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Show 1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 665 two fins of each side is nearby, though not quite, longitudinal, sloping somewhat obliquely ventralwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line shortly behind the anus. The embryonic muscle-plates, as I have elsewhere shown, grow into the bases of the fins; and the cells derived from these ingrowths, which are placed on the dorsal and ventral surfaces in immediate contact with the epiblast, probably give rise to the dorsal and ventral muscular layers of the limb, which are shown in section in Plate LVII. fig. 1 m and in Plate LVIII. fig. 7 m. The cartilaginous skeleton of the limbs is developed in the indifferent mesoblast cell between the two layers of muscles. Its early development in both the pectoral and the pelvic fins is very similar. W h e n first visible it differs histologically from the adjacent mesoblast simply in the fact of its cells being more concentrated ; while its boundary is not sharply marked. At this stage it can only be studied by means of sections. It arises simultaneously and continuously with the pectoral and pelvic girdles, and consists, in both fins, of a bar springing at right angles from the posterior side of the pectoral or pelvic girdle, and running parallel to the long axis of the body along the base of the fin. The outer side of this bar is continued into a thin plate, which extends into the fin. The structure of the skeleton of the fin slightly after its first differentiation will be best understood from Plate LVII. fig. 1, and Plate LVIII. fig. 7. These figures represent transverse sections through the pelvic and pectoral fins of the same embryo on the same scale. The basal bar is seen at bp, and the plate at this stage (which is considerably later than the first differentiation) already partially segmented into rays at br. Outside the region of the cartilaginous plate is seen the fringe with the horny fibres {h.f) ; and dorsally and ventrally to the cartilaginous skeleton are seen the already well-differentiated muscles (m). The pectoral fin is shown in horizontal section in Plate LVIII. fig. 6, at a somewhat earlier stage than that to which the transverse sections belong. The pectoral girdle (p.g) is cut transversely, and is seen to be perfectly continuous with the basal bar (bp) of the fin. A similar continuity between the basal bar of the pelvic fin and the pelvic girdle is shown in Plate LVII. fig. 2, at a somewhat later stage. The plate continuous with the basal bar of the fin is at first, to a considerable extent in the pectoral, and to eorae extent in the pelvic fin, a continuous lamina, which subsequently segments into rays. In the parts of the plate which eventually form distinct rays, however, almost from the first the cells are more concentrated than in those parts which will form the tissue between the rays; and I a m not inclined to lay any stress whatever upon the fact of the cartilaginous fin-rays berna; primitively part of a continuous lamina, but regard it as a secondary phenomenon, dependent on the mode of conversion of embryonic mesoblast cells into cartilage. In all cases the separation PROC. ZOOL. Soc-1881, No. XLIII. 43 |
