xii. 6-7 AMPHIBIAN LIMBS 307 



life, and is seen in many lines of amphibian and reptilian evolution. 

 It has proceeded farther in the frogs than in any other tetrapods. 



The second to eighth vertebrae of Rana are concave in front, con- 

 vex behind (procoelous), and have large transverse processes. In other 

 amphibians they may be amphicoelous or opisthocoelous. They fit 

 together by complex zygapophyses. The first vertebra has two con- 

 cave facets for articulation with the two condyles of the skull; its 

 centrum and transverse processes are much reduced. The ninth 



Fig. 181. Diagrams illustrating the probable changes in position during the 

 evolution of a pelvic fin into a tetrapod limb. A as in Ceratodus; B, double 

 flexure to give knee and ankle joints, leaving foot direct backwards; c and 

 D, rotation of tarsus and digits turning foot forward. (From Gregory and 

 Raven, after Romer and Byrne.) 



(sacral) vertebra has large transverse processes, which articulate with 

 the ilia of the pelvic girdle. There are free ribs in the primitive frogs 

 Ascaphus and Leiopelma. 



7. Evolution and plan of the limbs of Amphibia 



The girdles of the paired limbs have become much changed from 

 their fish-like condition (Figs. 179 and 180). Their basic pattern is 

 similar in the two limbs and has been retained throughout the w r hole 

 tetrapod series. Whereas in fishes the girdles are rather small carti- 

 lages and bones, the pelvic girdle being restricted to the ventral region 

 of the body, in amphibians they become enlarged in connexion with 

 the w T eight-bearing function of the limbs. 



The details of the sequence of stages by which a tetrapod limb 

 arose from a fish fin are still somewhat disputed. It is probable that 

 the ancestral crossopterygian possessed a lobed fin, rather like that 

 seen in *Eusthenopteron (Fig. 179). As the fishes came on land the fin 



