PAIRED FINS 



107 



ends diverging on either side (Fig. 67). 1 The view that the 

 nrchipterygium represents the ancestral form of the pelvic fin of the 

 Osteichthyes, and of the pectoral fin of both the Osteichthyes and 

 the Chondrichthyes, is thus quite reconcilable with a 'lateral-fold 

 theory.' But even so, it is probable that the ancestral type was 

 not as perfectly mesorachic as the modern Ceratodus fin, and that 

 a certain number of preaxial rays (pro- and mesopterygium) 



A, JL 



Diagrams to show the result of concentration on the skeleton and nerve-supply of a paired 

 tin. A, 15, C lead towards the Selachian type of fin ; A, 15, 1) towards the Dipnoan type. 

 n l -, nine spinal nerves supplying the fin ; n.c, nerve-cord ; r, radials represented as seen in 

 an early embryonic stage ; pr, preaxial, and pt, postaxial radials ; /./, fin fold. 



remained articulating with the girdle (as in Plewacanthus and 

 Cladodus, p. 183). 



The other theory, favoured by Balfour [28], Thacher [434], A. 

 Smith Woodward [503], and others, is that the originally separate 



1 If it is objected that in Ceratodus, where the adult fin has about thirty radials 

 and radial muscles, only about three segments have been shown to contribute muscle- 

 buds in the embryo (Semon [400]), it must be answered that this result is not trust- 

 worthy, since Davidoff [99] and Braus [46] have found twelve motor nerve-roots con- 

 tributing to the limb-plexus. With regard to the development of the cartilages, it 

 has already been shown that, in Elasmobranchs, the originally separate radials become 

 differentiated in a continuous procartilaginons rudiment, somewhat as in Ceratoili'* 

 (P- 73). 



