| OCR Text |
Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 31 filaments, the anterior cerebral vesicle is already distinctly divided into the thalamencephalon (from which the large infundibulum proceeds below, and the small tubular peduncle of the pineal gland above, while the optic nerves leave its sides) and a large single oval " vesicle of the hemispheres." O n the ventral face of the integument covering these are two oval depressions, the rudimentary olfactory sacs. As development proceeds, the vesicle of the hemispheres becomes divided by the ingrowth of a median longitudinal septum, and the olfactory lobes grow out from the posterior lateral regions of each "ventricle" thus formed, and eventually rise onto the dorsal faces of the hemispheres, instead of, as in most Vertebrata, remaining on their ventral sides. I m a y remark that I cannot accept the views of Miklucho-Maclay, whose proposal to alter the nomenclature of the parts of the Elasmobranch's brain appears to m e to be based upon a misinterpretation of the facts of development. II. The Skull. Dr. Gunther * distinguishes in the skull an " inner cartilaginous capsule and an outer incomplete osseous case, to which again some outer cartilaginous elements are appended. In the former the con- Fig. 4. Ceratodus forsteri. Lateral view of the chondrocranium, with most of the bones and fibrous tissues removed. ol, position of the olfactory sac; an, pn, position of the anterior and posterior nares; a, process of the cranial cartilage, d, between the two; 1, 2, upper labial cartilages; op, optic foramen; A.0, antorbital process; P.0, postorbital process ; P. Q, palato-quadrate process; Sq, Qu, remains of the bony plate which lies on the outer side of this process; 0. C, occipital prolongation of the chondrocranium roofing over the branchial cavity; D, dentary ; An, angular; Hy, hyoid ; I.op, interoperculum ; Op, operculum; B, B', cartilages representing hyoidean rays; HM, hyo-mandibular. fluence of cartilage is so complete, that no distinct divisions are traceable by sutures; its parts can be designated only by reference to the locally corresponding bones of the teleosteous skull." * L. c. p. 521. |
