NOTES ON MEDIAN AND PAIRED FINS OF FISH. 357 



several Luds. As a rule twenty to twenty-two pairs of radial 

 muscles can be made out in tlie adult pectoral and pelvic fins. 



A denser region of closely-packed nuclei at the base of the 

 fin-folds is tlie first sign of a skeleton, as in the case of the 

 median fins. Then the girdle, basals, and radials make their 

 appearance as procartilaginous tracts, all in continuity with 

 each other (fig. 9). The radials, however, arise as streaks of 

 denser mesenchyme, which are separate along the greater 

 part of their course. They are continuous only at their base, 

 where they join the girdle or the basipterygium (mcta- 

 pterygium). Even this basal shows faint indications of 

 segmentation as if it had been formed by the concentration 

 and fusion of the radials. 



Cartilage is first formed in the girdle and basipterygium 

 (fig. 10), then in the radials. The joints remain unchondrified. 



The Concordance between the Muscles and the 



Radials. 



In the adult fins there is almost perfect " concordance " of 

 these elements in the peripheral regions, one cartilage being 

 lodged between two corresponding muscles. But, especially 

 at the anterior edge of the paired fins, and both at the front 

 and hind ends of the dorsals, the agreement is imperfect. 

 Here excessive concentration has taken place ; the cartilages 

 are possibly compound, and the muscles are indistinctly sub- 

 divided into bundles, which do not agree exactly with them 

 either in number or in shape (figs. 11, 12, 16, 13, and 26). The 

 muscles also generally extend beyond the cartilages. There 

 can be little doubt that most fins contain more muscular than 

 skeletal metameres. Not every segment which contributes a 

 muscle-bud necessarily contributes a skeletal radial. 



Now, on tracing the development both of the paired and 

 of the median fins, we find that, so far as the two elements 

 co-exist, they are in exact correspondence. This concordance 

 of the muscles with the radials, which is, indeed, more perfect 

 in the young than in the adult, can bo clearly demonstrated 



