6l8 THE CHEIKOPTERYGIUM. 



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 that the fundamental point established by the development of these 

 parts in Scyllium is that the posterior border of the adult Elasmobranch fin 

 is the primitive base line, i.e. the 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 secondary character of the mesopterygium, and its 

 total absence in the embryo Scyllium, appears 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 Elasmobranchii 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. 



With reference to the development of the pectoral fin in the Teleostei 

 there are some observations of 'Swirski (No. 488) which unfortunately do 

 not throw very much light upon the nature of the limb. 



'Swirski finds that in the Pike the skeleton of the limb is formed of a 

 plate of cartilage, continuous with the pectoral girdle ; which soon becomes 

 divided into a proximal and a distal portion. The former is subsequently 

 segmented into five basal rays, and the latter into twelve parts, the number 

 of which subsequently becomes reduced. 



These investigations might be regarded as tending to shew that the 

 basipterygium of Elasmobranchii is not represented in Teleostei, owing to 

 the fin rays not having united into a continuous basal bar, but the obser- 

 vations are not sufficiently complete to admit of this conclusion being 

 founded upon them with any certainty. 



Tlie ckeiropterygium. 



Observations on the early development of the pentadactyloid 

 limbs of the higher Vertebrata are comparatively scanty. 



The limbs arise as simple outgrowths of the sides of the 

 body, formed both of epiblast and mesoblast. In the Amniota, 

 at all events, they are processes of a special longitudinal ridge 

 known as the Wolffian ridge. In the Amniota they also bear 

 at their extremity a thickened cap of epiblast, which may be 

 compared with the epiblastic fold at the apex of the Elasmo- 

 branch fin. 



Both limbs have at first a precisely similar position, both 

 being directed backwards and being parallel to the surface of 

 the body. 



