504 



TEE PELVIC FIN. 



The same may be said of the fact that the first part of each girdle 

 to be formed is that in the neighbourhood of the longitudinal bar 

 (basipterygium) of the fin, the dorsal and ventral prolongations being 

 subsequent growths. 



The later development of the skeleton of the two fins is more 

 conveniently treated separately. 



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 seg- 

 mented from it. Of the fin rays the anterior remains directly articu- 

 lated with the pelvic girdle on the separation of the basipterygium 

 (fig. 347), and the remaining rays finally become segmented from the 



FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG EMBRYO OF 



SCYLLIUM STELLARE. 



mpt. basipterygial bar (metapterygimn) ; fr. fin ray; m. muscles ; hf. horny fibres. 



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 pec- 

 toral girdle (figs. 344 and 348). 



