8o 



PAIRED FINS 



may contribute to the production of a limb (p. 75), and we find 

 that the segments of the region occupied by the limb in the adult 

 always share in its development. The limb, as a whole, retains its 

 position throughout ontogeny (Fig. 47). But if reduction takes 

 place in iront, and growth takes place behind or vice versa, if, in 

 other words, certain segments cease to contribute at one end, and 

 certain other segments begin to contribute at the other, then apparent 

 motion takes place backwards or forwards. This may be further 

 complicated by unequal concentration at the two ends. The nerve- 



A. 



B 



FIG. 53. 



Skeleton of the pectoral fin of A, Scymnus lichia, Cuv. (after Gegenbaur) ; B, Heterodontus 

 (Cestracion) 1'hilippi, Lac. (after Gegenbaur) ; C, Centrophorus calceus, Gthr. (after Woodland). 

 In the latter the web of the fin is represented. 6, basipterygium ; /, fin-web ; mt, metaptery- 

 giuin ; p.r, preaxial radials ; pt, propterygium ; pt.r, post-axial radials. 



supply of the adult limb is a sure guide to the identification of the 

 segments from which the muscles have been derived. Segments 

 before and behind the limb-plexus may no longer enter into the 

 formation of the limb owing to reduction ; but the adult nerves un- 

 doubtedly show which segments contribute most to the musculature. 

 That, in a series of metameric myotomes and nerves, each motor 

 nerve remains, on the whole, faithful to its myotome throughout 

 the vicissitudes of phylogenetic and ontogenetic modifications, may 

 be considered as established. However, by means of a nerve- 

 plexus, nerves may come into secondary connection with muscles not 

 originally belonging to them, at all events in the higher vertebrates 

 (Tetrapoda). The nerve-plexus of a limb is brought about not by 



