PHYLUM CHORDATA 



273 



coracoids is a rod of bone tipped by a rounded plate of cartilage, 

 the omosternum ; and passing backwards from their posterior ends 

 is a similar but larger bony rod, the sternum (st), also tipped by a 

 cartilaginous plate, to which the name xiphisternum (Kn) is applied. 

 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. 175). The omosternum 

 is developed as paired forwai'd 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 stenmm). 



Thefore-limbs deviate from the typical structure (p. 82) chiefly 

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

 (Fig. 922, EA. ul), and in the presence of only four complete 

 digits with a vestigial one on the radial side. In all probability 

 the latter represents 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. 927) 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. 922) and uniting posteriorly 

 in an irregular vertical disc of mingled 

 bone and cartilage which bears on each 

 side a deep, hemispherical acetabuhim (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 acetabulum. The posterior portions of the disc 

 and acetabulum are furnished by the ischia (Is), fused with one 

 another in the sagittal plane, their venti'al 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 



J^ 



Fig. 927.— Rana esculenta. 

 Pelvic girdle from the right 

 side. G, acetabulum ; It, P, 

 ilium ; Is. ischium ; Kn, 

 pubis. (From Wiedersheim's 

 Comparative Anatmny.) 



