280 Basic Structure of Vertebrates 



some modifications, to form a third kidney, the metanephros, which 

 becomes the adult kidney. The tubular epididymis, associated with 

 the testis of the adult amniote, is a part of the embryonic mesonephros 

 which otherwise disappears except for certain vestiges which are appar- 

 ently of little functional importance. 



While the more anterior or pronephric tubules are forming, the 

 mesomeric material on each side of the embryo gives rise to a longi- 

 tudinal tube (Figs. 228, 229) which extends from the pronephric region 

 to the cloaca, into which it finally opens. The pronephric tubules of each 

 side join the corresponding longitudinal pronephric duct, thus 

 putting the coelom into communication with the exterior by way of 

 the cloaca. The coelomic openings or nephrostomes (Figs. 80, 229) 

 of the pronephros are ciliated. The arrangement apparently serves for 

 drainage from the coelom to the exterior. 



The mesonephric tubules acquire connection with the already 

 formed longitudinal duct (Fig. 80) which, as the pronephros degener- 

 ates, then serves, at least in part, as the mesonephric or Wolffian 

 duct. In Anamnia each mesonephric tubule usually has a ciliated 

 nephrostome opening into the coelom. In the kidneys of amniotes, 

 nephrostomes rarely appear. 



Mesonephric and metanephric tubules usually form specialized ex- 

 cretory structures. The tubule (Figs. 81, 86) gives rise to a cup-shaped 

 expansion (Bowman's capsule). The hollow of the cup is occasioned 

 by ingrowth of a dense network of fine blood-vessels, the glomerulus. 

 The capsule and glomerulus together constitute a renal (or Mal- 

 pighian) corpuscle. The part of the tubule between the corpuscle 

 and the mesonephric duct eventually becomes much elongated, coiled, 

 and locally differentiated. 



In the absence of nephrostomes, drainage of waste from the coelom 

 does not occur and the function of excretion must be confined to the 

 renal corpuscle, where the glomerulus brings blood-vessels into close 

 relation to the lumen of a kidney tubule, and to other vascular regions 

 of the tubule. 



The metanephros has outlet by way of a duct, the ureter, which 

 develops as a forward-growing branch from the cloacal end of the 

 mesonephric duct of the same side of the embryo (Fig. 82). The tubular 

 structures of the metanephros are formed partly from mesomeric mate- 

 rial, but largely by outgrowth from the anterior end of the ureter. 



The adult kidney (Fig. 85) consists of the entire system of tubules 

 — mesonephric or metanephric — of one side of the embryo, increased 

 to great number by formation of secondary tubules from the primary 

 tubules, each tubule tremendously elongated and much coiled, the 

 tubules bound together by connective tissue with blood-vessels richly 



