The Development of the Frog 79 



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 connective tissue to 

 form the compact Wolffian bodies or kidneys of the 

 fully formed tadpole (Fig. 28, KM). At the same 

 time the head kidneys diminish in size, and undergo 

 degenerative changes, and by the time of the meta- 

 morphosis (Fig 29, K P) have almost completely dis- 

 appeared. 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 3-^ mm. length, /. e.^ some time 

 before hatching, a pair of longitudinal grooves appears 

 along the inner surface of the somatopleure, extending 

 from the neck to the hinder part of the body, and 

 lying a little distance to the right and left of the noto- 

 chord (Fig. 15, K B). 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 becom- 

 ing at the same time lengthened out into short tubes 

 (Fig. 30). This convoluted mass is the head kidney or 



