196 THE EXCRETORY SYSTEM 



Amphibian kidneys have a pecuhar blood supply. Branches of 

 the dorsal artery, called renal arteries, carry arterial blood to them, 

 and venous branches from the kidneys unite to form the postcaval 

 veins which return blood to the heart. In addition to these, renal- 

 portal veins arising from veins in the legs also pass to the kidneys. 

 The renal arteries are connected with the glomeruli of the renal 

 corpuscles. The capillary network around the tubules is derived 

 from the renal-portal veins. All the blood returns to the heart via 

 the postcava. 



Despite many ph}-siological experiments with the frog's kidney, 

 the manner of its functioning is not thoroughly understood. Obser- 

 vations of the li^'ing kidney have shown an intermittent flow of 

 blood through the glomeruli, with some glomeruli thus being retired 

 from or recalled to active functioning as conditions demand. The 

 neck region and the narrow ciliated portion at the lower end of the 

 proximal convoluted tubvile have been observed to contract, and 

 presumably regulate urinary flow. The flow of urine from the distal 

 convoluted tubule to the collecting duct is rapid. Experiments 

 indicate that the glomerulus filters urine from the blood, that the 

 proximal convoluted tubules concentrate this filtrate by resorbing 

 water to a greater extent than other substances, and that the 

 ciliated portions before and behind this region act as valves and, 

 together with the distal convoluted tubules, constitute the pressure 

 regulating portion of the kidney. 



THE METANEPHROS. 



The metanephros develops from the caudad nephrogenic tissue 

 near the cloaca at the base of the mesonephric duct, and is composed 

 of a compact mass of tubules. Like the mesone])hros, which pre- 

 cedes it in time and location, this kidney has a double origin. The 

 Bowman's capsule and secreting portions of the tubules are formed 

 from the caudad nephrogenic tissue, but the collecting ])ortions arise 

 as outgrowths of the mesonephric duct, which was formed at the time 

 of the pronephros appearance and retained after the degeneration 

 of the tubules of that kidney. The glandular portion of the entire 

 tubule, i. e., that part developing from nephrogenic tissue, is called 

 a nephron. At about the time when nephrogenic tissue begins 

 developing metanc])hric tubules, a bud evaginates from the bend 

 of the mesonephric duct where it joins the cloaca. This bud 

 pushes forward to the anterior lumbar region of the body cavity. 



