. The elasmobranch fishes . —'Ai--' ;.j_ui£i-i. Fig. 24. Embryo of ScyUium canicula showing early epidermal fin-folds. (From Mayer.) al., anal fin; df., dorsal fin-fold; dl., second dorsal fin; pL, pelvic fin; pt., pectoral fin; , ventral fin-fold. sory to the pectorals. Both the dorsals and the anal may be bound down with- out interfering seriously with propulsion. These fins are of service, however, in keeping the fish in a vertical plane. FOEM OF FIN IN ITS BEGINNING It is evident that the form of the fin in present-day Elasmobranchs differs from that of the ancestral type. And the ques
. The elasmobranch fishes . —'Ai--' ;.j_ui£i-i. Fig. 24. Embryo of ScyUium canicula showing early epidermal fin-folds. (From Mayer.) al., anal fin; df., dorsal fin-fold; dl., second dorsal fin; pL, pelvic fin; pt., pectoral fin; , ventral fin-fold. sory to the pectorals. Both the dorsals and the anal may be bound down with- out interfering seriously with propulsion. These fins are of service, however, in keeping the fish in a vertical plane. FOEM OF FIN IN ITS BEGINNING It is evident that the form of the fin in present-day Elasmobranchs differs from that of the ancestral type. And the question arises, What was the form of the ancestral fin ? To this inquiry many answers have been given, few of which have gained a hearing. We may mention briefly two of the best known theories for the origin of the paired fins. One is the lateral fin-fold theory of Balfour and Thacher; the other is the gill-arch theory of Gegenbaur. According to the fin-fold theory the ancestors of the present-day fishes are supposed to have possessed a median dorsal fold {df., fig. 24) which was con- tinued over the tail as the dorsal lobe of the caudal fin and then forward to the anal region as the ventral lobe, {); anterior to the anal region the ventral lobe separated into right and left lateral folds not greatly unlike the meta- pleural folds of Amphioxus. In certain regions of the dorsal and the lateral folds greater development ensued than at the interspaces. These parts of the fin-fold hence increased in size and became the present unpaired and paired fins, while the intermediate parts finally dropped out. The gill-arch theor}^ of Gegenbaur holds that the framework of the girdles for the paired fins and of the fins themselves have been derived from the gill arches (, fig. 25) and their attached branchial rays (). The arch itself represents the girdle and the formation of the skeleton of the fin proper was
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