204 ESSENTIALS OF ZOOLOGY 



this is the clavicle. A glenoid cavity appears between the 

 scapula and the coracoid and gives articulation to the 

 humerus. 



As the bud appears and lengthens, cartilages are formed in 

 succession and are the precursors of the bony skeleton of the 

 limb. 



The humerus of the male possesses a highly developed 

 deltoid crest. The radius and ulna are fused, and the ulna 

 presents an olecranon process. The carpus consists of six 

 small elements a radiale and an ulnare, a centrale and three 

 carpalia. Of these last, one each is related to the first and 

 second metacarpals and the third to the remaining meta- 

 carpals ; it is formed by the fusion of the outer carpalia and 

 includes also a centrale. There are five metacarpalia, but 

 toes are developed only on II to V. The phalanges number 

 0, 2, 2, 3, 3. The second metacarpal is enlarged in the 

 male to support the copulatory pad. 



The hind limbs appear externally before metamorphosis 

 in the frog, but the manner of development is essentially the 

 same as that of the anterior limbs. The thickening of the 

 somatopleure mesoderm extends upwards to form the ilium, 

 and below to form the ischium and the small pubis. The 

 ventral ends of the cartilaginous arches meet and fuse. The 

 acetabular cavity appears near the ventral end of the arch, and 

 the ilium and the ischium become ossified, the pubis remaining 

 cartilaginous. At first the girdle is vertically disposed, but it 

 is rotated backwards as the limb gains in length. 



The pelvic girdle of the adult frog is catapult-shaped. 

 The ilia are strongly developed as long processes which articu- 

 late at their anterior ends with the transverse processes of 

 the sacral vertebra. Posteriorly they are fused. The rounded 

 acetabular cavity is completed posteriorly by the fused ischia 

 and below by the fused cartilaginous pubes. The three 

 elements of the girdle thus meet in, and contribute to the 

 formation of, the acetabulum. 



The posterior limbs are made up of longer elements than 

 the anterior limbs. These are, in succession, the femur, 

 the fused tibia and fibula, the characteristically elongated 

 astragulus and calcaneum of the tarsus which are fused 



