PAIRED FINS 



the nerve deserting one muscle for another, but owing to the fusion 

 of muscle-buds, or cells. Strictly speaking, even then the nerves 

 probably remain faithful to the muscle substance of their own 

 segment, though it may be combined in one muscle with fibres 

 derived from other segments supplied by other nerves. The limb- 

 plexus is, however, formed not so much by an intermingling of the 

 nerves as by a gathering together of these nerves from a number 

 of segments into common collector trunks (Fig. 47). This collecting 

 does not necessarily disturb the metameric order of the structures 

 concerned. It is due to concentration (Mollier [301], Goodrich [176]) 

 in so far as it concerns the motor fibres, and alters neither their 



B. 



A. 



Fig. 54. 



Callorhynchus antarcticus, Lac. The web of the fin, and the ceratotrichia, have been cut 

 across. (After Mivart.) b, basi pterygium ; /, tin-web; mt, metapterygium ; p.r, preaxial 

 radials ; pt, propterygium ; pt.r, cartilages representing postasial radials. 



proximal roots nor their peripheral destination. Thus we find that 

 even in such a fin as that of Ceratodus, the development of which 

 is so abbreviated and obscured (Semon [400a], Braus [48]), the 

 preaxial and postaxial muscles are regularly supplied from nerves 

 belonging to segments in order from before backwards. 



Now, as Fiirbringer has shown in his admirable works [142, etc.], 

 a limb-plexus shifts backwards or forwards like the limb it 

 supplies. Its change of position can be accounted for neither by 

 the theory of inter- and excalation of segments, nor by the 

 supposition that the nerves actually move through the segments. 

 It is, therefore, by progressive growth in one direction, and by 

 corresponding reduction in the other, that change of position takes 

 place. The motion is only apparent, and is not due to the 



6 



