THE LATER DEVELOPMENT OF THE FROG 221 



The teeth are continually wearing away and dropping out of 

 the jaw; they are replaced by new teeth which develop similarly 

 deeper in the dermis. There are therefore always present in 

 the jaws, teeth in various stages of development. 



4. The Appendicular Skeleton 



The elements of the pectoral and pelvic arches and limbs do 

 not appear until just before metamorphosis. We shall make 

 only the briefest reference to these structures. 



The pectoral arch appears as a pair of crescentic cartilages 

 around the lateral and ventral parts of the body, opposite the 

 anterior end of the vertebral column. Just below its middle 

 each rod forms an articulation (glenoid cavity) with the head of 

 the humerus. Above this are formed the bony scapula and the 

 cartilaginous terminal suprascapula. Below the glenoid cavity 

 the arch is divided into the posterior coracoid and the anterior 

 procoracoid elements. The coracoid becomes bony, while in 

 connection with the procoracoid a dermal element, the clavicle, 

 develops later. The lower ends of the coracoids and procora- 

 coids become united on each side by a cartilaginous epicora- 

 coid. Later the two epicoracoids fuse together in the mid-line. 



Posterior to the epicoracoids a median cartilage develops 

 which is the rudiment of the sternum. Later this fuses with the 

 epicoracoids, and its proximal section becomes bony while 

 posteriorly it forms the cartilaginous xiphisternum. Anterior 

 to the epicoracoids a similar omosternum is formed. 



The pelvic arch also appears first as a pair of cartilaginous 

 rods, but these are early in contact medio-ventrajly, and soon 

 fuse together. These articulate with the femora, and the parts 

 dorsal to the articulations (acetabula) form the ilia, which con- 

 nect with the transverse processes of the last or ninth vertebra 

 (sacral vertebra). The postero-ventral region of each arch 

 becomes the bony ischium, while the antero-ventral part 

 remains cartilaginous as the pubis. 



While the pelvic arch is, like the pectoral, originally at right 

 angles to the vertebral column, after metamorphosis it rotates 



