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Show 184 ON '!'HE S'fRUC'l'UR.E OF THh SKULL. speeial structural pec~li:tritics are su~eraddcd in cac~1 case. Both types of skull exhibit ma~.v Lone.· In ·on1mon, but Jn each type, some of these bonr~ acquire sp CJal arrangon1en~ .and vet·y different relative magnitudes; and each type exhibits bones peculiar to itself, the number of those pros nt in the Fish and absent in the Man, being very JllllOh greater than of those present in the 1\rfan and absent in the Pish. A' n1ight Lo expected, the study of the developmcut of the Fi h'' cranium brings out into still stronger hgbt thA fundan1ental r cmblances of its structure with that of the higher Vertebrates. Tho pri1nitive groo,Te makes its appearance on the bla tod rm, and becomes converted into a canal by tho arching over and coalescence of the dorsallaminm. rrhe ant rior part of the canal dilates and becomes subdivided into cerebral v iel . Tho notochord appears and terminates: in Ii-ont, in a point b hind the pituitary body; while round its apex, that bend of tho primitive cranium takes place which constitutes the · phalic flexure. Tho organs of sense make their appearanc in the . a me r gions, and the visceral arches and clefts ar developed in tho mno way. But a greater number of them app ar, and the posterior ones, instead of vanishing, give ri e to the branehial skeleton and branchial clefts. Th mandible i d veloped in tho first visceral arch, and the hyoid apparatus in the second, as in Man; but the details of the modo of origin of the hyomandibular and symplectic, of the palatine and maxillary apparatu., and of tho naso-frontal process, have not been as yet worked out with sufficient thoroughness to enaLle u to determine with certainty the homologies of all the resulting parts. The craniun1 i at fir t wholly membranous, but after a time it becomes partially chondrified in the , aine way as in the high~r Vertebrates (Fig. 72 ). Cartilage appears in the base of the skull upon each side of the notochord, and surrounds tho great auditory capsules. Anteriorly it divid s into two processes, the trabeculEE cranii ( Tr.), which separate so as to inclose the pituitary fossa (P), and reunite, in front of it, to form the ethmovomerine rostrum. :From tho floor of the skull, at tho front and lateral part of eaeh auditory capsule, a cartilaginous process (HM.) is given off: and pascing downwardB and forwards ends in THE DEVELOPMEN'l, OF THE SKULLS OF FISHES. 185 a free styliform process, which lies parallel with, and is bound by connective tissue to, the free hinder crus of an inverted arch of cartilage, the anterior crus of which passes into the pre-frontal region of the ethmo-vomerine cartilage. The centre of this Fig. 72. D. Ch. pal~to-quadr~te arch is prolonged into a process ( Qu.), which articul~tes With the cartilaginous ramus of the mandible, while the upp~r p~rt of the cartilage (HM Sy.) gives attachment· to the cartilaginous hyoid (Hy.). This is the earliest condition of the cartilaginous cranium of the osseous fish that has yet been observed; but it can hardly be doubted that the hyomandibular and palato-quadrate cartilages. ~ave alr~ady deviated considerably from their primitive conditiOn, and It would be a rnatter of great interest to ascertain whether these cartilages a1·e primitively continuous; or whether, |