6l6 THE PECTORAL FIN. 



toral fin to the body wall becomes shortened from behind 

 forwards, the basipterygial bar is gradually rotated outwards, 

 its anterior end remaining attached to the pectoral girdle. 

 In this way this bar comes to form the posterior border of the 

 skeleton of the fin (figs. 348 and 349, mp\ constituting what 

 Gegenbaur called the metapterygium, and eventually becomes 

 segmented off from the pectoral girdle, simply articulating 

 with its hinder edge. 



The plate of cartilage, which is continued outwards from the 

 basipterygium, or as we may now call it, the metapterygium, 

 into the fin, is not nearly so completely 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.p), 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 

 important, and are easily understood by reference to fig. 349 

 representing the limb of a nearly full-grown embryo. The 

 front end of the anterior basal cartilage becomes segmented 

 off as a propterygium, bearing a single fin-ray, leaving the 

 remainder of the cartilage as a mesopterygium. 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 homo- 

 logue of the basal cartilage of the pelvic fin. 



From the mode of development 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 fin to be most nearly retained in Ceratodus, and to 

 consist of a central multisegmented axis with numerous rays. Gegenbaur 

 derives the Elasmobranch pectoral fin from a form which he calls the 

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



