THE SKELETAL SYSTEM 



237 



The alternative theory of Gegenbaur that the paired fins of fishes 

 arose from modified gill arches and their associated rays has too little 

 evidence in its support to make it worthy of serious consideration. 



The second step in this evolution was the invasion of connective tissue 

 and muscle into the fin-folds. A similar migration actually occurs in 



MEDIAN FIN-FOLD 



PAIRED FIN-FOLDS 



MEDIAN FIN 



ANUS 



PECTORAL FIN 

 PRIMITIVE RADIALS 



T PELVIC FIN 

 ANUS 



PELVIC GIRDLf 



Fig. 193. — Diagram illustrating the hypothetical evolution of the paired fins and 

 their skeletal supports. A represents the primitive stage of continuous fin-folds. The 

 dorsal fin and the ventral fin posterior to the anus are median and unpaired. B is the 

 definitive Elasmobranch stage. The paired fin folds persist only in the region of pectoral 

 and pelvic fins. The median fins also become discontinuous. C~R illustrate hypo- 

 thetical stages in the evolution of the skeleton of the pelvic fins of Elasmobranchs. 

 The right side of C and E represents a later stage in phylogenesis. In £ the skeletons 

 of the girdle and extremity are differentiated. (After Wiedersheim.) 



ontogenesis. In elasmobranchs the muscle buds which invade the fin- 

 folds are metamerically arranged. 



The third step in this evolution was the appearance of a series of inter- 

 myotomic cartilage rays like those which support both median and paired 

 fins of elasmobranchs. The universal occurrence of skeletal material in 

 connexion with muscle, and indeed wherever in an organism stresses occur, 

 may possibly be taken as explaining these radial cartilages. The position 

 between the myotomes is obviously adaptive, as is also their position 



