PHYLUM CHORD ATA 



271 



to the paraquadrate is a ring of cartilage, the annulus tympanicus 

 (Fig. 936, an. tymp.), which supports the tympanic membrane as 

 the frame of a tambourine supports the parchment. Inserted into 

 the fenestra ovalis is a nodule of cartilage, the stapes (stp), to 

 which is attached the inner end of a small hammer-shaped struc- 

 ture, the columella (COL), the handle of which is ossified, while its 

 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. 924) connected 

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

 of the Lamprey or the palatoquadrate of Chimsera or Ceratodus, 

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

 arch. The quadrate re- 

 gion (qu) of this primary <, A/> , 

 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 trabe- 

 cular region, the otic 

 process (ot. pr) which 

 unites it with the 

 auditory capsule being- 

 formed later. Behind 

 the suspensorium are distinct hyoid (c. hy) and branchial (br. 1 4) 

 arches supporting the gills by which the tadpole breathes. As 

 development goes on, the axis of the suspensorium is rotated 

 backwards, producing the wide gape of the adult, and the stout 

 palatopterygoid region of the subocular arch (pal. ptg) gradually 

 assumes the slender proportions 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* 



br, 



FIG. 924. Skull of Tadpole, au. cp. auditory capsule ; 

 br. 1 A, 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.) 



