URINOGENITAL ORGAN'S 



345 



The pronephros itself has only a transitory function as an 

 excretory organ. Its duct, however, always persists, and usually 

 undergoes important modifications, which are closely connected 

 with the appearance of a second and more extensive series of 



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excretory segmental tubules, which appear later, mainly posteriorly 

 to the pronephros, and constitute the mesonephros or mid- 

 kidney ; the pronephric duct now serves as a mesonephric duct. 



The mesonephros, often known as the Wolffian body (Figs. 

 273, 274, 277, B), is sometimes regarded as corresponding simply to 



