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Show 1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 667 The pelvic fin thus retains in all essential points its primitive structure. The Pectoral Fin.-The earliest stage of the pectoral fin differs, as I have shown, from that of the pelvic fin only in minor points (Plate LVIII. fig. 6). There is the same longitudinal or basipterygial bar (bp), to which the fin-rays are attached, which is continuous in front with the pectoral girdle {pg). The changes which take place in the course of the further development, however, are very much more considerable in the case of the pectoral than in that of the pelvic fin. The most important change in the external form of the fin is caused by a reduction in the length of its attachment to the body. At first (Plate LVIII. fig. 6), the base of the fin is as long as the greatest breadth of the fin; but it gradually becomes shortened by being constricted off from the body at its hinder end. In connexion with this process the posterior end of the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (Plate LVIII. figs. 8 & 9), constituting the metapterygium (mp). It becomes eventually segmented off from the pectoral girdle, simply articulating with its hinder edge. The plate of cartilage, which is continued outwards from the basipterygium, or, as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as the homologous part of the pelvic fin; and this is especially the case with the basal part of the plate. This basal part becomes, in fact, at first only divided into two parts (Plate LVIII. fig. 8)-a small anterior part at the front end {me.p), and a larger posterior along the base of the metapterygium {mp); and these two parts are not completely segmented from each other. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the (at this stage undivided) rudiment of the mesopterygium and propterygium of Gegenbaur. It bears in m y specimen of this age four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are prolongations outwards of the edge of the plate continuous with the metapterygium. These rays are at the stage figured more or less transversely segmented ; but at their outer edge they are united together by a nearly continuous rim of cartilage. The spaces between the fin-rays are relatively considerably larger than in the adult. The further changes in the cartilages of the pectoral limb are, morphologically speaking, not important, and are easily understood by reference to Plate LVIII. fig. 9 (representing the skeleton of the limb of a nearly ripe embryo). The front end of the anterior basal cartilage becomes segmented off as a propterygium {pp), bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium (mes). The remainder of the now considerably segmented fin-rays are borne by the metapterygium. General Conclusions.-From the above observations, conclusions of a positive kind may be drawn as to the primitive structure of the 43* |
