732 DEVELOPMENT OF FINS OF ELASMOBRANCHS. 



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 en- 

 tirely secondary character of the mesopterygium, and its total 

 absence in the young embryo Scyllium, appear to me as con- 

 clusive 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 shew that an homology between . the two is impossible. 

 In support of his view he states 1 that he has satisfied himself, 

 from embryos of Acanthias and Scy Ilium, that the rays of the 

 unpaired fins are undoubtedly products of the segmentation of tJie 

 dorsal and ventral spinous processes. 



This statement is wholly unintelligible to me. From my 

 examination of the development of the first dorsal and the anal 

 fins of Scyllium I find that their rays develop at a considerable 

 distance from, and quite independently of, the neural and haemal 

 arches, and that they are at an early stage of development dis- 

 tinctly in a more advanced state of histological differentiation 

 than the neural and haemal arches of the same region. I have 

 also found exactly the same in the embryos of Lepidostetis. 



I have, in fact, no doubt that the skeleton of both the paired 

 and the unpaired fins of Elasmobranchs and Lepidosteus is in 

 its development independent of the axial skeleton. The phylo- 

 genetic mode of origin of the skeleton both of the paired and of 

 the unpaired fins cannot, however, be made out without further 

 investigation. 



1 Loc. cit. p. 514. 



