THE SKULL. ^07 



of cartilage (Fig. 92, BR.I to URA). The dorsal ends of the four 

 bars of each side very early become connected together. The 

 ventral ends are at first independent, but the ends of the first 

 branchial bars early unite to form a broad median basibranchial 

 plate (Fig. 92, BB), with which the ventral ends of the hinder 

 branchial bars soon join. 



The chief characteristic of the skull of the tadpole is the 

 early development, and large size, of the cartilages of the extreme 

 anterior end, in connection with the jaws. The early formation 

 and great size of the anterior ends of the trabeculse, the large 

 labial cartilages, the early attachment of the mandibular arch to 

 the anterior end of the trabecula by the palato-pterygoid bar, 

 the enormous and singularly shaped orbital process of the 

 quadrate, and the massive character of the hyoid arch in the 

 floor of the mouth, are all connected with the need for a firm 

 supporting skeleton for the horny jaws, and an extensive sur- 

 face of origin for the muscles by which these jaws are worked. 

 All these are characters which concern the tadpole, and not the 

 frog ; and they are all lost, or greatly modified, in the adult 

 animal. 



b. The skull of the frog. About the time of the metamorphosis, 

 the skull undergoes important changes, by which the adult 

 condition and proportions are attained. 



A complete cartilaginous floor is formed to the skull, between 

 the two trabeculse. The hinder end of the skull acquires definite 

 shape : the parachordals increase greatly in size, growing inwards 

 so as to encroach upon, and ultimately to obliterate the notochord ; 

 they fuse with the auditory capsules and grow up behind these 

 to complete the occipital ring. The side walls and roof of the 

 skull are formed in the manner already described (p. 204). The 

 nasal capsules are formed by expansion of the anterior ends of 

 the trabeculse ; the upper labial cartilages persisting as the 

 pro-rhinal cartilages, which bound the anterior nostrils in front. 

 A thin cartilaginous shell is formed in the sclerotic coat of 

 each eye, but remains throughout life free from the skull proper. 



In the mandibular arch great modifications occur, associated 

 with the change from the small ventral mouth of the tadpole to 

 the wide, slit-like, and terminal mouth of the frog. The anterior 

 end of the quadrate rotates downwards and backwards, causing 

 great lengthening of the palato-pterygoid bar (Figs. 90 and 93, 



