xr URINOGENITAL 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 (c.^. 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 ccelonie : 

 the fore-kidney or pronephros (Fig. 151, A, p. nph), the mid 

 kidney or niesonephros (n/s. nph) and the hind-kidney or nieta- 

 nephros {nit. nph). Each of these is provided with a duct, 

 Ihe/w- (sg.d), n/eso- (msn.d), and inetancphric {nit. n. d) ducts, 

 which open into the cloaca. The gonads {gon) 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 niesonephros 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 (('.,;,''. frog). In the higher 

 forms the niesonephros is replaced in its excretory function 

 by the metanephros, the metanephric duct being the ureter 

 (6-.^. Rabbit). 



The development of the kidney reveals a resemblance to 

 the nephridia of worms which would hardly be suspected 

 from its adult structure. 'Vhc pronephros {¥\g. 151 A, />. 

 ''////) originates as two or three coiled tubes formed from 

 iliesoderm in the bod)-wall at the anterior end of the 

 coelome j the}- are arranged metamerically and each opens 



