PAIRED FINS 



77 



become differentiated in a continuous procartilaginous rudiment 

 (Balfour [28], Ruge [337a]), and not as separate rudiments, applies 

 equally to the median fins (Balfour, Brans [50, 176]). More- 

 over, in Teleosts, and possibly in many other fish, the radials are 

 separate from the first (Harrison [197]). It is also urged that the 

 metamerism of the radial muscles of the adult fin does not correspond 

 to that of the embryo (Braus [50]). Now, although it may be true 





'[HBiimiAiiim 



hiu ecu. C. 



Fig. 50. 



Acanthias vulgaris, Risso. A, dorsal fin and portion of the vertebral column which ha* 

 been cut through longitudinally in front. B, calcified cylinder of a centrum. C, transverse 

 section of the vertebral column, caudal region. D, surface view of denticles as disposed in 

 the skin, a, canal for aorta ; a.c, anterior cartilages (either modified radials or neural spines) ; 

 b.c, basal cartilage ; b.pl, basal plate ; ea, calcified cylinder ; ct, cartilaginous centrum ; il.f, 

 dorsal fin ; h.a, haemal arch ; in, interdorsal ; iv, intervertebral ligament ; li, dorsal ligament ; 

 n.a, neural arch ; n.c, neural canal ; nt, notochord ; s, blunt spine ; sp, tin spine, with base cut 

 away to expose cartilage core ; r, radial ; v, canal for caudal vein. The cartilage is dotted. 



that muscle-buds fuse at their base, that a nerve-plexus is formed, 

 and that the substance of an adult radial muscle may not be derived 

 entirely from one bud, yet it is doubtless derived mainly from 

 that bud the position of which it continues to occupy throughout 

 development, and the radial muscles correspond in number and 

 relative position to the buds from which they have been formed 

 (Figs. 47, 51). In this respect also the median resemble the paired 

 fins. Indeed, it is very doubtful whether any fusion of neighbouring 

 buds takes place at all in such low forms as the Elasmobranchs 



