280 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



highest placenta! mammals there is no urogenital sinus, but the urethra and the vagina are 

 separate and open separately to the exterior, the former anterior to the latter. In such cases 

 there are three openings in the perineum: the anus, the mouth of the vagina, and the mouth 

 of the urethra. 



For more complete accounts of the comparative anatomy of the urogenital system, the 

 appropriate chapters of K, W, or Wd should be consulted. 



B. THE UROGENITAL SYSTEM OF ELASMOBRANCHS 



The following outline applies to the smooth and spiny dogfish and the skate, 

 the latter being described separately. The majority of the dogfish used for dissec- 

 tion are immature, and it is therefore difficult or impossible to locate in them all 

 of the parts of the urogenital system. At least a few mature males and females 

 will be on hand for demonstration. The skates are sexually mature when still 

 relatively small, and most of the specimens used have fully developed repro- 

 ductive systems. 



Remove the digestive tract except cloaca and liver. 



i. The female urogenital system. 



Dogfish: The ovaries are a pair of soft bodies, situated dorsally. In the 

 spiny dogfish they are oval in form and located dorsal to the liver, each with a 

 mesentery, the mesovarium. In the smooth species they are long and slender, 

 extending the whole length of the coelom, more or less fused, and attached 

 posteriorly to the mesentery of the rectal gland. Their posterior ends are 

 toothed. There is single mesovarium for the two ovaries. In both species 

 when mature, the large eggs, consisting chiefly of yolk, are readily noted in the 

 ovaries. 



Locate the kidneys lying against the dorsal body wall, one to each side of 

 the dorsal aorta. They are long, slender, brown bodies. Their posterior portions 

 are broader and thicker than the anterior portions and probably perform most 

 of the work of excretion. The kidneys are retroperitoneal. Free their lateral 

 borders by slitting the pleuroperitoneum and note thickness of the organ at 

 different levels. The kidneys are mesonephroi; the posterior thicker part may 

 be named the caudal mesonephros, the anterior more slender part the cranial 

 mesonephros. 1 Between the two kidneys is a tough shining ligament which 

 should not be mistaken for a duct. 



The oviducts in immature females are slender tubes running along the ventral 

 face of the kidneys, without mesenteries. In mature females they are very large 

 tubes which spring free from the kidneys by means of well-developed mesenteries, 

 the mesotubaria. Trace the oviducts forward. They pass forward along the 

 dorsal coelomic wall, curve around the anterior border of the liver, and enter 

 the falciform ligament. Here the two oviducts are united to a common opening. 



1 The caudal mesonephros is often regarded as a metanephros and its duct as a ureter. Professor 

 Kingsley has kindly expressed his opinion that this usage is unjustifiable. 



