64 MR. F. M. BALFOUR ON THE SKELETON 



differs, as 1 have shown, from that of the pelvic fin only in 

 minor points (Plate V, fig. 6). There is the same longitu- 

 dinal or basipterygial bar [bp), to which the fin-rays are 

 attached, which is continuous in front with the pectoral 

 girdle (p g). The changes which take place in the course 

 of the further development, however, are very much more 

 considerable in the case of the pectoral that in that of the 

 pelvic fin. 



The most important change in the external form of the 

 fin is caused by a reduction in the length of its attachments 

 to the body. At first (Plate V, fig. 6), the base of the fin 

 is as long as the greatest breadth of the fin ; but it gradually 

 becomes shortened by being constricted off from the body at 

 its hinder end. In connection with this process the posterior 

 end of 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 (Plate V, figs. 8 & 9), constituting 

 the metapterygium (mp). It becomes eventually 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 meta- 

 pterygium, into the fin, is not nearly so completely divided 

 up into fin-rays as the homologous part 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 (Plate V, fig. 8) — a small anterior part at the 

 front end {me.p), and a larger posterior along the base of the 

 metapterygium [mp) ; and these two parts are not completely 

 segmented from each other. 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 

 (at this stage undivided) rudiment of the mesopterygium 

 and propterygium of Gegenbaur. It bears in my specimen 

 of this age four fin-rays at its extremity, the anterior not 

 being well marked. The remaining fin-rays are prolonga- 

 tions outwards of the edge of the plate continuous with the 

 metapterygium. These rays are at the stage figured more 

 or less transversely segmented, but at their outer edge they 

 are united together by a nearly continuous rim of cartilage. 

 The spaces between the fin-rays are relatively considerably 

 larger than in the adult. 



The further changes in the cartilages of the pectoral 

 limb are, morphologically speaking, not important, and are 

 easily understood by reference to Plate V, fig. 9 (representing 



