728 DEVELOPMENT OF THE SKELETON 



As may be gathered by comparing the figure of the fin at 

 the stage just described with that of the adult fin (woodcut, fig. 

 2), the remaining changes are very slight. The most important 

 is the segmentation of the basipterygial bar from the pelvic 

 girdle. 



The pelvic fin thus retains in all essential points its primitive 

 structure. 



The Pectoral Fin. The earliest stage of the pectoral fin dif- 

 fers, as I have shewn, from that of the pelvic fin only in minor 

 points (PI. 33, fig. 6). There is the same longitudinal or basip- 

 terygial 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, how- 

 ever, are very much more considerable in the case of the pectoral 

 than 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 attachment to the body. 

 At first (PI. 33, fig. 6), the base of the fin is as long as the great- 

 est breadth of the fin ; but it gradually becomes shortened by 

 being constricted off from the body at its hinder end. In con- 

 nection 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 (PI. 33, figs. 

 8 and 9), constituting the metapterygium (inp). It becomes 

 eventually segmented off from the pectoral girdle, simply articu- 

 lating 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 the homologous part of the pelvic fin; and this is especially 

 the case with the basal part of the plate. This basal part be- 

 comes, in fact, at first only divided into two parts (PI. 33, fig. 8) 

 a small anterior part at the front end (me. p\ and a larger pos- 

 terior 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, re- 

 sembling in this respect the anterior fin-ray of the pelvic girdle. 

 It constitutes the (at this stage undivided) rudiment of the meso- 



