THE UROGENITAL SYSTEM 23 



that it enlarges on the surface of the caudal mesonephros, 

 forming the seminal vesicle (best developed in adult speci- 

 mens). Remove the pleuroperitoneum from the ventral 

 surface of the caudal mesonephros and find that at the 

 posterior end and at the sides of the cloaca that the seminal 

 vesicle opens into the lower end of the sperm sac. These 

 lead into the urogenital sinus by a common opening. 



Split open the cloaca (in the male it is much smaller 

 than in the female) ventrally and notice on the dorsal 

 wall the projecting urogenital papilla. The urogenital 

 sinus leads into the cloaca at the tip of the urinary papilla. 

 Run a fine probe into the opening of the sinus and trace 

 the entrance of the sac and seminal vesicle. The sperm 

 cells pass out through the cloacal aperture and along the 

 grooved lateral margins of the claspers. 



The accessory mesonephric ducts are similar to those 

 of the female described above and are located by following 

 the same procedure. In the male skate several of these 

 accessory ducts will be found coming from the median side 

 of the mesonephros and going into the sperm sac. 



Drawing 12. Draw the male urogenital system. 



