148 DEVELOPMENT OF THE FROG. 



In connection with the lips two pairs of small labial cartilages 

 appear, serving to support the horny jaws of the tadpole. 



In the later stages the subocular or quadrate portion of the 

 mandibular bar acquires a very close connection at its hinder 

 end with the auditory capsule, and changes its direction, so 

 that in place of running horizontally forwards, it now runs 

 downwards and forwards. This change, which may be described 

 as a rotation backwards of the bar, causes lengthening of the 

 palato-pterygoid bar and of Meckel's cartilage : these latter 

 become respectively the basis of the upper and lower jaws of the 

 tadpole, which are completed later on by the development of 

 the pterygoid, squamosal, maxilla and other bones. 



This rotation backwards of the distal end of the quadrate, 

 with corresponding lengthening of the upper and lower jaws, 

 proceeds rapidly during and after the metamorphosis, so that 

 the quadrate, instead of being directed downwards and forwards, 

 soon runs vertically downwards, and later on downwards and 

 backwards as in the adult. ((7/. Fig. 10, p. 56.) 



ii. The hyoid bar also undergoes important changes. At 

 first it is a wide band of cartilage placed nearly vertically in 

 the side wall of the pharynx, immediately behind the mandibular 

 bar. When the mandibular arch becomes horizontal the hyoid 

 forms a broad stout bar of cartilage, articulating at its upper 

 end with the subocular part of the mandibular arch, and con- 

 nected at its ventral end with the hyoid bar of the other side 

 by a small median basi-hyal plate in the floor of the mouth. 



At the commencement of the metamorphosis the hyoid bar 

 becomes narrower, and begins to extend upwards towards the . 

 auditory capsule : and by the end of the metamorphosis this 

 upper part of the hyoid has become the long slender anterior 

 cornu of the hyoid, which acquires a loose connection at its 

 upper end with the cranium and with the quadrate cartilage. 



The development of the columella is imperfectly known. It 

 consists of two elements, one of which — the stapes — is a small 

 plate of cartilage partially filling a hole, the fenestra ovalis, 

 which appears in the lower and outer wall of the auditory cap- 

 sule about the time the opercular folds are growing back over 

 the gills. The other portion of the columella is a small rod, 

 partly cartilage, partly bone, which does not appear till some 

 months after the completion of the metamorphosis, and which 

 fuses with the stapes at its inner end, while its outer end becomes 



