APPENDICULAR SKELETON — ORIGIN 



243 



use of these folds in controlling the direction of motion was an additional agent 

 in preventing their union. From the median folds arise, dorsal, caudal and anal 

 fins, while from the paired folds on the lower side of the trunk, pectoral and pelvic 



Fig. 255. Diagram of origin of median and paired appendages from lateral fin-folds. 



fins have arisen, by the hypertrophy of parts and the obsolescence of the inter- 

 vening tracts. To this theory the objections are: 



(i) Ontogeny and paleontology show no evidence of the origin of median 

 fins from paired folds, except that the muscles are paired in origin, arising from 



Fig. 256. — Thacher's figures illustrating his theory of the origin of the pelvic 

 girdle by fusion of the basal elements of primitive paired fins, b, basipterygium; 

 ip, ischio-pubic bar; p, propterygium; o, obturator foramen. 



buds from the upper borders of the myotomes, just as do the muscles of the paired 

 fins from their lower borders (fig. 240). Then the actinotrichia and the dermal 

 bones of the median fins also are paired in origin. 



