EXCEETORY ORGANS OF VERTEBRATA. 607 



it both in space and time. No rudiment of the neplirostomata has 

 been observed, nor is the archinephric duct divided as in the 

 Anamnia; the Mullerian duct has a separate rudiment. 



The kidneys of Eeptiles and Birds somewhat resemble those of 

 Fishes in their size and position. They are placed far back and close to 

 the cloaca; in the Snakes only are they placed farther forwards, while 

 at the same time they are longer. They vary very greatly in form, in 

 consequence of the development of lobes. In Birds they are placed in 

 depressions between the transverse processes of the sacral vertebrae, 

 and are generally divided into three portions, which are sometimes 

 connected with one another, and which may vary greatly in size. 

 The ureters (Fig. 349, u) are generally placed on the inner edge of 

 the kidney, and receive at various points larger urinary canals 

 (Ophidii, Chelonii) ; or these canals are enclosed in the parenchyma of 

 the kidney, and do not leave the organ except at its termination 

 (Saurii, Crocodilini). In Birds a large part of the canal is outside 

 the kidney. In all cases they open separately into the cloaca, or 

 into a sinus urogenitalis, into which the genital ducts also open. 



The kidneys of the Mammalia vary in several points, and espe- 

 cially as to the characters of the orifice 'of the ureters, after the 

 differentiation of the rudiment which is known as the "renal 

 canal. ■'■' 



The kidneys, which are developed at the blind end of the 

 " urinary canal," are, after they are differentiated, placed behind 

 the primitive kidneys. At first they appear to have a smooth 

 surface, which becomes uneven when the glandular parenchyma is 

 developed into separate lobes. In either lobe the urinary canaliculi 

 are united together at a papilhform process, with which the common 

 efferent duct of the lobe is connected. It forms the pyramid, and a 

 number of these unite to form the pelvis of the kidney, from which 

 the ureter is given off. The permanently distinct lobes are very 

 numerous (about 200) in the Cetacea. There is a smaller number 

 in the Pinnipedia. In many Oarnivora, also, the lobes are separate 

 (Ursus, Lutra), while in others they are fused. This gives a 

 knobbed appearance to the surface of the kidney (e.g. in Hyaena, 

 Bos, Elephas). In others there is a condition of this kind for some 

 time, but when the cortical substance of the lobes is completely 

 fused, the surface of the kidney becomes smooth, although the 

 grooves that remain indicate its primitive division into lobes. 

 Within the organs, however, the division is more or less completely 

 retained, and the number of primitive lobes is implied by the 

 greater or less extent to which the papillae are fused together. 

 This fusion, further, may affect some, or all the lobes, so that the 

 number of renal papilla may be much reduced; at last, indeed, 

 they may all unite into one (Marsupialia, Edentata, Eodentia, several 

 Oarnivora and Primates). 



The ureters formed from the renal canals, after they are 

 separated from the archinephric duct, primitively pass into that 

 portion of the allantois which runs in the abdominal cavity of 



