The Development of the Frog 81 



pronephros. The hinder part of the duct is the archi- 

 nephric or segmental duct: it remains straight, or nearly 

 so, and shortly before the tadpole hatches acquires an 

 opening into the cloaca. 



u At the time of hatching, the excretory organs thus 

 consist on each side of (i) a head kidney, which is a 

 convoluted tube lined by a glandular epithelium, and 

 opening into the anterior end of the body-cavity by 

 three ciliated openings, the nephrostomes; and (2) the 

 archinephric or segmental duct, which is the posterior 

 part of the tube, and runs back along the dorsal body- 

 wall nearly straight to the cloaca, into which it opens. 



" The head kidney is closely surrounded by, indeed 

 almost imbedded in, the posterior cardinal vein (Fig. 

 31, VC), and it is from the blood of this vein that the 

 epithelial cells of the head kidney tubules separate the 

 excretory matters, which are then passed down the duct 

 to the exterior. 



" The head kidney continues to increase in size, the 

 tubules becoming still more convoluted, and lateral diver- 

 ticula arising from their sides, until the tadpole is about 

 12 mm. in length, and the hind limbs are just commen- 

 cing to appear. It remains stationary for a time and then, 

 in tadpoles of about 20 mm. length, begins to degen- 

 erate; the tubules become obstructed; some of them be- 

 come collapsed, others for a time irregularly dilated; 

 the whole organ steadily diminishes in size, and in tad- 

 poles of 40 mm. (Fig. 28, K P) is not more than half its 

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

 of the metamorphosis (Fig. 29, K P) has almost dis- 

 appeared, all three nephrostomes having closed up, and 

 the organ being reduced to a few small pigmented and 



6 



