66 



Morphology of the Fins 



by side, the type seen in modern sharks. Whatever the primi- 

 tive form of shark-fin, it may well be doubted whether any one 

 of these three {Cladoselache, Plenracanthns, or Heterodontus) 

 actually represents it. The beginning is therefore unknown, 

 though there is some evidence that Cladoselache is actually 

 more nearly primitive than any of the others. As we shall see, 

 the evidence of comparative anatomy may be consistent with 

 either of the two chief theories, while that of ontogeny or em- 

 bryology is apparently inconclusive, and that of paleontology 

 is apparently most easily reconciled with the theory of the fin- 

 fold. 



Development of the Paired Fins in the Embryo. — According to 

 Dr. John A. Ryder (" Embryography of Osseous Fishes," 1882) 

 "the paired fins in Teleostei arise locally, as short longitudinal 

 folds, with perhaps a few exceptions. The pectorals of Lepisostens 

 originate in the same way. Of the paired fins, the pectoral 



Fig. .5.3. — Pectoral fin of shark, Chiloscyllium. {Mter Parker and Haswell.) 



or anterior pair seems to be the first to be developed, the ventral 

 or pelvic pair often not making its appearance until after the 

 absorption of the yolk-sac has been completed, in other cases 

 before that event, as in Salino and in Gambusia. The pectoral 

 fin undergoes less alteration of position during its evolution 

 than the posterior pair." 



In the codfish (Gadns callarias) the pectoral fin-fold "ap- 

 pears as a slight longitudinal elevation of the skin on either 

 side of the body of the embryo a little way behind the auditory 

 vesicles, and shortly after the tail of the embryo begins to bud 

 out. At the very first it appears to be merely a dermal fold, 

 and in some forms a layer of cells extends out underneath it 

 from the sides of the body, but does not ascend into it. It 



