The pelvic fin. The changes in the pelvic fin are comparatively slight. The fin remains through life as a nearly horizontal lateral projection of the body, and the longitudinal bar—the basipterygium—at its base always remains as such. It is for a considerable period attached to the pelvic girdle, but eventually becomes segmented from it. Of the fin rays the anterior remains directly articulated with the pelvic girdle on the separation of the basipterygium ([fig. 347]), and the remaining rays finally become segmented from the basipterygium, though they remain articulated with it. They also become to some extent transversely segmented. The posterior end of the basipterygial bar also becomes segmented off as the terminal ray.
The pelvic fin thus retains in all essential points its primitive arrangement.
The pectoral fin. The earliest stage of the pectoral fin differs from that of the pelvic fin only in minor points. There is the same longitudinal or basipterygial bar to which the fin-rays are attached, whose position at the base of the fin is clearly seen in the transverse section ([fig. 346], mpt). In front the bar is continuous with the pectoral girdle ([figs. 344] and [348]).
Fig. 347. Pelvic fin of a young male embryo of Scyllium stellare.
bp. basipterygium; m.o. process of basipterygium continued into clasper; il. iliac process of pectoral girdle; pu. pubis.
Fig. 348. Pectoral fin of an embryo of Scyllium stellare.
mp. metapterygium (basipterygium of earlier stage); me.p. rudiment of future pro- and mesopterygium; sc. cut surface of scapular process; cr. coracoid process; fr. foramen; f. horny fibres.
The changes which take place in the course of the further development are however very much more considerable in the case of the pectoral than in that of the pelvic fin.