Paper - Concerning the development of the prechordal portion of the vertebrate head (1938)

Allis EP. Concerning the development of the prechordal portion of the vertebrate head. (1938) J Anat. 72: 584-607. PMID 17104729

Concerning The Development Of The Prechordal Portion Of The Vertebrate Head

By Edward Phelps Allis, Jr. Mentone, France

Ir has been suggested by an eminent authority that I give, with suitable figures in illustration, an account of my present views regarding the development of the prechordal portion of the vertebrate head. This requires, first, a careful reconsideration of the conditions in the Plagiostomi and then consideration of those in the Holocephali, Cyclostomata and Ganoidei, and it may be here stated that I am now able to give an explanation of the somewhat puzzling manner in which the hypophysis develops in the latter fishes. The conditions in the Teleostei, Dipnoi and Amphibia are not particularly considered. The drawings for the figures were prepared by Gen. G. Wannovsky.

PLAGIOSTOMI

In early embryos of Acanthias the alimentary canal is said by Platt (1891) to fill completely the head region. Theneural plate lies flat on the dorsal surface of the egg and does not extend forward quite to the anterior end of the alimentary canal.

In a 2-7 mm. embryo of Acanthias the neural groove is well developed, as shown in the accompanying Fig. 1, which is a copy of Scammon’s (1911) Fig. 8 giving a median view of an approximately bisected embryo. The groove lies directly upon the underlying portion of the alimentary canal and presents anterior and posterior sections which lie at a considerable angle to each other. The posterior section begins in the branchial region and from there the median line of its internal surface extends antero-ventrally in a smooth and even curve to the hind end of the premandibular (preoral) gut, where the anterior section begins and extends anteriorly slightly beyond the tip of the latter gut. The infundibular process of the brain develops at the angle between these two sections and accordingly lies approximately, if not actually, in the intersegmental line between the mandibular and premandibular sections of the alimentary canal. The anterior section of the groove is thus definitely premandibular in position in so far as its relations to the alimentary canal are concerned, and accordingly may be referred to for descriptive purposes as the premandibular section of the groove, and that part of the floor of the brain that is developed in relation to it may be referred to as the premandibular section of that floor. The optic chiasma is said by Platt to develop in this Prechordal Portion of the Vertebrate Head 585

premandibular section of the groove and the recessus preopticus to lie approximately dorsal to the tip of the premandibular gut. The premandibular gut has the appearance of being a diverticulum projecting anteriorly from the anterior wall of the foregut immediately above its floor, and I have heretofore considered this to have been due to compression caused by the sinking inward of the neural groove, and as it is the infundibular portion of the groove that sinks deepest into the underlying tissues I have referred to it as the descent of the infundibulum. Scammon, however, says (1911) that in a 2 mm. embryo of this fish the preoral gut is almost square in cross-section, and Platt refers to it as tubular in shape, neither of which is favourable to a marked dorsoventral compression of this part of the gut. It therefore seems probable that the conditions here are due to arrested development of the premandibular section of the alimentary canal and not especially to downward compression.

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Fig. 1. Median view of the bisected head of a 2-7 mm. Acanthias embryo. After Scammon.

That part of the neural groove that lay directly upon the dorsal surface of the premandibular portion of the alimentary canal would then have simply sunk inward as that part of the canal became reduced in relative diameter and have pulled the posterior portion of the groove downward after it.

The point X shown in the figure is assumed to lie in the line between the ectoderm covering the anterior surface of the brain and that covering the anterior surface of the premandibular visceral segment, and hence approximately ventral to the recessus preopticus. These relations of the point X to the cranial and visceral ectoderm suggest that it represents the tip of the later to be developed Rathke’s pocket, for that tip lies approximately between those two ectodermal surfaces, as will be explained when later describing a 7 mm. embryo. The tip of the pocket, however, lies in the latter embryo approximately external (ventral) to the hind end of the infundibulum, the external opening of the pocket lying external (ventral) to the recessus preopticus, the outer surface of the brain 586 Edward Phelps Allis, Jr.

between these two points being covered with cranial ectoderm. The brain of the 2-7 mm. embryo must accordingly in older embryos have shifted forward relatively to the premandibular visceral segment until the hind end of the infundibulum has come to lie dorsal to the point X. The ectoderm that in this 2-7 mm. embryo covers the anterior and ventral surfaces of the premandibular segment would then correspond to the buccopharyngeal upper lip of older embryos together with the primarily dorsal wall of Rathke’s pocket, as will be evident when older embryos are described. There are thus on the strongly curved anterior end of the head of this embryo the following several regions arranged successively from above downward; forebrain, neuropore with related olfactory placodes, tip of Rathke’s pocket, buccopharyngeal upper lip and the place of the future buccopharyngeal plate, and it will be

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Fig. 2. Median view of the bisected head of a 3-5 mm. Acanthias embryo. After Scammon.

later shown that in early embryos of the Ganoidei these same regions occur successively in the same order but on a much less strongly curved surface. In a 3-5 mm. embryo of Acanthias the brain has shifted forward to a considerable extent relative to the alimentary canal, as shown in the accompanying Fig. 2 which is a copy of Scammon’s Fig. 6 giving a median view of an approximately bisected embryo. In connexion with this shifting forward of the brain the premandibular gut has been withdrawn from the space it previously occupied immediately beneath the premandibular section of the neural groove, the tip of this space lying ventral to the hind edge of the optic chiasma instead of lying slightly anterior to the chiasma beneath the preoptic recess. The point now occupied by what will later become the tip of Rathke’s pocket cannot be recognized, but it must lie approximately beneath the optic chiasma and by the further shifting forward of the brain it will ultimately come to lie beneath the infundibulum. The space previously occupied by the premandibular gut is now shown filled with premandibular mesoderm. This Prechordal Portion of the Vertebrate Head 587

space is relatively small, and the dorsal and ventral ends of the band of mesodermal tissue on either side of the head in which the related premandibular branchial bar will later develop are relatively close to each other, the band quite certainly being slightly curved in form, the outer end of the curve directed somewhat anteriorly because of the rounded shape of the anterior end of the alimentary chamber. The floor of the premandibular gut is still practically on the level of that of the mandibular and more posterior portions of the alimentary canal, for these latter portions of the canal have not yet begun to expand ventrally as they do in older embryos. The floor of the mandibular section of the canal which will later become the internal layer of the buccopharyngeal plate is accordingly still in a nearly horizontal position and has not yet come into contact with the ectoderm immediately external to it.

In a 7 mm. embryo of Acanthias the neural groove has shifted relatively so far forward that the hind end of the infundibulum lies approximately directly internal (dorsal) to that point of the ectoderm that will later become the tip of Rathke’s pocket, as shown in Fig. 3 which is a copy of one of de Beer’s figures (1926, Fig. 85). What I have called the premandibular section of the neural groove is now completely lined externally with cranial ectoderm, and owing to the marked expansion of the forebrain the anterior portion of the head back to and including the premandibular section of the neural groove has swung downward and backward around a horizontal axis passing approximately through the future tip of Rathke’s pocket to such an extent that this cranial ectodermal surface is presented almost directly posteriorly. Between this Fig. 3. Median sagittal section of posteriorly directed cranial surface and the a 7 mm. embryo of Squalus. ectoderm covering the anterior portion of After de Beer. the ventral surface of the branchial chamber there is accordingly a large and approximately right angle. This large angle which will later become the Kieferaugenspalte of Haller’s (1923) descriptions of older embryos may be called the hypophysial fold, the tip of the fold forming the future tip of Rathke’s pocket. The mandibular and more posterior portions of the alimentary canal have begun to expand ventrally, this pressing the floor of the mandibular section of the canal downward and forward into a slightly anterodorsally directed position. The premandibular gut is not particularly described by de Beer and cannot be identified in his figure; it is, however, shown by Haller in a median section of a 4-5 mm. embryo of Raia as a small pointed process projecting forward from the dorso-anterior corner of the gut. It is embedded in a mass of cells that lies immediately internal to the dorsoposterior wall of the hypophysial fold, and that part of this mass of cells that

Anatomy LXxII 38 588 Edward Phelps Allis, Jr.

immediately surrounds the process later disappears excepting only a narrow strand the ultimate fate of which Haller could not determine. This little process of the gut is currently called Sessel’s pocket and its present position near the dorsal instead of the ventral edge of the mandibular gut is not the result of actual shifting of its position relative to that gut, but to the marked expansion ventrally of the mandibular and more posterior portions of the alimentary canal. The mandibular gut has actually no anterior surface excepting that represented by the line of contact with the hind edge of the premandibular gut, its exposed surfaces being either dorsal, lateral or ventral, the latter surface being perforated by the buccopharyngeal (oral) opening.

In an 8 mm. embryo of Raia described by Haller the anterior end of the

Fig. 4. Median section through the head of an 8 mm. embryo of Raia clavata. After Haller. Reversed in direction.

head has swung downward, backward and upward to such an extent that the two walls of the hypophysial fold have come together on either side of the median line and there completely fused with each other, as shown in the accompanying Fig. 4 which is a copy of one of Haller’s figures (1923, Fig. 1). These two surfaces of fusion extend from the transverse line of the hind end of the infundibulum to a point approximately in the transverse line of the preoptic recess, this indicating the full depth of the hypophysial fold. A median space called Rathke’s pocket is thus cut out of the latter fold, the depth of the pocket, like that of the fold, corresponding approximately to the length of what I have called the premandibular section of the floor of the brain, the pocket lying directly external to this section of the brain. What at this stage of development is the anteroventral wall of the pocket is thus of cranial ectoderm, the posterodorsal wall Prechordal Portion of the Vertebrate Head 589

being of visceral ectoderm and quite certainly derived from the anterior portion of the ectoderm that in early embryos covers the anterior and ventral surfaces of the preoral gut. At the external opening of Rathke’s pocket the median longitudinal line of the latter ectoderm turns abruptly dorsally and becomes continuous with the mandibular ectoderm that forms the dorso-anterior border of the buccopharyngeal opening. This part of the line of ectoderm is thus largely of premandibular origin but partly of mandibular origin, and it forms the external lining of what Haller calls in a 20 mm. Acanthias, described immediately below, the transverse hypophysial bolster. In the 8 mm. embryo of Raia the mandibular arch is said to show subdivision into four segments which are called by Haller from above downward, the Kieferaugenspaltstiick, Oberkieferstiick, Zwischenstiick and Unterkieferstiick. The Kieferaugenspaltstiick is said to be the pharyngeal element of the arch and it is definitely said to take no part in the formation of the upper jaw and some part of it aaa is just as definitely said to form the (( oo — floor and lateral walls of the pituitary fossa, this fossa being referred to by Haller as the posterior portion of the large Hirnbodenbucht peculiar to these fishes. The mandibular arch is considered by him to consist of two portions, one of which he calls the postorbital or primitive arch, the other portion being said to be of secondary origin and being called the suborbital portion of the arch. The larger Fig. 5. Median section through the hypophysial portion, pon body, of the Kiefer- region of a 20 mm. embryo of Acanthias. After . . . aller. Reversed in direction. augenspaltstiick is said to form the dorsal end of the primitive arch, the suborbital portion of the arch and the anterior end of the palatoquadrate (Oberkieferstiick) being developed from an anterior process of the Kieferaugenspaltstiick. This will be further explained when considering the conditions in 20 and 80 mm. embryos of Acanthias immediately below.

Ina 20mm. embryo of Acanthias the cranial flexure has begun to be reduced by the swinging forward and upward of the anterior end of the head, but the horizontal axis around which this takes place has shifted from the tip of Rathke’s pocket to a point approximately through the external opening of that pocket, as shown in the accompanying Fig. 5 which is copied from Haller’s Fig. 4. This shifting of the axis of rotation is due to the marked shortening in length of the ventral surface of this part of the head resulting from the infolding of the hypophysial fold and it is equal in amount to twice the depth of that fold. The fold is in fact a pleat taken in the ectoderm covering the

38—2 590 Edward Phelps Allis, Jr.

ventral surface of the head and projecting anterodorsally into the substance of the head. There has been no corresponding shortening in the length of the floor of the brain and because of the change in the axis of rotation, both what I have called the premandibular portion of the latter floor and Rathke’s pocket swing backward and downward as the remaining part of the anterior end of the head swings forward and upward. The hind end of the infundibulum is connected with the ventral end of the anterior limb of the plica encephali ventralis, and as the infundibulum swings backward and downward it carries with it the ventral end of that limb of the plica until they both reach their adult positions approximately ventral to the anterior end of the medulla oblongata. The so-called maxillary processes, one on either side of the head, are

Fig. 6. A. Median section through the head of a 30 mm. embryo of Acanthias. B. Enlarged view of the hypophysial region in the same embryo. After Haller. Reversed in direction.

said to grow forward from the anterior surface of the transverse hypophysial bolster, and they appear as horizontal ridges directed morphologically ventrally from the ventral surface of the anterior portion of the head. They are said to extend forward to the so-called lachrymal groove where each turns mesially and meets and fuses in the median line with its fellow of the opposite side. A prehypophysial recess bounded posteriorly by the hypophysial bolster and opening ventrally to the exterior is thus formed on the ventral surface of the head, Rathke’s pocket opening into it at its morphologically dorsoposterior edge. Haller says that at this stage there is no mesodermal tissue in either of these maxillary processes.

In a 80 mm. embryo of Acanthias the hypophysial bolster above referred to, which I have called in an earlier work (Allis, 1982) the buccopharyngeal Prechordal Portion of the Vertebrate Head 591

upper lip, has grown forward to such an extent that its morphologically ventral surface reaches nearly to the hind edge of a group of cells that will later become the palatine process of the palatoquadrate, as shown in the accompanying Fig. 6 which is a copy of Haller’s Fig. 5. The ventral edges of the maxillary processes have grown mesially and met and fused with each other in the median line. At the same time the mesial walls of the processes meet and fuse with each other in the median plane and break down and completely disappear excepting only certain parts that are preserved to form the anterior and lateral walls of the prehypophysial canal and probably also the enclosing walls of an anterior prolongation of the hypophysis which extends forward from Rathke’s pocket along the ventral surface of the brain. The hind wall of the prehypophysial canal is formed by the anterior surface of the hypophysial bolster and is accordingly of visceral ectoderm, its anterior wall being of cranial ectoderm, the canal extending forward from the external opening of Rathke’s pocket and opening into the buccal cavity slightly posterior to the palatine processes. Slightly ventral to the anterior prolongation of the hypophysis and extending forward slightly beyond its tip there is a long band of mesodermal cells separated into two parts by the prehypophysial canal. The two parts of this band are called by Haller the anterior and posterior trabeculae and are evidently, respectively, the trabecular and polar cartilages of current descriptions of the adults of these fishes. The polar cartilage lies wholly in the hypophysial bolster and is considered by Haller to represent the body of the Kieferaugenspaltstiick and is accordingly quite certainly the entire pharyngomandibular. Ventral to the anterior end of the trabecula there is a group of mesodermal cells that is said by Haller to represent the anterior end of the palatoquadrate (Oberkieferstiick) and this group of cells together with those that form the trabecula form what Haller calls the suborbital portion of the mandibular arch and are considered by him to have both been derived from a process of the Kieferaugenspaltstiick of its side of the head.

From this careful reconsideration of the conditions in these fishes I am convinced that, in them, the hypophysis is formed by the folding together of two ectodermal surfaces that cover at this stage of development the one a cranial and the other a visceral surface, that it is accordingly in no sense either an ingrowth or an invagination, and that it is also in no sense what de Beer calls an overgrowth, for in all of the embryos considered it always has approximately the length of what I have called the premandibular section of the floor of the brain, that is that part of that floor that extends approximately from the hind end of the infundibulum to the preoptic recess. Furthermore, I see no reason to change my opinion expressed in 1981 and 1932 that the polar cartilage is the pharyngeal element of the mandibular bar and the trabecular and palatine cartilages, respectively, the dorsal and ventral halves of the premandibular branchial bar, and I have attempted to show in the accompanying purely diagrammatic Fig. 7 the manner in which they have come to be developed from those branchial bars. This figure shows only the anterior 592 Edward Phelps Allis, Jr.

. portion of the alimentary canal and the premandibular, mandibular and hyal branchial bars. It is intended to present four successive stages in the progressive abortion of the premandibular section of the alimentary canal and the related changes in the shape and position of the mandibular and pre mandibular branchial bars. In the first stage shown the premandibular section of the canal is assumed to be of the same size as the posterior sections. In the second stage the premandibular section has already diminished considerably in relative size and the more posterior sections have expanded slightly dorsally. In the third stage the posterior sections of the canal expand ventrally and the premandibular branchial bar has become entirely independent of the related section of the canal, and the articular joint between the pharyngeal and epal elements of the mandibular arch lies opposite the closely adjoining dorsal and ventral ends of the premandibular bar. In the last stage the premandibular section of the alimentary canal has entirely disappeared and the dorsal and ventral halves of the premandibular branchial bar are in position to fuse, respectively, with the ventral end of the pharyngomandibular and the dorsal end of the epimandibular and so become the trabecular and palatine cartilages. De Beer (19315) considers the manner of development of the polar, trabecular and palatine cartilages as set forth in my earlier works to be based on purely morphological considerations and hence open to question. He accordingly attempted in 1981 to trace in Scyllium, as Haller had already done in Acanthias and Raia, the cells that enter into the

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polar and palatine cartilages.

composition of these several cartilages directly to them from their points of origin. A group of cells is said by him to arise independently in each maxillary process without proliferation from adjacent regions. These cells, when they first appear, are said to be of uniform character and indistinguishably mixed with each other, but they soon separate into two groups, a superficial and a deeper Prechordal Portion of the Vertebrate Head "593

one. The deeper group is considered to be of premandibular origin, and the trabecula which develops in it is considered to represent either the entire branchial bar of the premandibular arch, or its larger dorsal portion, the ventral portion of the bar in the latter case breaking down and being dispersed. The superficial group fuses with the pterygoquadrate and becomes its palatine process, this process being nevertheless said to be a definite but secondary outgrowth of the pterygoquadrate and hence apparently being in de Beer’s opinion of mandibular origin, although this is not definitely so stated. De Beer could not satisfactorily determine the origin of the polar cartilage, but he considers it probable that it is of mandibular origin. The pharyngo-mandibular is however considered by him to be quite probably represented in the little cartilage described by Sewertzoff & Diesler (1924) in the articular joint between the dorsal end of the orbital process of the palatoquadrate and the orbital wall, while both Haller and I considered this segment of the mandibular arch to have given origin to the polar cartilage. The conditions in Scyllium as thus described by de Beer would thus seem to be definitely favourable, rather than unfavourable, to my interpretation of the conditions in Acanthias and Raia as described by Haller. De Beer calls attention to the fact that as long ago as 1874 and 1875 Huxley contended that the trabeculo-polar bar, called by him simply the trabecula, forms a complete and single visceral arch. This, I regret to say, entirely escaped my notice when my earlier works relating to this subject were sent to press, and Haller does not refer to it. My work published in 1928 together with Haller’s work published later in the same year would seem to have definitely confirmed Huxley’s contention in so far as the visceral (branchial) origin of the bar is concerned, but if I am correct in my conclusion that this bar is formed by parts of two adjacent arches which fuse with each other, end to end, Huxley’s conclusion that the bar represents a single and complete “visceral arch”’ is certainly not correct. De Beer, however, contends, as just above stated, that the trabecular part alone of the trabeculo-polar bar represents the entire premandibular branchial bar and he considers this to definitely confirm Huxley’s view.

In embryos of the Plagiostomi older than those above considered the development Of the prechordal part of the chondrocranium is best described by de Beer in a work entitled ““The development of the skull of Scyllium canicula”’ (1981a). In a 34 mm. embryo of this fish the anterior ends of the trabeculae, curving mesially, have met and fused with the lateral edges of the hind end of a median cartilage which lies, at this stage, between the nasal sacs but later between the ventral edges of the nasal capsules. It is called by de Beer the rostral cartilage and is in no way an outgrowth of the trabeculae, the latter cartilages definitely ending, at this stage of development, approximately in the line between the orbital and ethmoidal regions, as they do in the 80 mm. embryo of Acanthias shown in Fig. 6. In a slightly earlier stage of Scyllium an ethmoidal process called by de Beer the lamina orbito-nasalis is said to extend laterally from the anterior end of the trabecula of its side of 594 Edward Phelps Allis, Jr.

the head, and somewhat later a planum antorbitale extends upward from it to form at the same time the hind wall of the nasal capsule and the anterior wall of the orbit. Mesial to the mesial edge of this planum another process called by de Beer the preoptic root of the orbital cartilage is said to grow upward from the anterior end of the trabecula and to meet and fuse dorsally with the orbital cartilage, so completing the dorsal and anterior boundaries of the orbit. An independent anterolateral wall to the nasal capsule has in the meantime developed, and the mesial wall of the latter capsule is then formed by a process that is said to grow upward and forward from the lateral edge of de Beer’s so-called rostral cartilage near its hind end and meet and fuse with the dorsal edge of the anterior wall of the capsule. From this point of fusion the dorsolateral limb of the three-limbed rostral basket grows anteromesially and meets and fuses in the median line with the anterior end of its fellow of the opposite side and also with , the anterior end of the ventro- Fic. oc. median limb of the rostral basket '

which grows forward from .the i

anterior end of de Beer’s rostral ‘

cartilage. There is now, as soon as the mesial walls of the nasal capsules have been completed, a tall cavum internasale similar to. = $7 ~~ --- gummy that described by Gaupp (1906) 97"

in certain of the Amphibia excepting in that it is closed | ; ventrally by de Beer’s so-called heal rostral. Posteriorly it opens

directly into the cranial cavity Fig. 8. Median view of the skull of a 45 mm. embryo

and anteriorly into the hollow o¢ Scyllium. After de Beer. Reversed in direction. of the three-limbed rostral With index letters used by him.

basket, that basket having been

considered by both Sewertzoff (1899) and myself to be the homologue of the cavum praecerebrale of the Notidanidae and Spinacidae. In a°slightly older embryo, shown in the accompanying Fig. 8, still another process has been developed in this region, but the manner of its development in this fish is not here definitely given. It is, however, said (de Beer, 19816) to correspond to two cartilages, one on either side of the head, developed in early embryos of Rana which are called the suprarostrals, each of them fusing with the anterior end of the corresponding trabecula. Stéhr (1882) is said to have considered these cartilages to be outgrowths of the trabeculae, but Spemann (1898) is said to have considered them to be of wholly independent origin. De Beer calls the single cartilage of Scyllium the suprarostral, but simply says of it that dorsal to his rostral process “a shorter and more blunt process extends forward between the median walls of the nasal capsules”. He calls it in the

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text the suprarostral, but in the figure given of the bisected skull of an adult (Fig. 9) it is index lettered as the ‘‘Median wall of nasal capsule’’. It has the appearance in the figure of arising from the dorsal surface of de Beer’s rostral cartilage approximately in the line of the anterior ends of the trabeculae. Running forward at about the middle of the height of the nasal capsules it cuts the cavum internasale into dorsal and ventral portions, the latter of which is thus cut off from the cranial cavity, but still retains its connexion with the cavum praecerebrale, while the dorsal portion retains its connexion with both these cavities. The origin of these several processes from the anterior ends of the trabeculae is evidently favourable to my conclusions that the ethmoidal

Fig. 9. Median view of the skull of an adult Scyllium. After de Reer. Reversed in direction. With index letters used by him.

region of the chondrocranium is of premandibular branchial bar or branchialray origin, the outer border of the antorbital wall being a branchial-ray bar the serial homologue of the branchial-ray bars of the mandibular and hyal arches which form, respectively, the lateral walls of the trigemino-facialis chamber and the hyomandibula of the Teleostomi (Allis, 1981). The so-called septum nasi of these fishes may now be considered, for there has been question of its relations to the trabeculae. That there is in Scyllium at no stage of development a median vertical internasal plate is evident from de Beer’s descriptions of the development of this fish. There is, however, an internasal wall formed by the mesial walls of the nasal capsules together with the two median longitudinal processes called by de Beer the rostral and supra rostral and that ventral portion of the cavum internasale that lies between 596 Edward Phelps Allis, Jr.

the two latter processes. This internasal wall or so-called septum has also been described in others of those of the Selachii that possess a three-limbed rostral basket and that there was primarily in all of these septa a narrow median cavum internasale would seem to be established by what I described in an earlier work (Allis, 1918) as the apparently imperfectly chondrified condition of the median plane of the septum. This is well shown in Gegenbaur’s (1872) and Parker’s (1876) figures of median sections through the skulls of Mustelus and Scyllium which are reproduced in the accompanying Figs. 10 and 11. Comparing these two figures with each other and with de Beer’s figure of a similar section through the head of an adult Scyllium reproduced in the accompanying Fig. 9, it is seen that there is marked difference in the names given by these three authors to the cartilages that bound or lie immediately anterior to the septum. In the Notidanidae, Spinacidae and Raiadae in which

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Fig. 10. Median section of the skull of an adult =‘ Fig. 11 Median view or the skull of an adult Mustelus. After Gegenbaur. Reversed in direction. Scyllium. After Parker. Reversed in direction. With index letters used by him. With index letters used by him.

a septum nasi has never been described, so far as I can find, the trabeculae have heretofore been considered to fuse to form a trabecular plate, and this plate has been considered to be prolonged anteriorly to the end of the snout, the preorbital portion of the plate being called its rostral portion. This is typically shown in Gegenbaur’s figure of a bisected skull of the adult Acanthias, reproduced in the accompanying Fig. 12 and also in Sewertzoff’s (1899) figures of embryos of this same fish. On the ventral surface of the rostral portion of this continuous trabeculo-rostral plate Gegenbaur shows a median longitudinal ridge which he calls the median internasal ridge and designates in his figures by the letter V. This ridge is not shown in Sewertzoff’s figures of embryos of this fish and would accordingly seem to be of later development. The nasal capsules of these fishes develop along the lateral edges of the rostral cartilage and project either lateroventrally or ventrally from those edges. This led me to conclude in an earlier work (Allis, 1917) that the internasal Prechordal Portion of the Vertebrate Head 597

septum of the Scylliidae and Carchariidae was developed as a result of a gradual shifting mesially of the capsules along the ventral surface of the trabeculorostral plate and their ultimate fusion in the median line with each other and also with the median ventral internasal ridge V of Gegenbaur’s description, a portion of the median ridge being squeezed ventrally to form the ventral border of the septum. I, however, now consider the median ridge V to have been, in the Notidanidae and Spinacidae, primarily an independent cartilage that has fused with the overlying rostral portion of the continuous trabeculorostral plate and that in the Carchariidae and Scylliidae the nasal capsules have pressed these two bars apart and so come to lie between them. Gegenbaur in fact considers the ventral border of the septum to correspond to the median internasal ridge of his descriptions of Acanthias as shown by the use of the index letter V to indicate it in his figure of Galeus (1872, Pl. 5, Fig. 2). The dorsal border of the septum was therefore certainly considered by him to correspond to the rostral portion of the trabeculorostral plate of his descriptions of Acanthias, the septum accordingly lying ventral to the latter plate. Parker considers both edges of the septum to beanterior prolongations of the trabeculae, the septum accordingly being, in his opinion, intratrabecular in position. I, in my earlier works relating to this subject, have adopted the nomenclature em ployed byGegenbaur andaccord- 5, 15 wedian view of the skull of an sdult

ingly made the general statement 4canihias. After Gegenbaur. Reversed in direction. . that in these fishes the septum With index letters used by him.

lies ventral to the trabeculae.

De Beer considers the ventral border, alone, of the septum to be the direct anterior prolongation of the trabeculae and hence maintains that the septum lies dorsal to the trabeculae, his opinion and mine, to which he takes marked exception, thus apparently differing simply because of a difference in the nomenclature employed and hence being of no particular morphological significance. If the ethmoidal region is largely of branchial-ray origin, as de Beer’s descriptions of Scylliwm would seem to indicate, the septum would in fact lie wholly anterior to the trabeculae and neither dorsal nor ventral to them.

HOLOCEPHALI

Schauinsland’s (1908) figures of early embryos of Callorhynchus show definitely that the hypophysis of these fishes is developed in a manner strictly comparable to that in the Plagiostomi, and his figures of older embryos of Callorhynchus and my figure of a bisected skull of the adult Chimaera colliei, 598 Edward Phelps Allis, Jr.

reproduced in the accompanying Fig. 18, show just as definitely that the marked cranial flexure essential to this manner of development of the hypophysis has not later been completely reduced. This is evidently due to the development of a large vesicle called by Dean (1906) the frontal knob which presses upon the downturned anterior portion of the head and prevents its continued swinging upward and forward into its primitive position in the line prolonged of the axis of the body. This does not particularly affect the chordal portion of the skull and because of this the forebrain projects anteroventrally at a marked angle to the mid and hind brains and is relatively long. The eyeballs retain their normal positions immediately anterior to the otic region and

Fig. 18. Median view of the skull of an adult Chimaera colliei. After Allis.

hence lie dorsal to the hind end of the forebrain, the orbits being separated from each other by a thin median interorbital septum. The supraorbital cartilages form the dorsal borders of the orbits and hence lie at a considerable distance dorsal to the forebrain. The walls of the forebrain cavity must accordingly either be of cranial origin developed in relation to the membranous tissues that primarily enclosed the brain, or be upgrowths of the trabeculae that have met and fused with each other dorsal to the forebrain. Assuming that they were of trabecular origin, I suggested in 1926 that the trabeculae themselves had shifted upward lateral to the forebrain and reaching its dorsal surface had there met and fused with each other in the median line to form the roof of the forebrain cavity. There is no mechanical objection whatever to this assumption, for in early embryonic stages the trabeculae do not extend Prechordal Portion of the Vertebrate Head 599

forward beyond the orbits, as shown in the 30 mm. embryo of Acanthias (Fig. 6), and in shifting upward they would in no way have interfered with the connexion of the nervi olfactorii with the nasal sacs, as suggested by de Beer & Moy-Thomas (1935). The condition and position of the nasal septum however strongly suggests that the trabeculae actually form the floor of the forebrain cavity and the bounding walls of the septum, as they do in the Plagiostomi, for in my figure of Chimaera, published in 1917, the median plane of the septum is shown in the same imperfectly chondrified condition that it has in the Plagiostomi. This evidently indicates that the septum primarily enclosed within it the ventral portion of a cavum internasale. The dorsal portion of that cavum which, in the Plagiostomi, forms part of the cavum praecerebrale, ' would then probably be included in the so-called ethmoidal chamber of Chimaera, the ethmoidal grooves on the dorsal surface of the snout of certain of the Selachii also being included in it, as I have suggested in certain earlier works (Allis, 1917, 1926). The cavum praecerebrale forms in early embryos of the Plagiostomi a direct anterior prolongation of the cranial cavity as shown in de Beer’s figure of a 45 mm. embryo of Scyllium (Fig. 8). This cavum praecerebrale is never lined with dura mater, and in the adult Chlamydoselachus I found (Allis, 1923) the fenestra praecerebralis plugged with tough tissue and the dura mater limited in its forward extension to the hind wall of the plug. There would accordingly seem to be nothing markedly improbable in the assumption that the ethmoidal chamber of Chimaera was primarily directly continuous with the actual postorbital portion of the cranial cavity. The cavum praecerebrale, if it actually forms part of the ethmoidal chamber, has, like the orbits, shifted upward to a certain extent after the general cranial flexure had ceased to be reduced, and hence does not in the adult form an anterior prolongation of the forebrain cavity. The presence in the Holocephali of a septum nasi of the selachian type suggests that they are more closely related to the Carchariidae and Scylliidae than to the Notidanidae and Spinacidae, and this further suggests that the frontal knob of the Holocephali may be in some way related to the three-limbed rostral basket of the Carchariidae and Scylliidae. CYCLOSTOMATA

In Petromyzon and hence probably in all of these fishes fertilization is external and the egg is enclosed in a thin and closely fitting membrane.

In a median sagittal section of a 5-day-old embryo of Petromyzon planeri, v. Kupffer (1894) shows the anterior end of the head projecting slightly dorso-anteriorly above the outer surface of the egg, as shown in the accompanying Fig. 14. The headfold is a large angular depression, the bottom of which projects posteriorly ventral to the preoral gut and slightly dorsal to it there is a triangular thickening of the ectoderm apparently wholly due to a lengthening of the related cells. It is called by v. Kupffer the hypophysis, lies approximately external to the recessus preopticus, is directed posteriorly dorsal to the preoral gut and hence apparently lies between cranial and visceral 600 Edward Phelps Allis, Jr.

ectodermal surfaces approximately in the position of the external opening of Rathke’s pocket in the Plagiostomi. It is later prolonged posteriorly to the hind end of the infundibulum, a central cavity develops in it and it then has to the infundibulum and the preoral gut the topographical relations of Rathke’s pocket in the Plagiostomi. It is therefore quite certainly the homologue of the latter pocket, for that the hypophysis was developed twice independently in the vertebrate line seems improbable. The probable explanation of its markedly different manner of development in this fish and in the Plagiostomi must then be that as the closely fitting egg membrane prevents the anterior end of the head from swinging downward, backward and upward to form a hypophysial fold, that fold is formed by what may be called the gradual and progressive pinching together on the internal surface of the headfold of two ectodermal surfaces, one cranial and the other visceral, that correspond to the two walls of the hypophysial fold of the Plagiostomi, this pinching off process continuing until the tip of the fold or pleat so-formed reaches the hind end of the infundibulum and becomes the tip of Rathke’s pocket.

Anterior to this hypophysis, between it and the median olfactory pit, maxillary processes must develop in later stages as they do in the Plagiostomi, but they have never been described so far as I can find. The buccopharyngeal upper Fig. 14. Median view of the bisected head of a 5-daylip then grows forward ventral to 4 embryo of Faron planeri. After v. Kupffer.

versed in direction.

these processes and the short pre hypophysial (intermaxillary) canal so formed is prolonged anteriorly to the end of the snout by the combined growth forward of the maxillary processes and the buccopharyngeal upper lip, as suggested in one of my earlier works (Allis, 1981a). This carries the olfactory pit forward to its ultimate position on the dorsal surface of the anterior end of the snout and as the buccopharyngeal upper lip grows forward slightly beyond the maxillary processes, the prehypophysial canal opens on the dorsal surface of the snout immediately anterior to the olfactory pit. The snout is thus now definitely of the viscero-trabeculocranial type and the conditions in this fish show a slight advance in this respect over those in the Plagiostomi.

The conditions in Bdellostoma have been described by v. Kupffer (1900), but he did. not have a complete set of embryos, and the conditions described by him differ so markedly in certain respects from those in Petromyzon that Prechordal Portion of the Vertebrate Head 601

any attempt to establish their definite homologies should await further investigation. It may, however, be stated that the prehypophysial canal lies in large part anterior to the single median nasal capsule instead of posterior to it, and that it is enclosed in longitudinal laminar processes, one on each side of the head, that are said to grow downward from the ventral surface of the prenasal portion of the snout. These processes would therefore seem to be maxillary processes similar to those in the Plagiostomi, and the cartilages that develop in them must be of premandibular origin and the cartilaginous rings that enclose the nasal tube of the adult would suggest origin from premandibular branchial rays. The extent to which the buccopharyngeal upper lip extends forward cannot be determined from the descriptions, but it certainly does not extend beyond the tip of the snout. The nasal aperture accordingly opens morphologically on the ventral surface of the snout, as the nasal apertures do in the Elasmobranchii.

GANOIDEI

Little is definitely known about the early development of the prechordal portion of the head of these fishes, but so far as can be judged from existing descriptions the manner of its development differs markedly in certain respects from that in either the Cyclostomata or the Elasmobranchii. It has been most fully and completely described in Acipenser, and as the manner of its development is probably, in principle, the same in all of these fishes, that in Acipenser alone will here be particularly considered.

In 1893 v. Kupffer gave a figure showing a median vertical section through the head of a 45-hr.-old embryo of Acipenser sturio, and, although this figure is probably incorrect in certain details, it presents conditions that can readily be compared with those in Petromyzon and is accordingly reproduced in

the accompanying Fig. 15. Fig. 15. Median view of the bisected head of a 45-hr.-old The embryo is said to lie flat embryo of Acipenser sturio. After v. Kupffer. Reversed in direction.

on the dorsal surface of the egg with its anterior end curving somewhat anteroventrally. The stomodaeum, which forms the expanded anterior end of the headfold, is a large angular depression near the anterior end of the embryo, between the anterior end of the head and that part of the body that encloses the heart. It is called 602 Edward Phelps Allis, Jr.

by v. Kupffer the ‘“‘Mund” and its inner end is in contact with the wall of a diverticulum of the gut which he calls the “‘ Vorderdarm”’, and as this contact persists in all of the older embryos described by him, it evidently represents the buccopharyngeal plate and is so designated by Neumayer (1932) in his descriptions of an embryo of this same age. This diverticulum of the gut, as described by Neumayer, has bulging lateral walls and as these walls apparently become closely pressed together in older embryos the point of the diverticulum and the tip of the stomodaeum, as shown in v. Kupffer’s figures, become for a time somewhat widely separated from each other.

The dorso-posterior wall of the stomodaeum forms the ventro-anterior wall of a mass of tissue roughly triangular in median sagittal section that is called by v. Kupffer the adhesive organ and which I have homologized in an earlier work (Allis, 1932 a) with the buccopharyngeal upper lip of Petromyzon and the Plagiostomi. The ventro-anterior and dorsal walls of this lip are of ectoderm, the ventro-posterior one being of endoderm and forming part of the outer wall of the gut. The hollow of the lip is said by both v. Kupffer and Neumayer to be filled with ectoderm, but Sawadsky (1912) and Holmgren (1981) say that it is filled with endoderm, this endoderm being shown in their figures simply as a pronounced thickening of the wall of the related part of the gut. In favour of the hollow of the lip being filled with endoderm instead of ectoderm is Reighard & Phelps’s (1908) statement that in Amia calva the adhesive apparatus is of endodermal origin. This marked development of endodermal tissue in the buccopharyngeal upper lip of these fishes doubtless accounts for the growth forward, in early embryonic stages, of the lip to the outer surface of the snout.

The median line of the dorsal wall of the buccopharyngeal upper lip forms the ventral half of a strand of cells called by v. Kupffer the hypophysis. This strand is cylindrical in form, with its cells arranged radially about its axis, and it is considered to have primarily enclosed a cavity which has been suppressed by the pressing together of its enclosing walls. This cylindrical strand is quite unquestionably the homologue of the prehypophysial canal of Petromyzon and the Plagiostomi and it will accordingly hereinafter be so referred to. The walls of this canal are directly continuous externally with the deeper layer of the ectoderm, a slight sharply pointed depression marking the place where the deeper layer of the ectoderm curves inward to join and become continuous with the walls of the canal. The walls of the inner end of the canal are said by v. Kupffer to be similarly continuous with the endoderm forming the dorsal wall of the gut and are so shown in his figures, but Neumayer says there is here fusion without perforation of the wall of the gut, while Holmgren says that there is contact but no actual fusion, and comparison with Petromyzon and the Plagiostomi indicates that this latter conclusion is quite certainly correct.

In slightly older embryos of Acipenser than that shown in the figure the inner end of the prehypophysial canal becomes detached from the endoderm, Prechordal Portion of the Vertebrate Head 603

turns posteriorly beneath the infundibulum and becoming enlarged acquires a central cavity and represents the hypophysis of the adult. This hypophysis is quite unquestionably the homologue of that of Petromyzon and hence quite certainly the homologue also of the elasmobranchian Rathke’s pocket. The dorsal wall of the prehypophysial canal of Acipenser must then correspond to the dorsal wall of the headfold of early embryos of Petromyzon and the definitive hypophysis be, morphologically, a fold in that wall, as it is in Petromyzon and the Elasmobranchii. Beyond the hypophysis the headfold of Actpenser must then turn outward close against its dorsal portion and when it reaches the outer surface of the head turn ventro-anteriorly across the outer surface of the buccopharyngeal upper lip and end in the stomodeal depression.

Internal to the inner end of the prehypophysial canal and hence internal to the anterior end of the definitive hypophysis, there is a small transverse V-shaped groove on the inner surface of the wall of the gut. It is called by Sawadsky the preoral gut and evidently corresponds to Sessel’s pocket in the Plagiostomi. That part of the outer wall of the gut that lies between it and the anteroventral edge of the buccopharyngeal plate must accordingly represent the floor of the mandibular section of the alimentary canal which has swung anterodorsally and become considerably elongated in connexion with the marked growth forward of the ventral ends of the mandibular and hyal arches, this all being in some way related to the enclosure of the egg in a relatively close-fitting membranous envelope.

The conditions and the relative positions of the organs here under consideration in carly embryos of Acipenser are thus markedly similar to those in much older embryos of Petromyzon, but the serial order and manner of their development is not the same in the two fishes. In earlier stages of development described by Sawadsky and Holmgren the buccopharyngeal upper lip has already extended forward to the outer surface of the head and is in position for the development of a functional adhesive organ, which, however, never develops in Acipenser. There is not the slightest indication of the definitive hypophysis. The headfold begins on the outer surface of the forebrain and extending anteroventrally across the outer surface of the buccopharyngeal upper lip ends in a slightly marked depression which indicates the position of the future stomodaeum. The hypophysis of this fish develops, as shown in the older embryos above considered, in relation to a hypophysial fold in that part of the headfold that covers the ventral surface of the trabeculocranial component of the snout. That component of the snout of these early embryos of Acipenser is already completely fused with the dorsoposterior surface of the buccopharyngeal upper lip which forms the visceral component of the snout. In order that a hypophysis may be developed the headfold must accordingly force its way in between these two already fused components of the snout until it reaches the region of the preoptic recess, and this is accomplished by the formation of what Holmgren calls a hypophysial cell plug. This plug is at first simply a marked thickening of the deeper layer of the ectoderm on

Anatomy LXXxII 39 604 Edward Phelps Allis, Jr.

the dorsal surface of the snout, the cells of the thickening being at first irregularly arranged. This mass of cells grows inward until its inner end reaches and comes into contact with the dorsal wall of the gut and, according to Sawadsky, it is only after this, at approximately the 45 hr. stage, that the cells assume the columnar shape and a central cavity in the column is to a certain extent indicated. The definitive hypophysis is then developed as a backward growth from the tip of the hypophysial cell plug beneath the infundibulum. The hypophysis of this fish, like that of the Plagiostomi, is thus definitely of ectodermal origin, endodermal cells, if actually present, having become secondarily attached to it.

The snout of Acipenser is primarily of the viscero-trabeculo-cranial type, but this cannot be a primitive condition for it is primarily of the trabeculocranial type in the Cyclostomata and Elasmobranchii. An adhesive organ apparently only develops on the outer end of the snout when it is of the viscero-trabeculo-cranial type, which would seem to indicate that this organ was not possessed by the ancestors of the Cyclostomata and Plagiostomi. An adhesive organ of this type is evidently needed immediately after the embryo leaves the membranous eggshell, this either accounting for or being the result of the marked early development of the buccopharyngeal upper lip, and as a result of the early development of the latter lip the prehypophysial canal develops before the hypophysis does, this being the reverse of the order of development of these two structures in Petromyzon and the Plagiostomi. The prehypophysial canal also here develops progressively from its outer to its inner end, which is the reverse of the manner of its development in Petromyzon and the Plagiostomi. The definitive hypophysis, like that of Petromyzon, also develops progressively from its outer to its inner end, which is the reverse of its manner of development in the Plagiostomi, and this reversal in the order of development of these structures is evidently due to the necessity of developing a hypophysis under changed conditions imposed by a closely fitting egg membrane.

In Amia calva the conditions, as described by Reighard & Mast (1908), Reighard & Phelps (1908) and de Beer (1923, 1926), are apparently strictly comparable with those in Acipenser, excepting in that the ventral ends of the mandibular and hyal arches do not grow forward to the level of the tip of the upper jaw at as early an age as they do in Acipenser. In Polypterus, which is also one of the Ganoidei, the early development has never been described so far as I can find. Certain authors have however said that the hypophysis of this fish opens on the roof of the buccal cavity, as it does in the Plagiostomi, but I found no indication of this in any of the several specimens that I examined in connexion with an earlier work (Allis, 1922).

CONCLUSIONS

In the Plagiostomi, and probably in all vertebrates, the polar cartilage is the pharyngeal element of the mandibular branchial bar, the trabecular Prechordal Portion of the Vertebrate Head 605

and palatine cartilages being the dorsal and ventral halves, respectively, of the premandibular branchial bar. The premandibular bars develop in the maxillary processes, the bend at the middle of the length of the curved bar directed anteriorly. The ethmoidal region of the chondrocranium is probably largely if not entirely of premandibular branchial-ray origin.

The so-called septum nasi of the Elasmobranchii, when present, is not a solid median plate of cartilage, always containing primarily a median cavity that represents the ventral portion of an embryonic cavum internasale.

The hypophysis, in the fishes above considered, is developed in relation to what is either actually or in principle a fold or pleat taken in the headfold at a definite place in its length. The tip of the fold or pleat lies approximately opposite the hind end of the infundibulum and approximately between cranial and visceral ectodermal surfaces, the external opening of the fold or pleat lying approximately opposite the preoptic recess. When the embryo, in early stages of development, lies in a body cavity or in a capacious and protective eggshell these two surfaces of the headfold actually swing together and fuse with each other excepting along the median line, but when in early stages of development the embryo is enclosed in a closely fitting and thin egg membrane this swinging together of these two parts of the headfold is replaced by a process of infolding which brings the two corresponding surfaces gradually and progressively in contact with each other internal instead of external to the line of the headfold. The actual swinging together of the two surfaces, which takes place in the Elasmobranchii and probably in all higher vertebrates, is probably always associated with internal fertilization, while the infolding which takes place in the Cyclostomata and Ganoidei is probably always associated with external fertilization.

The conditions in the Tcleostei, Dipnoi and Amphibia will be considered in a later work.

REFERENCES Auus, E. P., Jr. (1913). “The homologies of the ethmoidal region of the Selachian skull.” Anat.

Anz. Bd. xuiv.

—— (1917). “The preorbital portion of the chondrocranium of Chimaera colliei.”” Proc. Zool.

Soc. Lond.

—— (1922). “The cranial anatomy of Polypierus, with special reference to Polypterus bichir.”

J. Anat., Lond., vol. Lvt.

(1923). “Are the polar and trabecular cartilages of vertebrate embryos the pharyngeal

elements of the mandibular and premandibular arches?” J. Anat., Lond., vol. Lix.

—— (19236). “The cranial anatomy of Chlamydoselachus anguineus.”” Acta Zool. Bd. tv. —— (1926). ‘On the homologies of the prechordal portion of the skull of the Helocephali.”

J. Anat., Lond., vol. Lx.

(1981la@). ‘Concerning the homologies of the hypophysial pit and the polar and trabecular cartilages of fishes.” J. Anat., Lond., vol. LXv, pt. II.

(19316). “Concerning the mouth opening and certain features of the visceral endoskeleton of Cephalaspis.”” J. Anat., Lond., vol. LXV, pt. Iv.

(1932a). “‘Concerning the nasal apertures, the lachrymal canal and the buccopharyngeal

upper lip.” J. Anat., Lond., vol. LXVI, pt. IV.

—— (19326). The preoral gut, the buccal cavity and the buccopharyngeal opening in Ceratodus.”

J. Anat., Lond., vol. LXVI, pt. Iv.

39-2 606 Edward Phelps Allis, Jr.

Brrr, G. R., DE (1923). “Some observations on the hypophysis of Petromyzon and Amia. Quart. J. Micr. Sci. vol. Lxvu, pt. 11. —— (1926). The Comparative Anatomy, Histology and Development of the Pituitary Body. Biol. Monographs and Manuals. Edinburgh. —— (19314). ‘The development of the skull of Scyllium canicula.” Quart. J. Micr. Sei. vol. LXXIV, pt. Iv. —— (19316). “On the nature of the trabecula cranii.”” Quart. J. Micr. Sci. vol. LxxIVv, pt. IV. Brrr, G. R., DE & Moy-Tuomas, J. A. (1935). “On the skull of the Holocephali.” Philos. Trans. vol. ccoxxiv, No. 514. Dray, B. (1906). “‘Chimaeroid fishes and their development.” Publ. Carneg. Instn, vol. xxxtI. Gaurp, E. (1906). “Die Entwicklung des Kopfskelettes.” Handb. vergl. exp. Entwickel. v. O. Hertwig, Bd. mm. GEGENBAUR, C. (1872). ‘Das Kopfskelett der Selachier.” Untersuchungen vergl. Anat. Wirbeltiere, H. 3, Leipzig. : Ha ter, G. (1923). “‘ Ueber die Bildung der Hypophyse bei Selachiern.” Gegenbaurs Jb. Bd. Li. Hotmeren, N. (1931). “Note on the development of the hypophysis in Acipenser ruthenus.” Acta zool. Bd. x11. Houxtey, T. H. (1874). “On the structure of the skull and of the heart of Menobranchus lateralis.” Proc. roy. Soc. . “Preliminary note upon the brain and skull of Amphioxus lanceolatus.”’ Proc. roy. Soc. Kuprrer,C. v. (1893). ‘“ Die Entwicklung des Kopfes von Acipenser sturio.” Stud. vergl. EntwGesch. Kopfes Kranioten, H. 1. —— (1894). “Die Entwicklung des Kopfes von Ammocoetes planeri.” Stud. vergl. EntwGesch. Kopfes Kranioten, H. 2. —— (1900). “Zur Kopfentwicklung von Bdellostoma.” Stud. vergl. EntwGlesch. Kopfes Kranioten, H. 4. Neumayer, L. (1932). “Studien itiber die Entwicklung des Kopfes von Acipenser. I.” Acta zool. Bd. xm. Parker, W. K. (1876). “On the structure and development of the skull in sharks and skates.” Trans. Zool. Soc. Lond. vol. x. Pratt, J. (1891). ‘‘ Further contribution to the morphology of the vertebrate head.” Anat. Anz. Bd. v1. Reicuarp, J. & Mast, S. O. (1908). “Studies on Ganoid Fishes. II. The development of the hypophysis of Amia.” J. Morph. vol. xix. Reicuarp, J. & Puxtps, J. (1908). “The development of the adhesive organ and head mesoblast of Amia.” J. Morph. vol. x1x. Sawapsky, A. (1912). ‘“ Die Entwicklung des larvalen Haftapparates beim Sterlet.” Anat. Anz. Bd. xu. Scammon, R. E. (1911). “Normal plates of the development of Squalus Acanthias.” Normentaf. Wirbelt. Jena. Sonaurnsianp, H. (1903). “ Beitrige zur Entwicklungsgeschichte und Anatomie der Wirheltiere.”’ Zoologica, Bd. xvi. SEwertzorF, A. N. (1899). “Die Entwickelungsgeschichte des Selachierschadels.” Festschriften C. v. Kupffer. Jena. Szwerrtzorr, A. N. & Dresitzr, N. N. (1924). “Das Pharyngomandibulare der Selachier.” Anat. Anz. Bd. Lvim. Sremann, H. (1898). “Ueber die erste Entwicklung des Tuba Eustaschii und des Kopfskeletts von Rana temporalis.”” Zool. Jb. Bd. xt. Stéup, P. (1882). “Zur Entwicklungsgeschichte des Anurenschadels.” Z. wiss. Zool. Bd. xxxi11.

” a.c. bp.0. bp.ul. b.tr. Cp. emd. eth.c.

fo.

fb.c. fine. fo. fol. ft.

hp. ifd. Inc.

ng. non.

np.

Prechordal Portion of the Vertebrate Head

607

EXPLANATION OF LETTERING

Alimentary canal.

Buccopharyngeal opening.

Buccopharyngeal upper lip.

Basitrabecula (Parker).

Cephalic plate.

Epimandibula.

Ethmoidal chamber.

Forebrain.

Forebrain cavity.

Front wall of nasal capsule (de Beer).

Foramen for optic nerve (de Beer).

Olfactory foramen (de Beer).

Foramen for trochlear nerve (de Beer).

Headfold.

Hypophysis.

Infundibulum.

Side wall of nasal capsule (de Beer).

Schlussmembran der Prafrontalliicke (Gegenbaur).

Mandibular arch.

Mandibular bar.

Mandibular gut.

Median wall of nasal capsule (de Beer).

Maxillary process.

Naselkapsel (Gegenbaur).

Notochord.

Nasal cartilage (de Beer).

Outer process of nasal cartilage (de Beer).

Neural groove.

Notch for optic nerve (de Beer).

Neuropore.

Optic foramen (Gegenbaur).

oc. ol.

tr, f. Vv.

Orbital cartilage (de Beer).

Olfactorius Bucht der (Gegenbaur).

Olfactory plate.

Optic vesicle.

Orbit.

Polar cartilage.

Prehypophysial ganal.

Planum antorbitale (de Beer).

Premandibular bar.

Premandibular section of brain floor.

Premandibular gut.

Schadelhéle

. Premandibular mesoderm.

Palatine process.

Preoptic root of the orbital cartilage (de Beer). .

Praesphenoidvorsprung (Gegenbaur).

Rostrum (Gegenbaur).

Medialer Schenkel (Gegenbaur).

Lateraler Schenkel (Gegenbaur).

Rostrum.

Rathke’s pocket.

Lateral rostral process (de Beer).

Median rostral process (de Beer).

Septum nasi.

Suprarostral process (de Beer).

Trabecula (de Beer).

Trabecular plate (de Beer).

Trabecula.

Trabecular fenestra (Parker).

Mediane Leiste des Internasalknorpels (Gegenbaur).

Probable tip of Rathke’s pocket.

---

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