262 THE DOG-FISH. 



6. The urino-genital sinus of each side is the posterior 



end of the sperm-sac : the vesicula seminalis and 

 the ureter open into it. 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. 



Find the opening at the apex of the urino-genital papilla : 

 pass a seeker through the opening, and forwards along the 

 urino-genital sinus and sperm-sac of one side : slit up the 

 ventral wall of the sinus and sac along their whole length : 

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

 seminalis and ureter, 



7. 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 backwards along the inner side 

 and dorsal surface of the vesicula seminalis to open 

 into the urino-genital sinus, immediately to the 

 iimer side of the opening of the Wolffian duct ; just 

 before reaching the sinus the ureter is joined by the 

 fifth or hindmost duct, from the posterior portion of 

 the kidney. From three to five smaller ducts open 

 separately into the urino-genital sinus a little further 

 back. 



The metanephric ducts are more easily followed after 

 injecting them with a coloured fluid from the ureter, 'which 

 is a wide tube, lying along the 'inner side of the vesicula 

 seminalis. 



8. The cloaca is the shallow depression between the pelvic 



fins into which, in the male, the rectum opens in front 

 and the urino-genital sinus behind. 



9. The Miillerian ducts are abortive in the male, their 



