SKELETON OF THE FROG. 71 



the month widens, and the horny mandibles are shed. 

 As the mouth advances forwards, the tympanic pedicles 

 are elongated, and are placed more obliquely ; their 

 proximal end retrograding from the post-frontal to the 

 mastoid region of the skull, and their distal end inclining 

 forwards with the attached lower jaw, Nos. 29, 33, on 

 which the denticles now begin to be developed. For the 

 still more extraordinary changes of the hyoid arch, No. 

 41, and its branchial appendages, No. 46, the student is 

 referred to Duge^s Recherches sur V Osteologie des Bo.tra- 

 ciens, 4to., 1835 ; and to the writer's Archetype of the Ver- 

 tebrate Skeleton, pp. 70, 71. 



The scapular arch, which was close to the occiput, 

 whilst protecting and supporting the branchial heart — 

 its primary function — begins, as the rudiments of the 

 fore-limbs bud out, to recede backwards, like the mandi- 

 bular and branchial arches, but to a greater extent, the 

 attachment to the occipital segment being wholly lost. 

 The scapular and coracoid portions of the arch become 

 first ossified ; the suprascapular plate remains long carti- 

 laginous, and always partly so ; the sternum is developed 

 in proportion as the hyoid arch is reduced, and the bran- 

 chial arches are removed ; thus a strong fulcrum is com- 

 pleted for the articulation of the shoulder -joints. The 

 pelvic arch had previously been completed, and the iliac 

 bones and sides of the sacrum become coelongated : then 

 the ilia continue to extend backwards as the tail is being 

 absorbed, and the hind-limbs are lengthened out and 

 finished. 



Thus metamorphosed, the skeleton of the frog pre- 

 sents the following structure (Fig. 12): The number of 

 vertebr83 of the trunk, exclusive of the coxygeal style, c, 

 is nine ; the first, or atlas, has no diapophyses, but these 



