356 EUWIN S. GUODRICH. 



now faintly ill Jicated ; the radials liavc no definite outlines, 

 and merge together above and below. At this stage the 

 radial muscles are well differentiated (fig. 7). 



Cartilage begins to appear near the middle of each radial 

 when the embryo is about 36 mm. in length (fig. 10), thence 

 it spreads upwards and downwards to the joints, where it 

 stops. The proximal and the distal elements are separately 

 chondrified. The dorsal, or distal, edge is the last to become 

 cartilage (fig. 15). 



The skeleton of a first dorsal fin drawn in figs. 15 and 

 16 is of some interest, for unusual concrescence has taken 

 place. Not only are the first two and tlie last two radials fused 

 ventrally, as is usually the case, but also the 10th and the 

 11th, and the 10th has fused dorsally with the 9th. 



With regard to the very origin of the skeleton — whether 

 the radials appear as separate rods or not — the evidence 

 is somewhat obscure. It is true that a patch of denser 

 mesenchyme is first seen (fig. 17 h), but it can hardly be 

 called a plate of precartilage. It is merely a cloud of more 

 densely packed nuclei, and the radials, as such, make their 

 appearance as a series of denser zones separate from each 

 other, and extend outwards into a region not previously 

 occupied by the "plate." This stage may, perhaps, more 

 justly be considered as representing their first aj^pearance. 

 Their apparent continuity I believe to be due to their close 

 approximation, as MoUier has suggested (24). 



The Development of the Paired Fins. 



Little need be said about the paired fins, which luive been 

 so thoroughly studied by others. 



In Scyllium each muscle-bud, having divided into an ujiper 

 and a lower secondary bud, develo})s into a pair of radial 

 nuiscles. I^hese are regularly formed from their corresponding 

 buds throughout the greater part of the fin (iigs. 1, 25, 6, and 

 9). But at each end, especially the anterior end, is an indis- 

 tinctly segmented nia.ss of nuiscle-iibres, })robably derived from 



