MEDIAN FINS 71 



Since, in many fish, the unpaired fin extends uninterruptedly 

 along the dorsal mid-line to the tip of the tail, and along the 

 ventral mid-line forwards as far as the anal region or even beyond ; 

 and since, in many of those fish which in the adult state have 

 discontinuous dorsal, caudal, and anal fins, these separate fins 

 develop as differentiations in a continuous embryonic fin-fold, 

 it seems very probable that the median fins of the primitive 

 Gnathostomes formed continuous folds. By the obliteration of 

 certain regions they became subdivided. This view is further 

 borne out by the fact that we find quite or almost continuous 

 median fins amongst some of the most primitive groups of fish 

 (Pleuracanthini, Dipnoi). Moreover, traces of the fin-skeleton and 

 musculature are found between the discontinuous fins of such fish 

 as Acanthias, Rhina (Figs. 50, 52), Pristis (Goette [167], Thacher 

 [437], Mivart [300]), and Gadus. We may conclude, at all events, 

 that the unpaired fins are longitudinal median structures, internally 

 segmented like the body itself, and involving many segments. The 

 metamerism is evident in the skeleton, often composed of a series 

 of separate jointed rods corresponding in number to the neural 

 arches, or a multiple thereof. The muscles also are segmented, 

 being as a rule developed from special outgrowths of the 

 myotomes, the muscle-buds (Dohrn [118], Mayer [297], Harrison 

 [197], Goodrich [176]) (Fig. 47). A single bud grows into the fin- 

 fold from each myotome on each side {Scyllium, Salmo). The 

 muscle-forming cells may, however, be given off not as definite 

 buds, but from a mere proliferation of the myotome. The nerves 

 which supply the fin-muscles are branches of the spinal nerves of 

 the segments belonging to the myotomes from which they were 

 developed. The buds of the right and left sides correspond in pairs, 

 and between each such pair is developed a skeletal radial (Fig. 47). 



The exact correspondence between the body segments and the 

 fin segments is usually disturbed by ' concentration ' and by 

 concrescence, which accompany the breaking up of the continuous 

 fold into separate fins. The base of each separate portion tends to 

 become narrower, and thus a dorsal fin which in the embryo 

 developed from some dozen segments, may in the adult come to 

 occupy only some half-dozen or fewer. As a rule, more buds arise 

 in the embryo than come to full development in the adult, some 

 being suppressed at each end, where concentration is most pro- 

 nounced. The skeletal and muscular elements thus become 

 relatively compressed, and this concentration may take place more 

 at one end of the fin than at the other (Fig. 47). Owing to 

 concentration radial fin-muscles may become more or less widely 

 separated from the myotomes which gave rise to them, and the 

 nerves are made to converge towards the narrowed base of the fin 

 to supply -them; thus are formed longitudinal 'collector' nerves, 



