;8 Vertebrate Embryology 



THE DEVELOPMENT OF THE URO-GENITAL 

 ORGANS 



THE URINARY ORGANS 



There are several points, in connection with 

 the development of the urinary organs in the 

 frog, that have been differently described by 

 various investigators. We shall here follow 

 the description given by Marshall, whom we 

 shall quote at some length. 



i. 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 im- 

 bedded in the dorsal wall of the body at its anterior 

 end, immediately behind the constricted neck region 

 (Figs. 24 and 28, K P). Each head kidney is a con- 

 voluted tube with glandular walls, opening into the 

 body-cavity by three ciliated mouths or nephrostomes 

 (Fig. 24, K S), and continued back along the dorsal 

 wall as the archinephric or segmental duct, K A, to the 

 hinder end of the body, where it joins with the cor- 

 responding 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. 24, K M\ 



