THE URINARY SYSTEM 141 



the subocular part of the oral bar, and connected 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 filUng a hole, the fenestra ovalis, 

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

 sule about the time that the opercular folds are growing back 

 over the giUs. 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 connected with the tympanic membrane {cf. Fig. 

 10, p. 48) ; this outer element of the columella is commonly 

 regarded as formed from the uppermost part of the hyoid arch, 

 but appears to be really quite independent of it in the frog. 



iii. The branchial bars are at first simple flattened rods of 

 oartUage, independent of one another, but becoming early con- 

 nected with a median basi-branchial cartilage, which appears in 

 the floor of the mouth between the ventral ends of the first two 

 pairs of bars. 



As the hind-legs appear, the branchial bars of each side coalesce 

 with one another both at their dorsal and their ventral ends : 

 they also become strongly curved, and together form a complex 

 basket-work supporting the gills. Later on, as the gills begin to 

 shrink, the branchial bars become more slender : their dorsal ends 

 disappear, while their ventral ends fuse with the basi-hyal and 

 basi-branchial cartilages, and together give rise to the body of 

 the hyoid and its posterior coiTiua. 



N. The Development of the Urinary System. 



1. General Account. 



The excretory organs of the tadpole, during the early stages 

 of its existence, are the head kidneys or pronephra. These 

 are a pair of globular organs imbedded in the dorsal wall of the 



