ix.l THE COMMON FROG. 



M: 



The elimination of nitrogenous excreta becomes, 

 then, a very important process, and, indeed, in our- 

 selves death soon ensues when its elimination is 

 prevented by injury or disease. That part of the 

 body which is especially devoted to the excretion of 

 the nitrogenous waste of the tissues is the renal or 

 urinary organs — the two kidneys. Richly supplied 

 with blood, the kidneys have the power of straining 

 ofif from the blood {i.e. secreting) nitrogenous sub- 

 stances, w^hich are also in part eliminated by the skin 

 in perspiration. 



The material so strained off or secreted, is conveyed 

 by two long tubes, the ureters, which pass down, one 

 from each kidney, to the bladder or reservoir of the 

 nitrogenous excretion. 



In very many animals, as e.g. in birds, there is no 

 bladder, but the ureters terminate in a chamber, the 

 cloaca, which also receives the termination of those 

 canals (the oviducts) down which the eggs pass in 

 order to be " laid." 



In no adult bird, however, nor indeed in any adult 

 reptile, is there any closer connection between these 

 two sets of canals — the ureters and oviducts — which 

 terminate independently in the cloaca. In all such 

 animals, however, and in beasts also, at an early stage 

 of existence we do find a certain connection between 

 renal ducts and the oviducts or their analogues. This 

 coincidence is owing to the fact that in such higher 

 animals the urinary gland which ultimately exists in 

 the adult — namely, the kidney — is not that which pri- 

 mitively exists, but is a subsequent formation. The 

 primitive renal duct is not the ureter. The primitive 



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