506 THE PECTORAL FIN. 



divided up into fin-rays as in the case of the pelvic fin, and this is 

 especially the case with the basal part of the plate. This basal part 

 becomes in fact at first only divided into two parts (fig. 348) a small 

 anterior part at the front end (me.js), and a larger posterior along 

 the base of the remainder of the fin. The anterior part directly 

 joins the pectoral girdle at its base, resembling in this respect 

 the anterior fin-ray of the pelvic girdle. It constitutes the rudiment 

 of the mesopterygium and propterygium of Gegenbaur. It bears 

 four fin-rays at its extremity, the anterior not being well marked. 

 The remaining fin-rays are borne by the edge of the plate continuous 

 with the metapterygium. 



The further changes in the cartilages of the limb are not impor- 

 tant, and are easily understood by reference to fig. 349 representing 

 the limb of a nearly full-grown embryo. The front end of the ante- 

 rior basal cartilage becomes segmented off as a propterygium, bearing 

 a single fin-ray, leaving the remainder of the cartilage as a mesopte- 

 rygium. The remainder of the now considerably segmented fin-rays 

 are borne by the metapterygium. 



The mode of development of the pectoral fin demonstrates that, 

 as supposed by Mivart, the metapterygium is the homologue of the 

 basal cartilage of the pelvic fin. 



From the mode of developmeut of the fins of Scyllium conclusions 

 may be drawn adverse to the views recently put forward on the struc- 

 ture of the fin by Gegenbaur and Huxley, both of whom consider the 

 primitive type of fiu to be most nearly retained in Ceratodus, and to 

 consist of a ceati-al multisegmented axis with numerous rays. Gegenbaur 

 derives the El;<siiio.branch pectoral fin from a form which he calls the 

 archipterygiiam,, nearly like that of Ceratodus, with a median axis and two 

 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 con.sidei's that in the Elasmobranch 

 fin the majority of the lateral rays on the })Osterior (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 ScylUum ; not so much because the 

 pectoral tin 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 demonsti'ated to be really the base, and it is only in the adult that it is 

 conceivable that a second set of lateral ray.s 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'. With 



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



