FINS AND LIMBS 



3" 



and into it little " muscle-buds " make their way, having been 

 formed from the myotomes and separated off from them. 

 Cartilaginous radials then appear, and on each side of these, 

 the dermal fin-rays. The fins contain structures derived from 

 several segments of the body, and this is reflected in the number 

 of radial muscles, cartilaginous radials, dermal fin-rays and 

 nerves which the fin contains. 



In the most primitive forms, and in early stages of develop- 

 ment of other forms, it is common to find that the median 

 fins are continuous and form one fold which extends down the 



Fig. 157. — The pectoral fin of Cladoselache, showing the radials (r) project- 

 ing parallel to one another and perpendicular to the side of the body (b). 

 (Drawn from a cast.) 



dorsal side, round the tail and forwards again on the ventral 

 side. The presence of a number of separate and discontinuous 

 median fins in many fish is therefore probably due to the sub- 

 division of an originally continuous fin. 



If the median fin was primitively continuous, it is possible 

 that the paired fins also were originally continuous folds on 

 each side of the body, and that they became subsequently 

 divided into pectoral and pelvic sections. The fact that in 

 some fish such as Scyllium, there is in early stages of develop- 

 ment a continuous series of muscle-buds given off from all 

 the segments of the trunk makes this possibility fairly probable. 



