THE URINARY SYSTEM. 149 



connected with the tympanic membrane icf. Fig. 11, p. 58): 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. 



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

 cartilage, 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 cornua. 



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 

 body at its anterior end, immediately behind the constricted 

 neck region (Figs. 33 and 35 KP). Each head kidney is a 

 convoluted tube with glandular walls, opening into the body 

 cavity by three ciliated mouths or nephrostomes, and continued 

 back along the dorsal wall as the archinephric or segmental 

 duct, KA, to the hinder end of the body, where it joins with 

 the corresponding duct of the opposite side, and opens into the 

 cloaca. 



The head kidneys and their ducts are well developed in the 

 tadpole at the time of hatching : they subsequently increase 

 considerably in size, and are the sole excretory organs of the 

 tadpole during its early stages. In tadpoles of about 12 mm. 

 length the adult kidneys or Wolffian bodies (Fig. 33, KM), begin 

 to form in the hinder part of the body as a series of paired 

 tubules, which grow towards and open into the segmental duct. 

 These Wolffian tubules rapidly increase in number, as well as 

 in size and complexity, and become bound together by connec- 

 tive tissue to form the compact Wolffian bodies or kidneys of 



