OF THE PAIRED FINS OF ELASMOBRANCHS. 67 



fact is that, as may easily be seen by an inspection of figs 6 

 and 7, such a second set of lateral rays could not possibly 

 have existed in a type of fin like that found in the embryo. 

 With this view of Gegenbaur's it appears to me that the 

 theory held by this anatomist to the effect that the limbs 

 are modified gill-arches also falls, in that his method of 

 deriving the limbs from gill-arches ceases to be admissible, 

 while it is not easy to see how a limb, formed on the type of 

 the embryonic limb of Elasmobranchs, could be derived from 

 a gill-arch with its branchial rays. 



Gegenbaur's older view, that the Elasmobranch fin retains 

 a primitive uniserial type, appears to me to be nearer the 

 truth than his more recent view on this subject; though I 

 hold the fundamental point established by the development 

 of these parts in Scyllium to be that the posterior border of 

 the adult Elasmobranch pectoral fin is the primitive base- 

 line, i e. line of attachment of the fin to the side of the body. 



Huxley holds that the mesopterygium is the proximal 

 piece of the axial skeleton of the limb of Ceratodus, and 

 derives the Elasmobranch fin from that of Ceratodus by the 

 shortening of its axis and the coalescence of some of its 

 elements. The entirely secondary character of the meso- 

 pterygium, and its total absence in the young embryo 

 Scyllium, appear to me as conclusive against Huxley's view 

 as the character of the embryonic fin is against that of 

 Gegenbaur; and I should be much more inclined to hold 

 that the fin of Ceratodus has been derived from a fin like 

 that of the Elasmobranchs by a series of steps similar to 

 those which Huxley supposes to have led to the establishment 

 of the Elasmobranch fin, but in exactly the reverse order. 



There is one statement of Davidoff's which I cannot allow 

 to pass without challenge. In comparing the skeletons of 

 the paired and unpaired fins he is anxious to prove that the 

 former are independent of the axial skeleton in their origin, 

 and that the latter have been segmented from the axial 

 skeleton, and thus to show that an homology between the 

 two is impossible. In support of his view he states^ that he 

 has satisfied himself, from embryos oi Acanthias and Scyllium, 

 that the rays of the unpaired fins are undoubtedly products of 

 the segmentation of the dorsal and ventral spinous processes. 



This statement is wholly unintelligible to me.^ From my 

 examination of the development of the first dorsal and the 



^ Loc. cit., p. 514. 



» It is possible tliat DavidofF may have only studied the ventral lobe of 

 the caudal fiu, which differs from the other unpaired fins in the fact that 

 there are no interspinoas elements supporting the horny tin-rays. 



