142 DEVELOPMENT OF THE FROG 



four branchial arches respectively ; and they develop in order 

 from before backwards. 



i. The oral (mandibular) bar, which is the largest of the 

 series, lies at first parallel to the others, i.e., perpendicular 

 to the long axis of the body. It very early, however, under- 

 goes important changes, and by the time that the external gills 

 are developed, and before the appearance of the opercular folds, 

 it has altered its direction, and now runs almost horizontally 

 forwards, parallel to and below the trabeculre. 



It soon unites with the trabeculse, both behind and in front 

 of the eyeball, the latter union being effected by a short trans- 

 verse bar of cartilage the palato-pterygoid. In front of the 

 palato-pterygoid, the most anterior part of the oral bar becomes 

 segmented off as a short rod of cartilage, which is directed 

 upwards and forwards in the lower lip ; it is known as Meckcl's 

 cartilage, and forms the basis of the lower jaw or mandible. 

 That part of the oral bar with which this segment articulates 

 will give rise to the quadrate of the adult. 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 

 oral 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 membranous 

 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. (Cf. Fig. 10, p. 48.) 



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 oral bar. 

 When the mandibular arch becomes horizontal the hyoid forms 

 a broad stout bar of cartilage, articulating at its upper end with 



