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The Homologies of the Hyomandibula of the Gnathostome Fishes

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The Homologies of the Hyomandibula of the Gnathostome Fishes THE HOMOLOGIES OF THE HYOMANDIBUU OF THE GNATHOSTOME FISHES EDWARD PHELPS ALLIS, JR. Menton, France ONE FIGURE Work that I have under way on the cranial anatomy of Chla- mydoselachus has led to certain conclusions regarding the hyo- mandubula which, if correct, are of considerable morphological importance. They are based on the assumption, which seems practically established by my work, that the dorsal ends of all of the so-called inner cartilaginous bars of all of the visceral arches, in all of the gnathostome fishes, always lie ventral to the vena jugularis, and that when parts of the cartilaginous bars of the adult fish articulate or fuse with the neurocranium dorsal to that vein, those parts are derived either from the external cartilaginous bars of the arches, or from interarcual cartilages developed in, or in relation to, the dorsal interarcual ligaments. The terms inner and external cartilaginous bars, or arches, are here used with the significance commonly given to them, but my work tends decidedly to confirm Dohrn's conclusion that the so-called inner cartilaginous bars of the branchial arches of the gnathostome fishes, and not the external ones, are the homologues of the cartilaginous branchial arches of the Cyclostomata. The cartilages which form the so-called external arches of the gnathostome fishes are commonly called the extrabranchials in whatever arch they may be found. Parker ('76) limited the use of this term to the external cartilages of the gill-bearing, branchial and hyal arches, employing the term extraviscerals to designate, collectively, these cartilages and certain others, the so-called la- bial cartilages of his descriptions, which he considered to be their serial homologues in the prehyal arches. The use of the term 563 JOURNAL OP YORPHOLOQY. VOL. 26, NO. 4 DECEMBER, 1915 564 EDWARD PHELPS ALLIS, JR. extravisceral, thus employed, has been properly objected to, but as the term visceral is currently applied, not only to all of these arches themselves, but also to their inner cartilaginous bars, there would seem to be no good reason for not applying it also to the external cartilaginous bars. But as, with equal reason, the branchial and branchiostegal rays would then have to be collectively called the visceral and viscerostegal rays, I retain the term extrabranchial for these cartilages in whatever arch they may be found. Branchial, hyal and visceral I employ as proposed by Gaupp ('05). .The vena jugularis, which seems to have had a markedly im- portant influence on the development of the definitive carti- laginous bars of the prebranchial visceral arches, is not always, in the different orders of fishes, formed by the fusion of identical sections of the venae cardinalis anterior and capitis lateralis of embryos, the former vein being the primitive one and being said to lie ventro-mesial to the roots of all of the cranial nerves, while the latter one is of secondary formation and is said to lie dorso-lateral to those roots (Hochstetter '06). The definitive vein also has, in certain fishes, different relations to the hyoman- dibula, as I have quite recently had especially called to my at- tention, lying dorso-external to the hyomandibula in Chlamy- doselachus but ventro-internal to that element in Amia and tele- osts (Allis '14 b, p. 235). Wishing to know if these differences in the relations of the vein to the cranial nerves and to the hyomandibula were in any way related to each other, I have had the relations of the vein to the trigeminus and posttrigeminus nerves traced in a certain number of fishes. In the Selachii and Batoidei the definitive vena jugularis is said by Hochstetter ('06) to be formed entirely by the vena capi- tis lateralis and to agree in this with the definitive vein in Tro- pidonotus and the Mammslia. In Mustelus (probably laevis) I find the vein lying dorsal to all of the components of the nerves here under consideration, excepting only the latero-sensory fi- bers which, in their exit from the cranium and in their peripheral distribution, are associated with the nervi trigeminus, glossopha- ryngcus and vagus. The vein lies ventral to all of these latero- HYOMANDIBULA OF THE GNATHOSTOME FISHES 565 sensory nerves, but it lies dorsal to the latero-sensory fibers that issue with and as a part of the nervus hyomandibularis facialis. In the Teleostomi, Hochstetter does not give either the method of development or the composition of the definitive vena jugu- laris. I find this vein, on one side of the head of a 43 mm. em- bryo of Amia calva, running posteriorly ventral to the ganglion on the main root of the nervus trigeminus and then upward between that ganglion and the ganglion on the root of the nervus facialis; then posteriorly dorsal to the latter ganglion, and dorsal also to the latero-sensory fibers which issue with the nervus hyomandibularis facialis, but ventral to the latero-sensory fibers which issue with the nervus trigeminus; then downward between the nervi facialis and glossopharyngeus; and then posteriorly ventral to the latter nerve and the vagus: the first section of the vena capitis lateralis that is formed in this fish thus correspond- ing to the one said by Hochstetter to be first formed in reptiles. On the other side of the head of this embryo of Amia the vena jugularis had a similar course, but a large branch of it passed dor- sal to the nervus glossopharyngeus and then downward between that nerve and the vagus to fall again into the main vein, this thus showing a second section of the vena capitis lateralis in process of formation. In one adult specimen of Amia I find the vein running ventral to the nervus trigeminus, dorsal to the nervi facialis and glossopharyngeus but ventral or lateral, and hence morphologically ventral, to the latero-sensory nerves which issue with the nervi trigeminus and glossopharyngeus, and then ven- tral to the nervus vagus. The definitive vena jugularis of Amia is thus formed by the trigeminus and vagus sections of the vena cardinalis anterior and the facialis and glossophFyngeus sec- tions of the vena capitis lateralis, and it corresponds, not only to the second stage in the formation of the definitive vein in reptiles, as given by Hochstetter, but also to the definitive vein (vena petroso-lateralis) of amphibians as given by Driiner ('04). The conditions in the one embryo examined would seem to show that no other sections of the vena capitis lateralis are ever formed in Amia. 566 EDWARD PHELPS ALLIS, JR. In an 80 mm. specimen of Lepidosteus osseus and a 141 mni. specimen of Polyodon spathula the vena jugularis has the same relations to the several cranial nerves that it has in Amia, and I find similar conditions in young specimens of Hiodon tergisus, Scorpaena scrofa and Cottus aspera, and in the adults of Scom- bresox saurus, Gadus merlangus and Trigla hirundo. In a 57 mm. specimen of Catostoma occidentalis and a 40 mm. specimen of Gastrosteus aculeatus the glossopharyngeus section of the vein was the only one which lay dorsal to the related nerve. In an adult Cyprinus carpio and a 48 mm. specimen of Carassius auratus the facialis section of the nerve alone had this position. In embryos and the adult of Ameiurus nebulosus the vein lay ventral to all of the four nerves here under consideration, the entire vena cardinalis anterior thus here persisting as the defini- tive vein; Ameiurus, and hence probably all of the Siluridae, thus differing from other teleosts in this respect as well as in the arrangement of the pseudobranchial and carotid arteries, the innervation of the recti muscles of the eye-ball, and the condition of the myodome (Allis, '08, '09) In Ceratodus embryos of Semon's Stage 45, Greil ('13, figs. 2-3, pl. 51) shows the vena jugularis, called by him the capit'is lateralis, lying ventral to the nervus trigeminus and dorsal to the nervi facialis, glossopharyngeus and vagus, but, as in the Plagiostomi and Teleostomi, ventral to the latero-sensory nerves which issue with the nervi trigeminus, glossopharyngeus and vagus. In Stage 48 of this fish (1.c. fig. 2, pl. 55) the vein appar- ently still lies ventral to the nervus trigeminus and the same latero-sensory nerves, but it here lies ventral also to the nervus facialis, though still dorsal to the glossopharyngeus and vagus. This must, accordingly, be a less advanced stage than that shown in the embryo of Stage 45, unless it be that the vein change, a second time, its relation to the nervus facialis. But however this may be, it is evident that the definitive vein in this fish is of the plagiostoman and reptilian (Lacerta) type rather than the teleostean or amphibian. One further feature of this vein may here be mentioned. The large orbital venous sinus, found so well developed in the Plagi- HYOMANDIBULA OF THE GNATHOSTOME FISHES 567 ostomi, surrounds the nervus opticus, and the nervus opthalmi- cus profundus traverses it in its course through the orbit, thus in a measure being also surrounded by it. This sinus would t.hus seem to correspond to that stage in the development of the posterior sections of the vena capitis lateralis which Hochstetter (1. c., p. 133) describes as a 'Veneninsel' surrounding the related segmental nerve. A crescentic sinus is found, in the Plagios- tomi, similarly related to the nems olfactorius. These several variations in the relations of the vena jugularis to the cranial nerves, while they emphasize the facts that the Pla- giostomi form a group wholly apart from the other gnathostome fishes and that the Siluridae are similarly grouped apart from the other Teleostei, do not present any features which indicate that they have in any way influenced the development of the hyomandibula, as will be later evident.
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