LIMBS. 6ll 



Before proceeding to the development of the skeleton of 

 the fin it may be pointed out that the connection of -the two 

 rudimentary fins by a continuous epithelial line suggests the 

 hypothesis that they are the remnants of two continuous lateral 

 fins 1 . 



Shortly after the view that the paired fins were remnants of 

 continuous lateral fins had been put forward in my memoir on 

 Elasmobranch Fishes, two very interesting papers were published 

 by Thacker (No. 489) and Mivart (No. 484) advocating this 

 view on the entirely independent grounds of the adult structure 

 of the skeleton of the paired fins in comparison with that of the 

 unpaired fins 2 . 



The development of the skeleton has unfortunately not been 

 as yet very fully studied. I have however made some investi- 

 gations on this subject on Scyllium, and 'Swirski has also made 

 some on the Pike. 



In Scyllium the development of both the pectoral and pelvic 

 fins is very similar. 



In both fins the skeleton in its earliest stage consists of a bar 

 springing from the posterior side of the pectoral or pelvic girdle, 

 and running backwards parallel to the long axis of the body. 

 The outer side of this bar is continued into a plate which 



1 Both Maclise and Humphry (Journal of Anat. and Phys., Vol. v.) had 

 previously suggested that the paired fins were related to the unpaired fins. 



2 Davidoff in a Memoir (No. 477) which forms an important contribution to our 

 knowledge of the structure of the pelvic fins has attempted from his observations to 

 deduce certain arguments against the lateral fin theory of the limbs. His main 

 argument is based on the fact that a variable but often considerable number of the 

 spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with 

 the true plexus of the nerves supplying the fin. From this he concludes that the pelvic 

 fin has shifted its position, and that it may once therefore have been situated close 

 behind the visceral arches. If this is the strongest argument which can be brought 

 against the theory advocated in the text, there is I trust a considerable chance of its 

 being generally accepted. For even granting that DavidofTs deduction from the 

 character of the pelvic plexus is correct, there is, so far as I see, no reason in the 

 nature of the lateral fin theory why the pelvic fins should not have shifted, and on the 

 other hand the longitudinal cord connecting some of the spinal nerves in front of the 

 pelvic fin may have another explanation. It might for instance be a remnant of the 

 time when the pelvic fin had a more elongated form than at present, and accordingly 

 extended further forwards. 



In any case our knowledge of the nature and origin of nervous plexuses is far too 

 imperfect to found upon their character such conclusions as those of Davidoff. 



392 



