THE ORIGIN OF VERTEBRATE LIMBS 417 



of the paired fins apply equally well to the unpaired fins, which 

 are held, even by those who have raised the objections, to be 

 of strictly metameric and local origin. Other objections can 

 be shown to be based upon faulty evidence, or upon facts which 

 bear a different and more probable interpretation. 



The principal objections to the fin-fold theory are, briefly 

 stated, as follows: 



A. Relating to the comparison of the paired fin girdles with 

 gill arches the followers of Gegenbaur contend that : 



I. The pectoral girdle arises in serial order with the gill arches. 



II. The pectoral girdle makes its appearance earlier than the 

 basalia and rays. 



III. The basalia grow out of the girdles and the rays out of 

 the basalia. 



IV. The dorsal end of the pectoral arch comes into relation 

 with the visceral muscles. 



V. The pelvic girdle is the homolog of the pectoral in every 

 respect and so corresponds to a gill arch. 



B. Relating to the supposed migration of the paired fins 

 from the gill region : 



I. The pelvic fins have been shown to undergo slight migra- 

 tion or shifting during ontogeny. 



II. As a proof of migration there appears a strong collector 

 nerve in the anterior part of the pelvic fins. 



III. Also, in the early development of the pelvic fin the most 

 anterior muscle-buds degenerate without entering the fin, while 

 those just posterior to these are compelled to reach backward 

 to enter the fin. 



C. Relating to the contrast of paired with unpaired fins: 



I. The skeleton of the unpaired fins consists of modified 

 vertebral (spinous and hsemal) processes. 



II. The girdles of the paired fins have nothing to represent 

 them in the unpaired fins. 



III. The presence of post-axial rays in the pectoral fin proves 

 the primitive biseriality of the paired fins. 



IV. The fin-rays of the paired fins do not arise separately 

 and later become fused to form basalia as the fin-fold theory 

 would lead us to expect. 



