62 THE FROG 



The ischium forms the postero-dorsal portion of the 

 pelvis and the posterior third of the acetabulum. The 

 ischium is roughly oval in form, with a dorsal prominence. 

 The two ischia are united by the whole of their median sur- 

 faces, forming the ischial symphysis. 



The pubis is a triangular plate of cartilage, more or less 

 calcified, wedged in between the ilium and ischium ventrally 

 and forming about one-sixth of the acetabulum. The two 

 pubes are united by the whole of their median surfaces, 

 forming the pubic symphysis. 



Draw (X2) the pelvic girdle viewed from the lateral 

 aspect. 



Compare the pelvic girdle and its relation to the vertebral 

 column in the frog with the corresponding features in a 

 tailed amphibian (urodele). In the phylogenetic history 

 of the frog the loss of the tail and the fusion of the posterior 

 vertebrae to form the coccyx was doubtless associated with 

 a forward migration of the attachment of the pelvic girdle, 

 and the elongation of the ilia. The urodele presents the 

 more primitive condition, and the peculiar structure of this 

 part of the skeleton of the frog is the result of specialization 

 correlated with its leaping habits. Of what advantage to 

 the frog in leaping is the loss of the tail? the elongation of 

 the ilia and their attachment near the new center of gravity? 



Examine the pelvic girdle in skeletons of the turtle and of 

 man. 



Obviously the pectoral and the pelvic girdles are built on 

 essentially the same plan. Structures that are metamerically 

 repeated are said to be serially homologous. What ele- 

 ment in the pectoral girdle is homologous with each of the 

 elements of the pelvic girdle? Better examples of serially 



