146 DEVELOPMENT OF THE FROG 



from their sides, until the tadpole is about 12 mm. in length, 

 and the hind-limbs are just commencing to appear. It remains 

 stationary for a time and then, in tadpoles of about 20 mm. 

 length, begins to degenerate : the tubules become obstructed ; 

 some of them become collapsed, others for a time irregularly 

 dilated : the whole organ steadily diminishes in .size, and in 

 tadpoles of 40 mm. (Fig. 35, KP) is not more than half its 

 former size. It now shrinks rapidly, and at the time of the 

 metamorphosis (Fig. 36, KP) has almost disappeared, all three 

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

 few small pigmented and irregularly 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 

 to 37, 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 develop 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 



