244 



VERTEBRATE SKELETON 



(2) The most primitive Chordates {Amphioxus and the Cx'clostomes) show 

 no trace of paired appendages, save the improbable suggestion that the meta- 

 pleural folds of the former, and folds near the anus of Petromyzon be such. 



(3) The deeper parts of the skeleton of the median fins (basalia and radialia) 

 are unpaired in development. 



(4) Except the embryo of Torpedo, no Vertebrate is known to have such 

 longitudinal folds in the trunk region as the hypothesis would imply. 



On the other hand a variety of the Japanese goldfish has fins which are in full 

 harmony with the view that the median fins are double in origin, both anal and 

 caudal sometimes being double throughout, a condition expHcable on the 

 assumption that these normally azygous appendages have had a paired ancestry. 

 The skeleton of the more primitive paired fins {e.g., Cladoselache, fig. 244) is 



Fig. 257. Fig. 258. 



Fig. 257.— Fore foot of Amphibian showing relations of persisting parts with archi- 

 pterygium (Gegenbaur, '77; compare with fig. 254). The heavy line is archipterygial 

 axis, the dotted lines the uniserial radii on the preaxial side; the postaxial being lost. 

 Later, Gegenbaur had other interpretations of parts. 



Fig. 258. — Pectoral girdle and base of appendage of Saiiripierus (Adams, in Gregory, 

 '15). c, coracoid; cl. cleithrum; cv, clavicle; h, humerus; r, radius; 5, scapula; scl, 

 supracleithrum ; u, ulna. 



composed of metameric radials, some articulated to basal cartilages, others to 

 plates, evidently the result of fusion of basals. Fusion of these plates of the 

 two sides in the middle line would result in a pubo-ischiadic bar, while the 

 necessity of a firm origin for the levator muscles of the fin would explain the iliac 

 process of the pelvis. The same conditions in front would give the shoulder 

 girdle and the skeleton of the pectoral fin. 



The second problem of the paired appendages concerns the evolution of the 

 Tetrapod podia from the pterygia of fishes. That podia and pterygia are broadly 

 homologous is self evident, but the steps by which the latter have been trans- 

 formed into the jointed limbs of the higher \'ertebrates are still very uncertain. 



