APPENDICULAR SKELETON ORIGIN 



241 



the preaxial, those of the pelvic fin on the postaxial side, the pre- 

 axials being reduced in size and number. Lepidosiren and Protopterus 

 have retained only the jointed axis, all radials being lost. The paired 

 fins of fossils which are clearly Dipnoan show no essential difference 

 from Ceratodus. 



The fins of Ceratodus represent Gegenbaur's archipterygium, and pla.v a 

 most important part in his theory of appendages, of which biseriality is the very 

 essence. It is more probable that biseriality is not primitive, but is the extreme 

 of conditions in some sharks. It is a question whether the axis of the Dipnoan 

 appendage is a radial or the homologue of the metapterygium of Elasmobranchs. 



THE ORIGIN OF VERTEBRATE APPENDAGES 



Two prominent hypotheses attempt to explain the origin of the paired 

 appendages of Vertebrates, the fin-fold theory of Thacher, supported later by 

 Mivart and Balfour, and the gill-arch or archiptervgial theory of Gegenbaur. 



mm.. 



E 



D 





% 



Fig. 254. — Diagrams illustrating theories of origin of paired limbs. A-G, archi- 

 ptervgial theory; H, another view. A, typical gill arch and gill rays; B, outgrowth of 

 middle radial, drawing with it adjacent rays; C, gill arch a girdle, rays now forming a 

 biserial appendage as in Z?; £, loss of most postaxial radials; F, formation of basal 

 cartilages of pectoral fin; G, origin of Tetrapod appendage from fin, persisting parts with 

 dotted lines; H, persisting parts stippled. 



Both agree that the fins of fishes are more primitive and ancestral to the legs of 

 Tetrapoda. The first theory regards both median and paired fins, which have 

 many points in common, as having a common ancestry. The archipterygial 

 theory, at least by implication, denies any homology between the two and only 

 tries to account for the paired appendages. 



The archipterygial theory is based on the biserial fin of Ceratodus, its girdle 

 bearing the axis which bears radials on either side. This is the archipterygixmi 

 and from it he believes that all paired appendages of Vertebrates — pterygia and 

 podia — have been derived. He argues that the archipterygium and its girdle 

 have developed from a gill septum Hke that of an Elasmobranch (fig. 254, A), 



16 



