| OCR Text |
Show 30 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, the separation of which into fourth and third ventricles was indicated only by slight constrictions of the roof and side walls. The large ventricle of the prosencephalon is partially separated into two chambers by a median septum, formed by the infolding of its ventral wall; and the spacious ventricle of each olfactory lobe opens into the dorsolateral part of each of these chambers. The place and mode of origin of the olfactory and of the optic nerves have already been mentioned. The third nerve is indicated in the figure; but I am somewhat doubtful as to the nature of the cord thus marked. No fourth or sixth nerve was observed. The fifth arises by a single large cylindrical root just below the anterior end of the cornu of the tela vasculosa of the fourth ventricle. The seventh and eighth leave the medulla by a common root just behind this ; and the roots of the ninth and tenth nerves, divided into three bundles, arise from a tract at the sides of the medulla which extends from the last to the hinder limit of the tela vasculosa, and incline obliquely backwards to their exit. The brain of Ceratodus is very singular and interesting, inasmuch as it presents resemblances to that of the "Marsipobranchii on one side, to that of the Ganoids and Amphibia on another, and to that of the Chimaeroids and Plagiostomi on a third. As in the brain of the Marsipobranchii, the pineal gland is relatively very large, with its pointed dorsal end inclined upwards and forwards, and the roof of the fourth ventricle is almost entirely formed by the tela vasculosa; but, as in the Ganoidei and Amphibia, the cerebellum is larger than in the Lampreys. In Ceratodus it is similar to, though proportionally less than, that of Lepidosteus, and still more like that of Polypterus. In the proportions of the thalamencephalon the brain of Ceratodus resembles that of the Sturgeon and that of the Ray; while in the representation of the cerebral hemispheres, or prosencephalon, by a large imperfectly divided lobus communis, from the dorso-lateral regions of which the olfactory lobes take their rise, the brain of Ceratodus presents a feature hitherto known, so far as I am aware, only in the Plagiostomi*. Thus, in its cerebral characters, Ceratodus occupies a central place in the class Pisces. The development of the cerebral hemispheres in Plagiostome fishes differs from the process by which they arise in the higher Vertebrata. In a very early stage, when the first and second visceral clefts of the embryo of Scyllium are provided with only a few short branchial * So far as I can judge from the examination of a small but well-preserved specimen of Lepidosiren anncctcns, for which I a m indebted to M r . Sclater, the brain of this fish is similar, in all essential respects, to that of Ceratodus. The figure of the brain of Lepidosiren given by Prof. O w e n in his ' Anatomy of Vertebrates ' is susceptible of interpretation in this sense. Hyrtl's description and figure of the brain of Lepidosiren paradoxa (Abhandlungen der koniglichen bohmischen Gesellschaft, Bd. iii. 1845), on the other hand, leave m e in doubt whether, apart from its curious asymmetry, the brain of this fish does or does not present important differences from that of Ceratodus and that of Lepidosiren annectcns. |
