254 THE DOG-FISH. 



vertical slit in the anterioi* wall, to the left of the 

 ventral end of which is a small aperture from the 

 coronary sinus, which returns to the sinus venosus 

 the blood from the walls of the heart. 



VII. DISSECTION OF THE EENAL AND EEPEODUCTIVE 



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 o£ 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 wall of the abdominal cavity. 

 Each consists at an early age 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, in many vertebrates, each kidney 

 appears in three portions : an anterior part or pronephros, 

 which is small and abortive in the dog-fish and other Elas- 

 mobranchs ; a middle portion, the mesonephros or Wolffian 

 body, including the greater part of the length of the primary 

 kidney (largely developed in most ordinary fish)"; and a 

 posterior portion or metanephros, formed from its hinder 

 part. 



The ducts also undergo division. The segmental duct 

 becomes split longitudinally into two separate ducts : the Miil- 

 lerian duct, which loses its connection with the kidney ; and 

 the Wolffian duct, into which the kidney tubules open. At a 

 'later stage several of the hinder kidney tubules unite to form 

 an independent duct, the metanephric duct or ureter. 



In the female the right and left Miillerian ducts unite 

 in front, so as to open into the abdominal cavity by a single 



