j6 STErCTUEE OF THE PAIEED FIXS OE CEEATODUS, 



is to be regarded as derivable from sucli simpler limb-skeletons as 

 those of the Selachii. 



In his work on the "Development of Elasmobranch Fishes " and 

 in his "Comparative Embryology" F. M. Balfour gives an account 

 of the development of the fins of Scy Ilium, which is important as 

 throwing considerable light upon this question. Externally each 

 fin first shews itself as a ridge of epiblast, subsequently enclosing 

 mesoblast ; the front and hind fins of each side arc connected at 

 first in some JElasmohranchs by a very low ridge of epiblast cells ; 

 but this connecting ridge soon disappears. Its existence, however, 

 at this early stage would seem to render it probable, as pointed out 

 by Balfour, that the pectoral and pelvic fins of each side had 

 originally the form of a continuous fold similar in nature to the 

 unpaired fins. The first rudiment of the skeleton of the fin 

 consists of a bar of cartilage attached in front to the limb-arch 

 and running backwards parallel with the long axis of the body ; 

 and a plate which extends into the fin and very soon becomes 

 divided into a series of cartilaginous rods placed at right angles 

 to the longitudinal bar. By a series of changes which are greater 

 in the pectoral than in the pelvic fins, and include in the former 

 the rotation outwards of the basal bar or hcmpterycjium which 

 becomes converted into the onetapterijglum, and the introduction 

 of additional basal elements, the primitive longitudinal bar and 

 segmented plate become converted into the skeleton of the adult fin. 



The most important result of these researches, as regards the 

 present question, was the proof that the paired fins are developed 

 from structures which are essentially very similar to the unpaired 

 fins. More recently Mivart* in a memoir entitled " On the Eins 

 of Elasmobranchs, with Considerations on the Nature and 

 Homologues of Vertebrate Limbs " (Trans. Zool. Soc, vol. x.) 

 has endeavoured to shew that this fundamental identity of the 



* And independently J. K. Thacker in a memoir on the Median and Paired 

 Fins in the Transactions of the Connecticut Academy 



