428 OSBURN 



in number, but that is no indication that the number of rays 

 in the true gills has been diminished by more than one-half. 

 Certainly in Cestracion there is no evidence of any reduction in 

 the number of rays in the true gills. 



II. It would seem that, if the paired limb had been derived 

 in the way the gill-arch theorists maintain, there should remain 

 in ontogeny some indication of the intermediate steps through 

 which the gill passed while becoming a fin. That no such steps 

 are known to occur in embryology, palaeontology, or comparative 

 anatomy is a very forcible argument against such an origin of 

 the fins. 



Fusions of gill rays resembling somewhat by their branching 

 structure the basalia and rays of a fin have been described in 

 sharks (Braus '04). However, the only cases of this kind thus 

 far made known have occurred in the hyoid arch, and even 

 the most sanguine adherent of the gill-arch theory would scarcely 

 maintain that the hyoid is progressing in the direction of be- 

 coming a fin. 



III. Because both the spinal and the visceral (trapezius) 

 muscles attach to the dorsal end of the pectoral arch, we are 

 hardly justified in supposing that the one is abstracting it from 

 the other. Yet on this basis the gill-arch theorists assume that 

 the anterior spinal muscles not only deprived the branchial 

 region of the pelvic arch but passed it over to their neighbors 

 and proceeded to abstract another. Why this kleptomania 

 should have been satiated with two arches, while half a dozen 

 yet remained, does not appear. 



If the above assumption were true, it might indeed make a 

 strong argument for the gill-arch origin of the paired limbs, but 

 that it is without foundation appears in the light of the following 

 facts in selachian embryology: 



(1) As we have shown, the pectoral girdle is differentiated 

 from a thickening of mesenchyme cells which grows inward 

 from the region of the epidermis. 



(2) According to my observations on Cestracion and Spinax, 

 the first anlage of the pectoral fin is situated entirely ventral 

 to the place of origin of the trapezius muscle, and it is by the 

 later growth of both these structures that they finally come 



