234 THE DOG-FISH 



become connected, owing to the genital ducts being formed 

 from parts of the kidney-ducts. 



The kidneys are paired organs, lying side by side along the 

 whole length of the dorsal surface of the body-cavity. Each 

 consists at an early stage of its development of a number of 

 convoluted tubules, placed one behind another, and opening 

 at one end into the body- cavity : at their other ends they open 

 into a longitudinal duct, the segmental duct, which runs the 

 whole length of the kidney, and opens in front into the body- 

 cavity and behind into the cloaca. 



During development, each kidney becomes divided into three 

 parts ; an anterior part or pronephros, which is rudimentary 

 or absent in the dog-fish and other Elasniobranchs, but largely 

 developed in most ordinary fish ; a middle portion or meso- 

 nephros, including the greater ,part of the length of the 

 primary kidney; and a posterior portion or metanephros, 

 formed from its hinder part. 



A corresponding division of the duct takes place, so that 

 in place of the single segmental duct, three ducts are formed ; 

 a pronephric or Miillerian duct, which opens in front into the 

 body-cavity, but, owing to the absence of a proiiephros, loses 

 all connection with the kidney ; a mesonephric or Wolffian 

 duct, which runs from the mesonephros to the cloaca ; and a 

 metanephric duct or ducts, which convey the excretion of the 

 metanephros to the cloaca. 



In the female the right and left pronephric ducts unite in 

 front, so as to open into the body-cavity by a single median 

 aperture, and become converted into the oviducts ; the meso- 

 nephros and its duct are small and unimportant ; while the 

 metanephros and its ducts form the functional kidney and 

 ureters of the adult. 



In the male the pronephric ducts are rudimentary and 

 functionless ; the mesonephric tubules become connected with 

 the testis, and act as vasa efferentia, conveying the sperma- 

 tozoa to the mesonephric duct which acts as vas deferejis. 

 The metanephros and metanephric ducts, as in the female, 

 become the functional kidney and ureters. 



