. The elasmobranch fishes. Chondrichthyes. 110 THE ELASMOBRANCH FISHES It is evident that the number of segments tliat take part in the formation of buds for the pectoral fin is fewer in the sharks than in the rays. This fact is clear when we consider two types like Mustelus and Torpedo, in the former of which the fin is relatively narrow and in the latter is of great extent. Accord- ing to Maurer (1912), in the embryo of Mustelus only ten segments contribute to the formation of the musculature of the pectoral fin; while in Torpedo there are twenty-six such segments. The further course of the
. The elasmobranch fishes. Chondrichthyes. 110 THE ELASMOBRANCH FISHES It is evident that the number of segments tliat take part in the formation of buds for the pectoral fin is fewer in the sharks than in the rays. This fact is clear when we consider two types like Mustelus and Torpedo, in the former of which the fin is relatively narrow and in the latter is of great extent. Accord- ing to Maurer (1912), in the embryo of Mustelus only ten segments contribute to the formation of the musculature of the pectoral fin; while in Torpedo there are twenty-six such segments. The further course of the development of these buds in two forms like the above has been studied in great detail because of the bearing which such development has on the lateral fin-fold theory. That in a type like Mustelus segments (myotomes) an- terior to the pectoral fin and between the pectoral and pelvic fins form buds which atrophy without enter- ing the fin is taken by those wdio accept the lateral fin-fold theory to mean that the fin previously had a much greater anteroposterior extent than at present; and it is hence in agreement with what would be ex- pected from that theory. Muscles of the unpaired fins are formed in essential respects like those of paired fins. As the myotome grows dorsally to the middorsal line it gives off buds in the regions of the dorsal fins and the dorsal lobe of the caudal fin. Each bud for the un- paired fins divides into an anterior and a posterior bud but no further division takes place since the buds from opposite sides unite to form the muscu- lature of the unpaired fins. Ventral buds arise from the tip of the tail forward to the anal region. Tlie more posterior of these supply the ventral lobe of the caudal fin, while those in the region of the anal fin, in forms in which an anal fin develops, provide musculature for that Fig. 112. Adult pectoral muscles, Sqtialus sucJdii. (Evelyn Forsythe, orig.) cL, gill cleft; , dorsal bundle; , dorsal radial
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