238 THE DOG-FISH 



vertical slit in the anterior wall, to the left of the 

 ventral end of which is a small aperture leading 

 into the coronary sinus, which returns to the sinus 

 venosus the blood from the walls of the heart. 



VII. DISSECTION OF THE EENAL AND BEPBODUCTIVB 



SYSTEMS. 



These two systems have already been seen in part, but 

 may now be examined more fully. They are originally in- 

 dependent of each other, but in the course of development 

 become associated, 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 abdominal 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 abdominal 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 abdominal 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 Elasmobranchs, 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 

 abdominal cavity, but, owing to the absence of a pronephros, 

 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 abdominal cavity by a single 



