THE URINARY SYSTEM. 153 



nephrostomes having closed up, and the organ being reduced to a 

 few small pigmented and irregulary twisted tubules, which have 

 separated from the duct, and which soon disappear completely. 



Opposite the head kidney an irregular sacculated outgrowth, 

 the glomerulus, arises from the aorta on each side (Figs. 31 

 and 33 GM) : this appears first about the time of hatching, and 

 its development keeps pace with that of the head kidney. It 

 lies immediately opposite the nephrostomes, and very close to 

 these, though not touching them. It begins to diminish in size 

 about the same time as the head kidney. At the time of the 

 metamorphosis (Fig. 36 GM) it is very small, and after the first 

 year it can no longer be recognised. Its close relation to the 

 head kidney, and the fact that its growth and subsequent 

 degeneration, keep pace with those of the head kidney, point 

 to a close physiological connection between the two organs, 

 though it is not easy to imagine what precise function the 

 glomerulus subserves. 



3. The Wolffian Body. 



The Wolffian body, or kidney, first appears in tadpoles of 

 from 10 to 12 mm. in length. It arises on each side as a series 

 of small solid masses of mesoblast cells lying along the inner 

 side of the segmental duct, between this and the aorta (Figs. 

 33 and 35). They develope from behind forwards, the hindmost 

 pair being a short distance in front of the cloaca, and the most 

 anterior ones about three segments behind the head kidney. 



These solid masses soon become elongated into twisted rods, 

 which then become tubular, and growing towards the segmental 

 ■duct meet and open into it. At their opposite ends these 

 Wolffian tubules, as they are termed, dilate into bulb-like 

 expansions, which become doubled up by ingrowth of little 

 knots of bloodvessels, derived from the dorsal aorta, and so 

 form Malpighian bodies. From the necks of the Malpighiau 

 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 



