LIMBS 



105 



far dorsally ; but when the limb is destined to perform more im- 

 portant movements in locomotion, or to give a more definite support 

 to the body, the arch may extend upwanls so as to come into 

 connection with the axial skeleton as well as meeting with 

 its fellow ventrally, thus forming an almost complete girdle 

 around the body. The limb skeleton may become ossified later. 



Fig. 82.— a, 



DiAGKAM OF Three Successive Stages ix the Develop- 

 ment OF THE Pelvic Fik of a Shakk. 



rd, primitive radii, wliich in A are beginning to fuse into a basal plate (6.s). In B 

 tliis fusion lias taken place on both sides, and at * the proximal ends of tlie 

 two basals are approximating to form the arch. In C the process is com- 

 pleted, and at t 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 second- 

 arily segmented, fo, obturator foramen ; cl, cloacal aperture. 



In the case of Fishes, the pelvic fin as a rule remains at a simpler 

 and more embryonic stage than the pectoral. 



The paired limbs are not connected with any particular body- 

 segments, but vary greatly in their relative positions and in the 

 number of nerves which supply them. 



The essential part of this conception as to the origin of the paired limbs 

 is due to Thacher, Mivart, Balfour, Has well, and Dohrn.^ Gegenbaur had 



^ A somewhat similar idea was put forward h\ Goodsii- as early as 1856. 



