THE ORGANS OF THE MIDDLE GERM-LAYER. 367 



The further fate of the primitive kidney is very different in the 

 separate classes of Vertebrates. In the Ananmia, i.e., in Fishes and 

 Amphibia, it becomes the permanent urinary organ, through which 

 the excretions of the body are eliminated ; but besides that, it also 

 acquires relations to the sexual apparatus, upon which, however, I 

 shall not enter until later. In Birds and Mammals, on the contrary, 

 the primitive kidney is functional only a short time during embryonic 

 life ; soon after its establishment it undergoes profound regressive 

 changes, and at last is preserved only in part, in so far as it enters 

 into the service of the sexual apparatus, and, as we shall likewise see 

 later, participates in conducting away the sexual products. 



(c) The Kidney. (Metanephros.) 



The secretion of urine is assumed in the higher Vertebrates by 

 a third gland, which is established at the posterior end of the meso- 

 nephric duct the permanent kidney. The method of its formation, 

 which appears to differ at first from that of the mesonephros, presents 

 great obstacles to its investigation. It is most accurately known 

 from studies on the development of the Chick through the works of 

 SEDGWICK. At the beginning of the third day of incubation in the 

 Chick there grows out of the [posterior] end of the mesonephric duct, 

 from its dorsal wall, an evagination the excretory duct of the kidney 

 or ureter. 



There are two conflicting views relative to its connection with the 

 development of the kidney. According to the older view, which is 

 still shared by many, the kidney is formed from the ureter in the 

 manner of an ordinary glandular growth. It is maintained that 

 paginations take place which give rise to other evaginations, and 

 thus produce the whole parenchyma of the kidney. According to the 

 second view, which has been formulated especially by the more recent 

 ^mbryologists, by SEMPER, BRAUN, FURBRINGER, SEDGWICK, and 

 BALFOUR, the permanent kidney is, on the contrary, developed out 

 of two different fundaments, which come into relation with each other 

 only secondarily : the medullary substance with its collecting tubules 

 out of the ureter, the cortical substance with the tortuous tubules 

 and the loops of HENLE, on the other hand, out of a special fundament. 

 According to this view there would be an agreement between the 

 development of the kidney and primitive kidney, in as far as in the 

 latter the mesonephric duct and the mesonephric tubules also arise 

 separately, and only secondarily enter into relation with each other 



