258 STRUCTURE OF THE VERTEBRATES 



B. Development of the Urinogenital System 



The simplest urinogenital system among the vertebrates is 

 found in the cyclostomes. In this class the mesonephros, and the 

 few retained pronephric tubules, are drained by the mesonephric 

 ducts which pass posteriorly and unite to form a urinogenital 

 sinus which has a genital pore on either side. Consequently the 

 coelomic cavity is connected with the outside of the body by the 

 pores which open from the coelom to the sinus, and then to 

 the outside through the urinogenital duct. The reproductive 

 cells as they mature in the gonad break through the closely 

 adherent peritoneum and fall into the coelomic cavity. With the 

 movement of the animal they pass posteriorly, through the geni- 

 tal pores into the sinus, and out into the water with the urinary 

 wastes. Fertilization is external. The system is structurally 

 identical in the two sexes, the difference lying in the type of 

 reproductive cells which are produced; and, although the same 

 animal may produce both sperms and eggs, in most cases these 

 mature at different times and self fertilization does not occur. 



Dogfish. The embryo of the dogfish shows the fundamental 

 vertebrate plan of the urinogenital system. There are two im- 

 portant changes from the system as found in the cyclostomes: 

 (1) the genital pores lose their connection with the sinus, and 

 remain as apparently functionless abdominal pores connecting 

 the coelomic cavity with the cloaca; and (2) the pronephric 

 ducts split longitudinally to form two ducts, (A) the meso- 

 nephric ducts and (B) a new pair of ducts, the Milllerian or 

 oviducts. Each oviduct has an anterior ostium opening into 

 the coelomic cavity, and a posterior outlet into the cloaca. The 

 mesonephric tubules, as in the more primitive condition, drain 

 into the mesonephric duct. This undifferentiated stage is found 

 in the embryos of both sexes. 



As development proceeds the female shows no material change. 

 The Wolffian (mesonephric) ducts continue to drain the kid- 

 nej'S; they fuse posteriorly, and open to the outside through a 

 urinary papilla which projects into the cloaca. The oviducts 

 develop wide ostia in the anterior part of the coelom, and in 

 Squalus these fuse in the adult to form a single opening. As the 



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