THE METANEPHROS 197 



where it expands and becomes associated with the nephrogenic 

 tissue. The (hict ])ortion of this prolonged evagination becomes 

 the lU'eter, or main excretory (hict of tlie metanephros. The ex- 

 panded anterior portion is the primiti\'e pehis and gi\'es off anterior 

 buds that form the calyces from which branching evaginations give 

 rise to systems of branched collecting tubules. The latter fuse with 

 the nephridial tubules that ha\e been developing during this same 

 period. This collecting system forms a conical mass of tree-like 

 branched collecting tubules that constitute the major portion of 

 the medulla of the kidney and extend radially into the cortical 

 portion occupied mainly by the secretory tubules with which they 

 fuse. 



The nephridial tubules of the metanephros are more complex and 

 more numerous than those of the mesonephros and do not at any 

 time have a nephrostome associated with them. Bowman's capsule 

 closely surrounds the glomerulus and continues distally to the neck 

 of the tubule. Continuing from the neck, the tubide becomes con- 

 voluted for some distance, then constricts and straightens to descend 

 and loop liack to form a wider distal convoluted portion. The 

 distal convoluted portion finally joins with a collecting tubule by 

 a junctional tubide or in some cases by a short arched portion. 



This type is the permanent kidney of reptiles, birds, aufl mammals, 

 but before its appearance the other two types successively' precede 

 it in embryological de^'elopment. The caudad portion of the pro- 

 nephric duct remains as the mesonephric duct, and this in turn by 

 evagination from its caudad portion gives rise to the ureter of the 

 metanephros, and the original main portion of it becomes converted 

 into the male genital duct, the ^'as deferens, for these forms. 



As an example of this type the mammalian kidney may be used, 

 since details of its structure are well known and give all the struc- 

 tural characters. 



Mammahan Kidney.— Mammalian kidneys are bean-shaped organs 

 located in the lumbar region against the dorsal body wall. (Fig. 

 124.) They appear to l)e free in the body cavity, but are held in 

 place by the sturdy trunks of the renal arteries, renal veins, and 

 by the ureters. The peritoneiun, which splits into connecti\'e-tissue 

 sheets or fascia at the outside border of each kidne\', also aids 

 in support. The posterior fascia unites with the front of the spinal 

 column, and the anterior fascia covers the front of the kidney and 

 its vessels, passing over the aorta to meet the corresponding fascia 

 from the other kidney. There is usually considerable fatty tissue 



