418 OSBURN 



V. The fusion of the muscle-buds in the paired fins before 

 the appearance of the skeleton precludes the possibility of the 

 metameric origin of the latter. 



VI. The early discrepancy between muscles and rays in the 

 pelvic fin proves the primitive dysmetamery and independent 

 origin of the paired fin skeleton. 



Let us now examine the foregoing objections point by point 

 and determine whether they are well founded. 

 A. 



I. The pectoral girdle cannot be considered serially homolo- 

 gous with the gill arches for the following reasons : 



(i) The first anlage of the pectoral girdle lies almost its 

 whole length below the gill arches, as shown by Braus's recon- 

 structions ('04) of Spinax, and by my own observations on 

 Cestracion. 



(2) It arises near the external wall of the body, while the 

 gill arches arise near the pharyngeal wall, according to my 

 studies on Cestracion. 



(3) The thickening of mesenchyme from which the girdle is 

 differentiated takes its origin next to the ectoderm and spreads 

 in an inward direction till it occupies all the region in which the 

 pectoral fin skeleton arises, which latter is therefore of external 

 origin, since it arises out of this mesenchyme thickening. The 

 gill arches, on the other hand, arise next to the enteron. 



(4) The study of Cestracion shows that the first anlage of the 

 pectoral fin lies wholly within the region of spinal muscles. 



(5) In Cestracion at least, the pectoral girdle is relatively 

 much farther from the last gill arch at its first appearance than it is 

 during later growth. That is, it grows toward the gill-arch region. 



(6) The first four points apply with even more force to the 

 pelvic girdle which in a general way must be considered the 

 homolog of the pectoral. 



II. The observations of E. Ruge ('02) and Braus ('04) that 

 the pectoral girdle of Spinax niger is the first part of the fin 

 skeleton to make its appearance are probably not to be ques- 

 tioned, but since Spinax is the only form so far examined in 

 which the time relations are thus, and as in Cestracion, which 



