THE KIDNEYS 127 



in that class. The mesonephros is, however, the adult kidney 

 in fishes and amphibians, where its duct serves in the male as 

 both reproductive duct and ureter. The presence of this kidney 

 and of its duct in embryonic mammals determines the form of 

 the ductus deferens and its connections with the terminal portions 

 of the urinary system. The pronephros, on the other hand, is in 



Fig. 74. Photomicrograph of a small part of a section of the kidney of 

 a rabbit. X 360. A collecting tubule appears at the right, the structure of 

 its epithelial lining being distinct. At the left is a glomerulus lying within 

 its Bowman's capsule and round it are several sections of convoluted tubules. 



all vertebrates a vestigial kidney present only in embryos. Its 

 duct system, however, which opens proximally to the body cavity, 

 plays an important part in the formation of the oviduct in the 

 female of all classes. The tubules of the pronephros and often 

 those of the mesonephros connect with the coelom by ciliated 

 openings, the nephrostomes, which do not occur in more special- 

 ized kidneys. Thus the development and structure of the prone- 

 phros and, in a less perfect way, those of the mesonephros show 



