LIMBS. 



507 



FIG. 349. SKELETON OF THE PECTORAL FIN AND 

 PART OF PECTORAL GIRDLE OF A NEARLY RIPE EMBRYO 

 OF SCYLLIUM STELLARE. 



mp. metapterygium ; me. p. mesopterygium ; 

 pp. propterygium ; cr. coracoid process. 



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 Elas- 



mobranchs, could be derived 



from a visceral 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 that 

 the fundamental point estab- 

 lished 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. 



in some Elasmobranch pectoral fins as rudiments of a second set of rays on the 

 posterior side of the metapterygium, these rays will have to be regarded as structures 

 in the act of being evolved, and not as persisting traces of a biserial fin. 



i Some arguments in favour of Gegenbaur's theory adduced by Wiedersheim as 

 a result of his researches on Protopterus are interesting. The attachment which he 

 describes between the external gills and the pectoral girdle is no doubt remarkable, 

 but I would suggest that the observations \\e have on the vascular supply of these 

 gills demonstrate that this attachment is secondary. 



