NOTES ON MEDIAN AND PAIRED FINS OF FISH. 355 



the second dorsal, which differs only from the first in being 

 rather smaller, and in developing a little later. 



Passing now to later stages (figs. 1, 11, 16, and 13), the 

 radial muscles are seen to become thoroughly differentiated, 

 retaining all the while their individuality. The little mass 

 described above at each end develops into a bundle of radial 

 muscle-fibres, which in some cases appears to represent only a 

 single segment. For instance, in the dorsal fins of a Scyllium 

 canicula, about a foot long, dissected in Naples, and shown 

 in fig. 23, there are only twelve muscles altogether. Possibly, 

 however, even here the anterior muscle is compounded of two 

 at least. As a rule the anterior and posterior bundles of 

 muscle-fibres show traces of subdivision in the adult. 



The adult dorsal fins vary in their extent and in the 

 number of their constituent parts. Neopolitan specimens 

 generally have twelve radial cartilages, with ten or eleven 

 clearly-defined radial muscles (figs. 22 and 23). Dog-fish 

 from Plymouth usually have thirteen radial cartilages, with 

 eleven or twelve distinct muscles, not counting the complex 

 muscle bundle at each end. Sometimes there are fifteen 

 radials and sixteen muscles (fig. 26). 



Development of the Skeleton of the Median Fins. 



The first indication of the endoskeleton of the median fins 

 is seen in embryos about 30 mm. long. By this time the 

 muscle-buds are concentrated, but are only just beginning to 

 become converted into muscle-fibres. A slightly darker zone 

 appears near the base of the fin (fig. 17 h). Here the nuclei of 

 the mesenchymatous layer filling the fin-fold are rather more 

 closely crowded together. This denser zone spreads a little, 

 and soon between each pair of right and left muscle-buds is 

 seen a dark streak of crowded nuclei, the first rudiment of 

 the radials (tig. 5) ; this embryo, 28 mm. long, is, however, 

 more advanced than the previous one. In an embryo 

 33 mm. long the procartilaginous rudiments of the radials 

 are clearly shown (fig. 20). The whole future skeleton is 



