COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 281 



the ostium. This is a wide, funnel-shaped opening lying in the falciform liga- 

 ment, the opening directed posteriorly. To find the opening it is generally 

 necessary to separate the walls of the ostium, as these tend to adhere. Trace 

 the oviducts posteriorly. They are narrow tubes at first, soon presenting in 

 mature specimens a slight enlargement, the shell gland. Posterior to this they 

 narrow again and then in mature females enlarge greatly to form the uterus, 

 which swings free by means of the mesotubarium. In immature females there 

 is no such enlargement, nor is there any mesotubarium, but the oviducts widen 

 slightly as they proceed posteriorly along the ventral faces of the kidneys. 



Trace the oviducts to the cloaca. Cut open the cloaca by a median ventral 

 slit which opens up the intestine. Note the opening of the intestine into the 

 ventral part of the cloaca and the slight fold which separates this from the dorsal 

 urogenital region of the cloaca. Note the urinary papilla in the median dorsal 

 wall of the cloaca; it is a conical papilla in the spiny dogfish, a low elongated 

 ridge in the smooth species. In mature specimens the large openings of the 

 oviducts are readily seen to each side of the urinary papilla. In immature 

 specimens they are in the same position but quite small. To find them it is 

 best to cut into the posterior ends of the oviducts and probe toward the cloaca. 



The ducts of the kidneys are more difficult to locate in females. The cranial 

 mesonephros is provided with a Wolffian duct. This lies along the ventral face 

 of the kidney exactly dorsal to the oviduct in immature specimens, or in mature 

 ones along the fine of attachment of the mesotubarium. Locate it in immature 

 specimens by carefully stripping off the oviduct and also freeing the pleuroperi- 

 toneum from the ventral face of the kidney. The Wolffian duct is a slender 

 duct proceeding posteriorly to the cloaca. The caudal mesonephros (posterior 

 third) is also provided with a duct, the accessory mesonephric duct. In the 

 smooth dogfish this lies alongside. of the Wolffian duct and has probably been 

 noted already. In the spiny species it is more deeply located imbedded in the 

 kidney tissue; it will be found by dissecting in this tissue near the median fine. 

 The accessory ducts are delicate tubes receiving ducts from the kidney at intervals. 

 Both Wolffian ducts and accessory ducts proceed to the cloaca lying in contact 

 with and on the dorsal surface of the oviducts. To follow them cut along one 

 side of the cloaca, separating the cloaca from the body wall. They all terminate 

 by a single median pore in the urinary papilla. 



Skate: The ovaries are a pair of elongated soft bodies containing large yellow 

 eggs; they are situated dorsally in the anterior half of the pleuroperitoneal 

 cavity. The large oviducts pass dorsal to them. Follow one oviduct forward. 

 Its narrow anterior portion passes along the dorsal coelomic wall, curves around 

 the anterior margin of the liver, and passing into the falciform ligament unites 

 with its fellow to a single common opening or ostium. The ostium is a wide 

 funnel-like aperture situated in the ligament and facing posteriorly. Trace the 

 oviducts caudad. After a short distance they widen greatly to a uterus, the 



