242 VERTEBRATE SKELETON 



its skeleton coming from the branchial arch and its rays. Each septum is 

 supported near its inner wall by an arch from which rays extend into the lateral 

 part of the septum. Lateral growth of the septum would produce a fin-like 

 process and would draw the radials out into it, the medial ray forming an axis, 

 those above and below becoming arranged on either side of it (fig. 254, C, D). 

 Jointing of the axis would give a skeleton like that of the fin of Ceratodus. 

 Migration caudally of two such modified septa along the sides of the body would 

 furnish pectoral and pelvic fins. 



Somewhat aUied to this (and subject to the same objections) is the view that 

 the paired appendages have come from external gills hke those of Amphibia. 



To the part of the archipterygial theory outlined above, several objections 

 have been advanced and have been practically ignored by its author in his last 

 presentation of the subject. Briefly these are: 



(i) Development shows (fig. 70) that the branchial arches arise in the 

 splanchnic wall of the coelom of the branchial region; while the girdles of the 

 appendages are clearly somatic in origin, arising lateral to the coelom. Hence 

 the two cannot be equivalent. 



(2) The gill septum, as shown by myotomes, blood vessels, nerves and 

 skeletal parts does not include structures from more than a single metamere, 

 while paired appendages- — ^both pterygia and podia — are by the same test, poly- 

 meric, arising from several somites (fig. 240). 



(3) The archipterygial theory demands that the primitive line of attachment 

 of appendage to trunk shall be vertical; according to ontogeny, whatever the 

 adult condition, the original attachment is horizontal, extending over several 

 somites. 



(4) There is not the slightest evidence of such shifting over several or many 

 somites as the theory demands, especially for the pelvic member, and the innerva- 

 tion totally negatives any such hypothesis. The appendages actually arise from 

 the same somites with which they are connected in the adult, and the nerves 

 are those of the corresponding segments. 



(5) Ontogeny and paleontology agree that the biserial condition is secondary 

 and not primitive. 



(6) The gill-arch theory does not explain the several resemblances between 

 median and paired fins. 



The alternative (fin-fold) theory assumes that the ancestral vertebrate had 

 two longitudinal folds, one more dorsal, the other more ventral, on either side 

 of trunk and tail. These folds were supported by metameric skeletal rods Hke 

 the radials of primitive fins. The folds migrated, the upper towards the dorsal 

 surface, the ventral in the opposite direction (fig. 255). There being no obstruc- 

 tion to the dorsal migration, right and left folds met in the mid-dorsal line, 

 forming a continuous fin along the back from head to tip of tail. Behind the 

 anus, in the same way, the ventral folds would form a similar median fin on the 

 ventral side of the tail. 



The anus, however would prevent the ventral folds meeting at that point, 

 and the influence of that opening prevented their meeting farther forwards, so 

 that there resulted paired folds on the lower lateral sides of the trunk, while the 



