330 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The Wolffian ducts persist only as the ducts of the testes (vasa deferentia) and 

 the ureters take their place as carriers of the nitrogenous waste. These latter 

 tubes open separately into the cloaca. An (allantoic) urinary bladder is found 

 only in lizards and turtles (fig. 313). The urine is semisolid and consists largely 

 of uric acid. 



MAMMALS. In the mammals but two tubules are outlined in the 

 pronephros and these never become functional. The pronephric duct 

 is formed as a solid cord on the surface of the nephrotomic segments 

 which later becomes canalized. Of the fate of the pronephros 

 nothing certain is known. The mesonephros (fig. 329), on the other 



hand, is an important structure in foetal 

 life, and in the monotremes and mar- 

 supials it continues to function in the im- 

 mature stages. Later it largely disap- 

 pears in all, with the exception of the 

 parts concerned in the formation of the 

 efferent ductules of the testes and some 

 inconsiderable remnants in both sexes. 

 Only in Echidna are nephrostomes 

 formed and in some rodents there is no 

 formation of glomeruli. 



The peculiar development of the 

 mammalian metanephros (p. 316) results 

 in the kidney of the young stages having 

 a lobulated appearance, the lobules cor- 

 responding to the ducts given off from 

 the end of the ureter, so that each has 

 its own duct. This condition is retained 

 in the adult elephants, some ungulates, 

 carnivores (fig. 330) and primates, and especially in the aquatic 

 species (whales, seals), the lobules being most numerous in some of 

 the whales. In all other forms the ducts fuse later and the lobules 

 unite into a compact mass lying in the lumbar region near the last 

 rib. Each kidney has a peculiar shape (giving rise to the adjec 

 tive reniform), convex on the lateral, concave on the medial sur- 

 face, the latter being called the hilum and receiving the excretory 

 duct (ureter) and the blood-vessels of the organ (hepatic artery 

 and vein). Just inside the hilum is a cavity, the pelvis of the 

 kidney, into which one or several papillae project, each bearing the 



FIG. 330. Lobulated kidney 

 (metanephros) of otter, Lutra cana- 

 densis (Princeton, 2234). a, aorta; 

 u, ureter; v, postcava. 



