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 nitrogénous 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 
aed cicee De the end of the ureter, so that each has 
densis (Princeton, 2234). a, aorta; itsownduct. This condition is retained 
#, ureter; v, postcava. A 
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 papille project, each bearing the 
