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Show 670 SKELETON OF FINS OF ELASMOBRANCHS. [June 7» to the establishment of the Elasmobranch fin, but in exactly the reverse order. There is one statement of Davidoff's which I cannot allow to pass without challenge. In comparing the skeletons of tbe paired and unpaired fins he is anxious to prove that the former are independent of the axial skeleton in their origin and that the latter have been segmented from the axial skeleton, and thus to show that an homology between the two is impossible. In support of his view he states1 that he has satisfied himself, from embryos of Acanthias and Scyllium, that the rays of the unpaired fins are undoubtedly products of the segmentation of the dorsal and ventral spinous processes. This statement is wholly unintelligible to me. From my examination of the development of the first dorsal and the anal fins of Scyllium I find that their rays develop at a considerable distance from, and quite independently of, the neural and haemal arches, and that they are at an early stage of development distinctly in a more advanced state of histological differentiation than the neural and haemal arches of the same region. I have also found exactly the same in the embryos of Lepidosteus. I have, in fact, no doubt that the skeleton of both the paired and the unpaired fins of Elasmobranchs and Lepidosteus is in its development independent of the axial skeleton. The phylogenetic mode of origin of the skeleton both of the paired and of the unpaired fins cannot, however, be made out without further investigation. EXPLANATION OP THE PLATES. PLATE LVII. Fig. 1. Transverse section through the pelvic fin of an embryo of Scyllium belonging to stage V2, magnified 50 diameters, bp, basipterygium; br, fin ray; m, muscle ; hf horny fibres supporting the peripheral part of the fin. 2. Pelvic fin of a very young female embryo of Scyllium stellare, magnified 16 diameters, bp, basipterygium ; pu, pubic process of pelvic girdle (cut across below) ; il, iliac process of pelvic girdle ; fo, foramen. 3. Pelvic fin of a young male embryo of Scyllium stellare, magnified 16 diameters. bp, basipterygium; mo, process of basipterygium continued into clasper : il, iliac process of pelvic girdle ; pu, pubic section of pelvic girdle. 4. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pectoral fins, to show how the fins are attached to the body, magnified 18 diameters, br, cartilaginous fin-ray ; bp, basipterygium ; m, muscle of fin ; mp, muscle-plate. 5. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pelvic fin, on the same scale as fig. 4. bp, basipterygium ; br, cartilaginous fin-rays ; m, muscle of the fins; mp, muscle-plate. PLATE LVIII. Fig. 6. Pectoral fin of an embryo of Scyllium canieula, of a stage between O 1 Loc. cit. p. 514. 2 I employ here the same letters to indicate the stages as in m y Monograph on Elasmobranch Fishes. |