A treatise on zoology . Fui. 49. Left-side view of a portion of the tail of Raja. The vertebi-al column has been exposed infront, also the en<loskeleton of the two dorsal fins, showing the concrescence of the radials attlieir base to form a posterior axis. necessary to assume this even on Gegenbaurs theory, which derivesthe fin-skeleton from originally separate gill-rays. The objection often raised that, in Elasmobranchs, the fin-radials PAIRED FINS 77 become differentiated in a continuous procartilaginous rudiment(Balfour [28], Kuge [337a]), and not as separate rudiments, appliesequally to
A treatise on zoology . Fui. 49. Left-side view of a portion of the tail of Raja. The vertebi-al column has been exposed infront, also the en<loskeleton of the two dorsal fins, showing the concrescence of the radials attlieir base to form a posterior axis. necessary to assume this even on Gegenbaurs theory, which derivesthe fin-skeleton from originally separate gill-rays. The objection often raised that, in Elasmobranchs, the fin-radials PAIRED FINS 77 become differentiated in a continuous procartilaginous rudiment(Balfour [28], Kuge [337a]), and not as separate rudiments, appliesequally to the median fins (Balfour, Braus [50, 176]). More-over, in Teleosts, and possibly in many other fish, the radials areseparate from the first (Harrison [197]). It is also urged that themetamerism of the radial muscles of the adult fin does not correspondto that of the embryo (Braus [50]). Now, although it may be true. Aa, CO. Fig. 50. Acanthias vuhjarit:, Risso. A, dorsal tiu and portion of the vertebral column which hasbeen cut through longitudinally in front. B, calcitied cylinder of a centrum. C, transversesection of the vertebral column, caudal region. D, surface view of denticles as disposed inthe skin, a, canal for aorta ; , anterior cartilages (either modified radials or neural spines);, basal cartilage; )l, basal plate; ca, calcified cylinder; ct, cartilaginous centrum; ,dorsal fin ; , haemal arch ; in, interdorsal; iv, intervertebral ligament; li, dorsal ligament;, neural arcii; , neural canal; nt, notochord ; s, blunt spine ; sp, fin spine, with base cutaway to expose cartilage core ; r, radial; v, canal for caudal vein. The cartilage is dotted. that muscle-buds fuse at their base, that a nerve-plexus is formed,and that the substance of an adult radial muscle may not be derivedentirely from one bud, yet it is doubtless derived mainly fromthat bud the position of which
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
