270 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 ureters are usually five in number on each side ; they 

 arise from the outer border of the ventral surface 

 of the secreting portion of the kidney at intervals 

 along its length. The four anterior ducts, after 

 a short course, unite to form a wide thin-walled 

 tube which runs backwards along the inner side 

 and dorsal surface of the vesicula seminalis to open 

 into the urino-genital sinus, immediately to the 

 inner side of the opening of the Wolffian duct ; just 

 before reaching the sinus this tube 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 ureters are more easily followed after injecting 

 them with a coloured fluid from the main 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 



