282 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



beginning of which bears a conspicuous bilobed swelling, the oviducal gland : 

 which secretes the horny case in which the eggs are laid (N, p. 164, Fig. 96). 

 The uteri proceed to the cloaca supported by the thickened mesotubaria. Cut 

 open the cloaca in the median ventral line, slitting open the intestine. Note 

 the opening of the intestine into the ventral part of the cloaca and the conspicuous 

 horizontal fold which separates this from the dorsal urogenital part. Cut into 

 the latter by cutting forward through this fold. The cloaca is greatly extended 

 and thickened in the anterior direction. Find the openings of the oviducts, 

 one to each side of this thickened part of the cloaca. Halfway between the 

 openings of the oviducts is the urogenital opening in the median dorsal wall. 



The main part of the kidneys in female skates consists of a thick rounded 

 lobe lying against the dorsal wall at each side of the cloaca. These lobes are 

 revealed by stripping off the thick pleuroperitoneum which covers their ventral 

 surfaces. These lobes may be named the caudal mesonephros. The anterior 

 part of the mesonephros or cranial mesonephros is nearly degenerate in females 

 but will be found as diffuse brownish tissue extending forward ventral to the 

 dorsal aorta. From the median surface of the caudal mesonephros several ducts, 

 the accessory mesonephric ducts, pass anteriorly and medially in contact with 

 the posterior cardinal vein and open into a small chamber, the urinary sinus, 

 situated on the dorsal surface of the anterior end of the cloaca. The two urinary 

 sinuses of the two sides unite into a common chamber, which is sometimes called 

 the urinary bladder. It does not correspond to the bladder of higher forms, 

 since it consists of the enlarged terminations of the mesonephric ducts. Cut 

 into this, note the entrance into it of the two urinary sinuses, and find the opening 

 in its mid-dorsal wall by which it opens into the cloaca. The Wolffian ducts or 

 ducts of the cranial mesonephros are slender tubes extending anteriorly from the 

 urinary bladder, lying on the dorsal surface of the strong white portions of the 

 mesotubaria. 



Draw, showing kidneys, gonads, and their ducts, and the opened cloaca. 



2. The male urogenital system. The testes are a pair of soft bodies dorsally 

 situated. In the spiny dogfish they are located in the anterior part of the 

 pleuroperitoneal cavity, dorsal to the liver; each has a mesorchium. In 

 the smooth dogfish they are long and slender bodies extending the length of the 

 pleuroperitoneal cavity, their toothed posterior ends attached to the mesentery 

 of the rectal gland. They are more or less fused and are supported by a single 

 mesorchium. In the skate the testes are broad, flat bodies against the dorsal 

 wall; each is provided with a mesorchium. 



The kidneys are identical with those of the females and should be next 

 examined according to the directions given under females. In the male skate, 

 however, the cranial part of the kidney is very much better developed than in 

 the female and extends forward as a firm cylindrical body on either side of the 

 mid-dorsal line. 



