The elasmobranch fishes (1934) The elasmobranch fishes elasmobranchfish03dani Year: 1934 THE ELASMOBRANCII FISHES 15 understood to have resulted from a fusion of the branchial rays at their bases into a main axis which by extension drew out the adjoining rays so that they arose from this main axis (a-c) . When we consider that the pectoral girdle in form and position is much like a gill arch (see p. 49, fig. 54), and that the fin rays may be arranged both pre- and postaxially, Gegenbaur's theory appeals to us with much force. The appli- cation of the theory to tlie pelvic fin is more difficul
The elasmobranch fishes (1934) The elasmobranch fishes elasmobranchfish03dani Year: 1934 THE ELASMOBRANCII FISHES 15 understood to have resulted from a fusion of the branchial rays at their bases into a main axis which by extension drew out the adjoining rays so that they arose from this main axis (a-c) . When we consider that the pectoral girdle in form and position is much like a gill arch (see p. 49, fig. 54), and that the fin rays may be arranged both pre- and postaxially, Gegenbaur's theory appeals to us with much force. The appli- cation of the theory to tlie pelvic fin is more difficult, however, for the fin is assumed to have reached its present position by migrating from the branchial region. This assumption strikes one as far-fetched. Indeed, Dean (19026) has studied the problem of migration of the fins and concluded for Heterodontus, at least, that there is no evidence of migration. ^/y//.. D Fig. Diagram A-D illustrating the gill-arch theory of Gegenbaur for the origin of paired fins, , branchial ray; , gill arch. Further objections have been offered against the gill-arch theory. If the skeleton of the pectoral fin arose from the branchial rays of a single arch doubtless it at first occupied a dorsoventral position, a position disadvanta- geous to the fin as a directing organ. Moreover, if the pectoral arose as a modi- fied gill arch, why are there so many segments involved in the fin? Again, paired fins both in their development and in their structure resemble in a remarkable degree unpaired fins. But unpaired fins certainly have not arisen as modified branchial arches. Still another objection which has been urged against the theory is that the branchial arches lie in the walls of the digestive tract (pharynx) and hence within the aortic arches of the blood system. The pectoral girdle, on the contrary, is superficial to both the arteries and nerves as is shown by the fact that it is perforated hj these as they pass out to the fin (p. 78,
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