KATABOLISM. 



433 



tity of CO„ and NH 3 are embodied and disembodied 

 many times in the history of the organic kingdom. 



COMPARATIVE MORPHOLOGY OF THE KIDNEYS. 



Vertebrates : Mammals.— Little need be said on 

 the mammalian kidney. The organ has a similar posi- 

 tion in all mammals, and in most a similar shape and 

 structure to that of man. 

 In the ox the surface is 

 mammillated as if there 

 were a commencing sepa- 

 ration of the cones. This 

 separation is realized in 

 some living water mam- 

 mals, such as the otter 

 and the porpoise (Fig. 303). 

 But in these cases the dif- 

 ference is not significant. 

 Each cone consists of a 

 medullary and a cortical 

 portion, and they all dis- 

 charge into one pelvis and 

 ureter. 



Fig. 303. — Section of the kidney of a 

 porpoise, showing its structure : 

 u, ureter. (From Owen.) 



Birds. — The first important change in this, as in so 

 many other characters, is found in birds. In these (1) 

 the kidneys are not yet clearly differentiated into two 

 parts, a cortical and a medullary, having different func- 

 tions. (2) In these there is no bladder, but the ureters 

 empty into a cloaca or enlargement of the rectum just 

 within the vent. 



Reptiles and Amphibians (i. e., all cold-blooded 

 land vertebrates) have a cloaca ; but some — e. g., snakes 

 and lizards — like birds, have no bladder, the ureters 

 emptying directly into the cloaca; while others — e. g., 

 tortoises and frogs — have a bladder. In these cases the 



