THE URINARY SYSTEM. 151 



and by the time of the metamorphosis (Fig. 36) have almost 

 completely disappeared. 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 J mm. length, i.e.^ some time before 

 hatching, a pair of longitudinal grooves appear along the inner 

 surface of the somatopleure, extending from the neck to the 

 hinder end of the body, and lying a little distance to the right 

 and left of the notochord. 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 becoming at the same time 

 lengthened out into short tubes. This convoluted mass is the 

 head kidney or pronephros. The hinder part of the duct is 

 the archinephric or segmental duct ; it remains straight, or 

 nearly so, and shortly before the tadpole hatches acquires an 

 opening into the cloaca. 



At the time of hatching, the excretory organs thus consist on 

 each side of (1) 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 nephro- 

 stomes ; 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, 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 diverticula arising 

 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 : 



