodermcommissur.'Groningen. (Also 'Morph. Jahrb.' 56, 1926.)

Johnston, J. B. (1907).—'The nervous system of vertebrates.' London.Koltzoff, N. (1901).—"Development of the head of the lamprey" (in

Russian), 'Mem. Sci. Univ. Moscou', 16.Lundborg, H. (1899).—"Studien liber die Betheiligung des Ektoderms an

der Bildung des Mesenchyms bei den niederen Vertebraten", 'Morph.Jahrb.', 27.

730 G. R. DE BEER

Parker, W. K. (1878).—"On the structure and development of the skullin sharks and skates", 'Trans. Zool. Soo.', 10.

(1883).—"On the skeleton of the Marsipobranch fishes", 'Phil.Trans. Roy. Soc'

Platt, J. B. (1891).—"Further Contribution to the Morphology of theVertebrate Head", 'Anat. Anz.', 6.

(1893).—"Eetodermio origin of the cartilages of the head", 'Anat.Anz.', 8.

(1898).—"The development of the cartilaginous skull and of thebranchial and hypoglossal musculature in Nectums", 'Morph. Jahrb.', 25.

Rathke, H. (1839).—'Entwickelungsgeschichte der Unke.' Kbnigsberg.Schrick, F. G. van (1927).—"Uber den Schwund der praehyalen Vis-

ceraltasche bei Lepidosteus", 'Morph. Jahrb.', 58.Sewertzoff, A. N. (1916).—"Etudes sur revolution des vertebres inferieurs.

I. Morphologie du squelette et de la musculature de la tete chez lesCyclostomes", 'Arch. Russes d'Anat. et d'Embryol.', 1.

(1917).—"Etudes sur revolution des vertebres inferieurs. II. Or-ganisation des ancetres des vertebres actuels", ibid.

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(1882).—"Zur Entwickelungsgeschichte des Anurenschadels", ibid.,36.

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TRABECULAE CRANII 731

6. EXPLANATION OP PLATES 40 TO 46.PLATE 40. All figures of Scyl l ium can i cu l a .

Fig. 1.—Longitudinal section, stage J.Fig. 2.—Longitudinal section, 26 mm., stage 0.Fig. 3.—Transverse section, tangential to ventral surface of head,

stage M.Fig. 4.—Horizontal section, transverse to head, stage M.

PLATE 41. All figures of Scyl l ium can icu l a .

Fig. 5.—Transverse section, tangential to ventral surface of head, 23 mm.Fig. 6.—Horizontal section, transverse to head, 23 mm.Fig. 7.—Longitudinal section, 25 mm.Fig. 8.—Transverse section, tangential to ventral surface of head, 25 mm.

PLATE 42. All figures of Scyl l ium can icu la .Fig. 9.—Transverse section, tangential to ventral surface of head, 26 mm.Fig. 10.—Horizontal section, transverse to head, 26 mm.Figs. 11, 12.—Transverse sections, 29 mm.

PLATE 43. All figures of Salmo fa r io .Fig. 13.—Transverse section (trout 24-2).Fig. 14.—Transverse section (trout 25-2).Fig. 15.—Longitudinal section (trout 23).Fig. 16.—Transverse section (trout 25 A).Fig. 17.—Transverse section (trout 26-1).Fig. 18.—Transverse section (trout 29 II).

PLATE 44. All figures of R a n a t e m p o r a r i a .Fig. 19.—Transverse section (frog AA 2).Figs. 20, 21.—Transverse sections (frog AA 3).Fig. 22.—Horizontal section (frog II B).Figs. 23, 24.—Transverse sections (frog B T).

PLATE 45. All figures of Amblys toma t ig r in i im.Fig. 25.—Horizontal section (Axolotl A I).Fig. 26.—Transverse section (Axolotl B T I ).Fig. 27.—Longitudinal section (Axolotl Aoo II).Fig. 28.—Longitudinal section (Axolotl h L).Fig. 29.—Transverse section (Axolotl B J K).

PLATE 46. All figures of P e t r o m y z o n p l a n e r i .Fig. 30.—Horizontal section.Fig. 31.—Longitudinal section.

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