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Show 664 MR. F. M. BALFOUR ON THE SKELETON [June 7, pterygium, the mesopterygium, and the metapterygium; and these names are now generally adopted. The metapterygium is by far the most important of the three, and in Scyllium canieula supports 12 or 13 rays1. It forms a large part of the posterior boundary of the fin, and bears rays only on its anterior border. The mesopterygium supports 2 or 3 rays, in the basal parts of which the segmentation into distinct rays is imperfect; and the pro-pterygium supports only a single ray. The pelvic fins are horizontally placed, like the pectoral fins, but differ from the latter in nearly meeting each other along the median ventral line of the body. They also differ from the pectoral fins in having a relatively much broader base of attachment to the sides of the body. Their cartilaginous skeleton (woodcut, fig. 2) consists of a basal bar, placed parallel to the base of the fin, and articulated in front with the pelvic girdle. On its outer border it articulates with a series of cartilaginous fin-rays. I shall call the basal bar the basipterygium. The rays which it bears are most of them less segmented than those of the pectoral fin, being only divided into two; and the posterior ray, which is placed in the free posterior border of the fin, continues the axis of the basipterygium. In the male it is modified in connexion with the so-called clasper. The anterior fin-ray of the pelvic fin, which is broader than the other rays, articulates directly with the pelvic girdle, instead of with the basipterygium. This ray, in the female of Scyllium canieula and in the male of Scyllium catulus (Gegenbaur), is peculiar in the fact that its distal segment is longitudinally divided into two or more pieces, instead of being single as is the case with the remaining rays. It is probably equivalent to two of the posterior rays. Development of the paired Fins.-The first rudiments of the limbs appear in Scyllium, as in other fishes, as slight longitudinal ridgelike thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins. These ridges are two in number on each side-an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected; but in some Elasmobranch embryos, more especially in that of Torpedo, they are connected together at their first development by a line of columnar-epiblast cells. This connecting line of columnar epiblast, however, is a very transitory structure. The rudimentary fins soon become more prominent, consisting of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold, and the fin becomes a simple ridge of mesoblast covered by epiblast. The pectoral fins are at first considerably ahead of the pelvic fins in development. The direction of the original epithelial line which connected the -* In one example where the metapterygium had 13 rays the mesopterveium had only 2 rays. l ° |