XIII 



PHYLUM CHORDATA 



265 



These two structures are the first indication of a sternum we have yet 

 met with, with the possible exception of the median ventral element 

 of the shoulder-girdle of Heptanchus (p. 170). The omosternum is 

 developed as paired forward extensions of the epicoracoids which 

 undergo fusion : the sternum and xiphisternum arise as paired rods 

 lying posterior to the epicoracoids, and subsequently uniting with 

 one another. This sternal apparatus of the Frog (and of the 

 Amphibia in general) differs developmentally from the structures 

 in the higher Vertebrates to which the same name is applied the 

 latter being formed from separated-off portions of embryonic ribs 

 (costal sternum). 



The fore-limbs deviate from the typical structure (p. 81) chiefly 

 in the fusion of the radius and ulna into a single radio-ulna (Fig. 

 933, RA. UL), and in the presence of only four complete digits with 

 a vestigial one on the radial side. In all probability the last repre- 

 sents the pollex, and the complete digits are the second to the fifth 

 of the typical hand. Six carpals only are present, the third, fourth, 

 and fifth digits articulating with a single bone which has apparently 

 arisen by the fusion of the third, fourth and fifth distalia and of at 

 least one centrale. 



The pelvic girdle (Fig. 938) is very peculiarly modified ; it resembles 

 in form a Bird's " merrythought," consisting of two long, curved 

 bars articulating in front with the transverse processes of the sacral 

 vertebra (Fig. 933) and uniting posteriorly in an irregular vertical 

 disc of mingled bone and cartilage which 

 bears on each side a deep, hemispherical 

 acetabulum (G) for the articulation of the 

 thigh-bone. The curved rods are the ilia 

 (II., P) ; they expand posteriorly and 

 unite with one another in the median 

 plane to form the dorsal portion of the 

 disc and about one-half of the acetabu- 

 lum. The posterior portions of the disc 

 and acetabulum are furnished by the 

 ischia (Is), fused with one another in the 

 sagittal plane, their ventral portions by 

 the similarly united pubes (Kn). The 

 ilium and ischium are formed of true bone, 

 the pubis of calcified cartilage ; the union 

 of the elements in the median plane is 

 called the symphysis. In the larva the 

 ilium is vertical, but during development 

 it becomes lengthened and at the same time rotated backwards, 

 thus bringing the articulation of the hind-limbs as far back as 

 possible. 



In the hind-limb the tibia and fibula are fused to form a single 

 tibio-fibula (Fig. 933, TI. FI), and the two bones in the proximal 



G 



FIG. 938. Rana esculenta. 



Pelvic girdle from the right 

 side. G, acetabulum ; //, P, 

 ilium ; 7s. ischium ; Kn, 

 pubis. (From Wiedersheim's 

 Comparative Anatomy.) 



