238 THE DOG-FISH 



5. The vesicula seminalis is a thin- walled tube lying along 



the outer side and dorsal surface of the dilated por- 

 tion of the mesonephric duct, which it about equals 

 in length. It ends blindly in front, and opens 

 behind, with the mesonephric duct, into the urino- 

 genital sinus. 



Dissect away the vesicula seminalis from the mesonephric 

 duct, and follow it back to its opening into the urino-genital 

 sinus. 



6. The metanephric ducts are usually five in number on 



each side : they arise from the outer border of the 

 ventral surface of the metanephros at intervals along 

 its length. The four anterior ducts, after a short 

 course, unite to form a wide thin-walled tube, the 

 ureter, which runs back along the inner side of the 

 dilated mesonephric duct to open into the urino- 

 genital sinus : just before reaching the sinus the 

 ureter is joined by the fifth or most posterior duct, 

 which is larger than the others, and has a very 

 convoluted course. 



The metanephric duvets are most readily followed by inject- 

 ing them with a coloured fluid from the ureter, which is a 

 large wide tube, readily found along the inner side of the 

 mesonephric duct. 



7. The urino-genital sinus is the posterior end of the meso- 



nephric duct into which open the vesicula seminalis 

 and the ureter. About a quarter of an inch behind 

 these openings the sinuses of the two sides unite, 

 and then open into the cloaca by a small aperture 

 at the apex of the urino-genital papilla, which is 

 situated immediately behind the opening of the 

 rectum. 



Slit open the ventral wall of the posterior end of the meso- 

 nephric duct, and of the urino-genital sinus on one side : 

 examine, and pass a seeker into the orifices of the vesicula 

 seminalis and ureter; and slit open the urino-genital papilla. 



