THE URINARY ORGANS. 259 



kidney. The glomerulus in this case is situated in 

 the wall of the nephrotome, which makes the meso- 

 nephros a more efficient organ. The mesonephros 

 is an elongated segmented organ and a permanent 

 structure in amphibians. It is an embryonic organ 

 in birds and mammals, in which it is replaced by 

 the metanephros. 



3. The metanephros, or permanent kidney, develops 

 as a diverticulum from the cloacal end of the Wolffian 

 duct. The diverticulum lengthens into a tube, the 

 ureter. The upper or anterior end of the tube 



Artery. 



Vein. 



Fig. 197. Kidney of new-born infant, showing a distinct separation 

 into reniculi; natural size. At a is seen the consolidation of two adjacent 

 reniculi (Bohm and Davidoff). 



branches to form a number of smaller tubes, the uri- 

 niferous tubules of the kidney. The surrounding 

 mesoderm becomes condensed and vascular, inter- 

 lacing between the tubules to form the adult kidney. 



The development of the urinary system is closely 

 associated with that of the generative system, and 

 will be referred to again when the latter is described. 



Structure. The hilus of the kidney is an opening 

 through which the ureter and blood-vessels pass. 

 On making a longitudinal section of the kidney, it 

 will be seen that the hilus leads to an expanded fis- 



