THE UKINARY SYSTEM 143 



convoluted tube with glandular walls, opening into the body 

 cavity by three ciliated mouths or nephrostomes (Fig. 33, KS), 

 and continued back along the dorsal wall as the archinepliric 

 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. 83, KM), 

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

 tubules, which grow towar'ds 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 

 the fully formed tadpole (Fig. 35, KM). At the same time the 

 head kidneys diminish in size, and undergo degenerative changes, 

 and by the time of the metamorphosis (Fig. 36) have almost 

 completely disappeared. The Wolffian bodies persist as the 

 kidneys of the frog ; and by a series of further changes the 

 ureters and generative ducts of the adult become established. 



2. The Head Kidney and its duct. 



In tadpoles of about 3J mm. length, i.e., some time before 

 hatching, a pair of longitudinal grooves appear along the inner 

 surface of the somatopleure, extending from the neck to the 

 hinder end of the body, and lying a little distance to the right 

 and left of the notoohord. The lips of each groove soon meet 

 and fuse so as to convert the groove into a tube or duct. The 

 closure of the tube takes place from behind forwards, and at the 

 anterior end is effected imperfectly, three holes or nephrostomes, 

 one behind another, being left, through which the tube opens 

 into the body cavity. As the embryo grows, the anterior end 

 of the duct becomes convoluted and twisted on itself to form a 

 ball, the three nephrostomes becoming at the same time 

 lengthened out into short tubes. This convoluted mass is the 

 head kidney or pronephros. The hinder part of the duct is 

 the archinephric or segmental duct ; it remains straight, or 

 nearly so, and shortly before the tadpole hatches acquires an 

 opening into the cloaca. 



