THE URINARY SYSTEM 149 



form Malpighian bodies. From the necks of the Malpighian 

 bodies, short solid rods of cells grow towards the peritoneal 

 epithelium and fuse with it. These rods soon become hollow, 

 and open into the body cavity by ciliated funnel-shaped mouths 

 or nephrostomes : their opposite ends break away from the 

 Wolffian tubules and open directly into the renal veins on the 

 ventral surface of the kidney. The Wolffian tubules rapidly 

 increase in number ; they also branch freely, and so give rise to 

 a complicated system of glandular tubules, which, when bound 

 together by bloodvessels and connective tissue, form the 

 Wolffian body or kidney of the frog. The nephrostomes persist : 

 and in the adult frog as many as 200 or more are present on 

 the ventral surface of the kidney, as minute funnel-like ciliated 

 openings, leading by short tubes into the renal veins. 



4. The Wolffian and Miillerian ducts. 



So far we have only described one duct on each side, the 

 segmental duct, which acts as the excretory duct first of the 

 head kidney, and then of the Wolffian body as well. We have 

 now to consider in what way the ureters and generative ducts 

 of the adult frog are formed. 



About the time of the metamorphosis the head kidney, 

 which has become rudimentary, separates completely from the 

 duct, which now ends blindly a short distance in front of the 

 Wolffian body. 



A little later, after completion of the metamorphosis and 

 entire disappearance of the tail, this anterior end of the 

 segmental duct, in front of the Wolffian body, becomes divided 

 somewhat obliquely into two ; an anterior part, which is now 

 isolated from the Wolffian body, and will be called the 

 Miillerian duct ; and a posterior part, the Wolffian duct, which 

 is simply the posterior part of the original segmental duct, and 

 receives the Wolffian tubules of the kidney. 



The Miillerian duct becomes connected in front with the 

 peritoneal epithelium, and acquires an opening into the anterior 

 end of the body cavity. At its hinder end it grows back along 

 the outer side of the Wolffian duct to the cloaca, into which it 

 opens. So far the changes are the same in both sexes. In the 

 male frog the Miillerian duct persists in this condition through- 

 out life, and may be recognised as a slender longitudinal streak 

 lying in the thickness of the peritoneum a short distance to the 



