LIMBS. 



6I 7 



rows of rays ; but holds that in addition to the rays attached to the median 

 axis, which are alone found in Ceratodus, there were other rays directly 

 articulated to the shoulder-girdle. He considers that in the Elasmobranch 

 fin the majority of the lateral rays on the posterior (median or inner 

 according to his view of the position of the limb) side have become 

 aborted, and that the central axis is represented by the metapterygium ; 

 while the pro- and mesopterygium and their rays are, he believes, derived 

 from those rays of the archipterygium which originally articulated directly 

 with the shoulder-girdle. 



Gegenbaur's view appears to me to be absolutely negatived by the facts 

 of development of the pectoral fin in Scyllium ; not so much because the 

 pectoral fin in this form is necessarily to be regarded as primitive, but 

 because what Gegenbaur holds to be the primitive axis of the biserial fin 

 is demonstrated to be really the base, and it is only in the adult that it is 

 conceivable that a second set of lateral rays could have existed on the 

 posterior side of the metapterygium. If Gegenbaur's view were correct 

 we should expect to find in the embryo, if anywhere, traces of the second 

 set of lateral rays ; but the fact is that, as may easily be seen by an inspec- 

 tion of figs. 344 and 346, such a second set of lateral rays could not pos- 

 sibly have existed in a type . 

 of fin like that found in the 

 embryo 1 . With 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 Elasmobranchs, could be 

 derived from a visceral arch 

 with its branchial rays 2 . 



Gegenbaur's older view 



FIG. 349. SKELETON OF THE PECTORAL FIN 

 AND PART OF PECTORAL GIRDLE OF A NEARLY 

 RIPE EMBRYO OF SCYLLIUM STELLARE. 



m.p. metapterygium ; me.p. mesopterygium ; 

 //. propterygium ; cr. coracoid process. 



1 If, which I very much doubt, Gegenbaur is right in regarding certain rays found 

 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. 



2 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 we have on the vascular supply of these 

 gills demonstrate that this attachment is secondary. 



