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. 2.5. Diagram A-D illustrating the gill-arch theory of Gegenbaur for the origin of paired 

 fins, h.r., branchial ray; g.a., 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, fig. 84a). It is therefore difficult to see how the pectoral girdle could 

 have reached its present position outside of blood vessels and nerves. 



Evidence supporting the lateral fin-fold theory has been obtained from sev- 

 eral sources. It has been shown by Dean (1909) , for example, that in Cladose- 

 lachiis (see p. 2, fig. 10), one of the most ancient sharks yet discovered, the 

 paired fins are essentially lateral folds unconstricted at the base and with 

 radial supports nearly parallel. From development also comes evidence for 

 the lateral fin-fold theory. In a type like Scyllmm (fig. 24) or Prist iur us or 

 Torpedo there may be in the embryo an epiblastic fold along the back and 

 over the tail, and in Torpedo embryonic lateral folds may extend from the 

 pelvic region behind to the pectoral region in front. Furthermore, Dohrn has 

 shown that the skeleton of the fin in Pristiurus arises as a series of parallel 

 radials and that the girdle at the same time arises from a fusion of the bases 



