Jti tJRlNOGENlTAL ORGANS 573 



more or less entirely replaced functionally by the develop- 

 ment of a postcaval (compare p. 440) ; the anterior part of 

 one or both cardinals may, however, persist as the azygos 

 vein or veins (e.s;. Rabbit, p. 509). 



Urinogenital organs. The excretory organ, speaking of 

 craniate Vertebrates as a whole, consists of three parts, all 

 paired and situated along the dorsal wall of the coelome : 

 the fore-kidney ot pronephros (Fig. 151, h.,p. nph), the riud 

 kidney or mesonepkros (ms. nph) and the hind-kidney or ineta- 

 nephros (int. nph). Each of these is provided with a duct, 

 \hepro- (sg.d), meso- {msn.d), and metanephric (int. n. d) ducts, 

 which open into the cloaca. The gonads (goti) lie in the 

 coelome suspended to its dorsal wall by a fold of peritoneum ; 

 they are developed as ridges covered by coelomic epithelium 

 (compare pp. 194 — 196 and 336). 



The pronephros is nearly always functionless in the adult 

 and often even in the embryo, and usually disappears alto- 

 gether : in the young tadpole it acts as the sole excretory 

 organ for some time. The mesonephros is usually the 

 functional kidney in the lower Craniata, in which as a rule 

 no metanephros is developed (see p. 576), and the meso- 

 nephric duct acts as a ureter, often in addition carrying off 

 the seminal fluid of the male (e.g. frog). In the higher 

 forms the mesonephros is replaced in its excretory function 

 by the metanephros, the metanephric duct being the ureter 

 (e.g. Rabbit). 



The development of the kidney reveals a resemblance to 

 the nephridia of worms which would hardly be suspected 

 from its adult structure. The pronephros (Fig. 151 h, p. 

 nph) originates as two or three coiled tubes formed from 

 mesoderm in the body-wall at the anterior end of the 

 coelome j they are arranged metamerically and each opens 



