252 



ZOOLOGY 



SECT. 



cartilaginous head, or extra-columella, is fixed to the inner surface 

 of the tympanic membrane. 



The comparison of the Frog's skull with those of Fishes is 

 facilitated by a study of its development. In the Tadpole or 

 larval Frog there is a cartilaginous cranium (Fig. 872) connected 

 on each side with a stout inverted arch, like the subocular arch 

 of the Lamprey or the palato-quadrate of Chimsera or Ceratodus, 

 and, like them, developed from the dorsal region of the mandibular 

 arch. The quadrate region (qu) of this primary upper jaw is well 

 in front of the eye, the axis of the suspensorium being inclined 

 forwards and the mandible very short, in correspondence with the 

 small size of the Tadpole's mouth. The quadrate is fused by its 

 pedicle with the trabecular region, the otic process (ot.pr) which 

 unites it with the auditory capsule being formed later. Behind 



the suspensorium are 

 >tpr~ distinct hyoid (c. hy) and 



branchial (br. 1-4) arches 

 supporting the gills by 

 which the tadpole 

 breathes. As develop- 

 ment goes on, the axis 

 of the suspensorium is 

 rotated backwards, pro- 

 ducing the wide gape 

 of the adult, and the 

 stout palatopterygoid 

 region of the subocular 

 arch (paLptg) gradually 

 assumes the slender pro- 

 portions it has in the 

 adult. The greater 



part of the hyoid arch gives rise to the anterior cornua of the 

 adult hyoid apparatus, the body of which is formed from the basi- 

 hyal and basi-branchials. and its posterior cornua probably from 

 the fourth branchial arch. The columella is developed inde- 

 pendently, but may perhaps represent a pharyngo-hyal or dorsal 

 segment of the hyoid arch. The stapes is a detached portion of 

 the outer wall of the auditory capsule. Thus, with the assumption 

 of purely aerial respiration, the complex branchial skeleton is 

 reduced to a simple structure for the support of the tongue. 



The shoulder-girdle has essentially the structure already de- 

 scribed (p. 77) in general terms as characteristic of the penta- 

 dactyle Craniata. The scapula (Fig. 873, S) is ossified, and is 

 connected by its dorsal edge with a supra-scapula (Fig. 870, s. SCP) 

 formed partly of bone, partly of calcified cartilage, and developed 

 from the dorsal region of the embryonic shoulder-girdle. The 

 coracoid (Co) is also ossified, but the procoracoid is represented by 



bri 



FIG. 872. Skull of Tadpole, an. cp. auditory capsule ; 

 l'- 1 4, branchial arches ; c. hy. ceratohyal ; col. 

 columella ; mck. Meckel's cartilage ; olf. cp. olfactory 

 capsule ; opt. for. optic foramen ; or. pr. orbital pro- 

 cess of suspensorium ; ot. pr. otic process ; pal. ptg. 

 palato-pterygoid bar; qu. quadrate; stp. stapes. 

 (After Marshall, slightly altered.) 



