66 ANA TOMY OF AMPHIOXUS. 



the kidney, the tubules emerge from the latter as a series 

 of efferent ducts placed one behind the other, and these 

 again open into a common longitudinal duct on each side 

 of the body, known as the ureter, which leads the products 

 of excretion backwards to the cloaca. 



The permanently functional kidney of Fishes and Am- 

 phibia is known as the mesonephros. In Reptiles, Birds, 

 and Mammals, this is only functional during the embryonic 

 period, and later is replaced in a way not yet fully eluci- 

 dated by the permanent kidney of these forms which is 

 known as the metanephros. 



The ureter, or duct, of the mesonephros, is spoken of as 

 the mcsonepJiric duct, while the renal tubules constitute, 

 collectively, the glandular portion of the kidney. 



The permanent kidney of the craniate Vertebrates is ab- 

 solutely unique among all the other glands of the body, in 

 the fact that the glandular portion of the organ arises 

 independently of the duct, and only communicates secon- 

 darily with it. Moreover, the duct develops in point of 

 time before the gland. This is a very extraordinary fact, 

 and taken alone would be quite inexplicable. It has been 

 found, however, that the mesonephric duct has primary 

 relations with a totally distinct set of excretory tubules, 

 which differ from those mentioned above, both in their 

 position in the body and in their mode of development. 

 These primitive tubules, which mark the first appearance 

 of a renal organ in the Vertebrate embryo, constitute the 

 pronephros. 



The degree of development attained by the pronephros, 

 or primitive kidney, in the life-history of the various types 

 of Vertebrates, is very different in the different classes. 



Frequently, as with the Selachians (sharks), Birds, most 

 Reptiles, and with the Mammals, the pronephros is an 



