COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 283 



As explained in the introduction the male ducts in the majority of vertebrates 

 are the mesonephric or Wolffian ducts. In mature males these ducts are con- 

 sequently much larger than in females. The Wolffian ducts run posteriorly 

 along the ventral face of the kidneys. In immature specimens each is a slender, 

 straight tube, similar to that of the female, but in mature males, it is greatly 

 coiled. The testis is connected with the cranial mesonephros by means of 

 delicate ducts, the vasa ejferentia, which run in the mesorchium and can sometimes 

 be seen by holding the mesorchium up to the light. The vasa efferentia connect 

 with the tubules of the mesonephros. The greater part of the cranial meso- 

 nephros apparently serves in male elasmobranchs for transmitting the sperm, 

 and is sometimes called the epididymis, since it corresponds to the head of the 

 epididymis of mammals. The sperm-bearing tubules of the mesonephros then 

 connect with the Wolffian duct. Trace this duct posteriorly. Its anterior part 

 is greatly coiled in mature males, but in dogfishes (not in skate) straightens as 

 it approaches the caudal mesonephros and in all three animals enlarges upon the 

 surface of the latter to form the seminal vesicle. Trace it by removing the 

 pleuroperitoneum from the ventral face of the caudal mesonephros. At its 

 posterior ends on the sides of the cloaca, the seminal vesicle terminates in a sac, 

 the sperm sac, which projects forward as a blind sac lying against the ventral 

 surface of the seminal vesicle. 



Cut open the cloaca as directed under the female and identify its parts as 

 directed there. There is no difference in the cloaca of the dogfish between the 

 males and females, but in the male skate the cloaca is very much smaller than 

 in the female and is not divided into intestinal and urogenital parts. In the 

 median dorsal line there is in the male skate a urogenital papilla. 



The sperm sacs should now be cut open and the papillae, where the seminal 

 vesicles open into them, identified. The two sperm sacs unite at their posterior 

 ends to form a urogenital sinus which opens at the tip of the urinary papilla. 



The accessory mesonephric ducts are similar to those of the females; see the 

 description under females. In male dogfishes each runs along the medial side 

 of the seminal vesicle and enters the sperm sac near the opening of the vesicle. 

 In the male skate several accessory ducts pass from the caudal mesonephros 

 into the sperm sac. 



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



Look in male dogfishes for remnants of the ostium and oviducts in the region 

 of the liver. 



C. THE UROGENITAL SYSTEM OF NECTURUS 



1. The female urogenital system. — The ovaries have already been noted as 

 elongated saclike bodies bearing eggs of various sizes. Note the mesovarium. 

 Lateral to each ovary running along the dorsal body wall is the oviduct, a thick, 

 white, coiled tube supported by the mesotubarium. Follow it anteriorly. 



