262 THE DOG-FISH. 



vertical slit in the anterior 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 RENAL AND REPRODUCTIVE 

 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 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 two portions : an anterior part or pronephros, 

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

 mobranchs ; and a posterior portion, the mesonephros or 

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

 primary kidney (largely developed in most ordinary fish). 



The ducts also undergo division. The segmental duct 

 becomes split longitudinally into two separate ducts : the 

 Mullerian 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 one or more independent ducts, or ureters. 



In the female the right and left Mullerian ducts unite 

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

 median aperture, and become converted into the oviducts ; 



