APPENDICULAR SKELETON 



139 



support to the body, in addition to meeting with its fellow 

 ventrally, the arch may extend upwards so as to come into 

 connection with the axial skeleton, thus forming an almost 

 complete girdle around the body. The parts of the limb-skeleton 



FIG. 98. A, B, C. DIAGRAM OF THREE SUCCESSIVE STAGES IN THE DEVELOP- 

 MENT OF THE PELVIC FIN OF A SHARK. 



cl, cloacal aperture ; fo, obturator foramen ; rd, primitive radii, which in A are 

 beginning to fuse into a basal plate (bs). In B this fusion has taken place 

 on both sides, and at * the proximal ends of the two basals are approximat- 

 ing to form the arch. In C the process is completed, and at f an articulation 

 has been formed between the arch and the free portion of the fin. On the 

 left side in C the radii are becoming secondarily segmented. 



may become ossified later. The pelvic fin of Fishes as a rule 

 remains at a simpler and more embryonic stage than the pec- 

 toral fin. 



The paired extremities are not connected with any particular 

 body-segments, but vary greatly as to their relative positions and 

 the nerves which supply them, 



