xii URINOGENITAL ORGANS 591 



veins. The chief somatopleuric veins in all embryonic 

 Craniates are, as in the dogfish, the jugulars and the cardinals, 

 but in all those above the fishes, the cardinals became 

 subsequently more or less entirely replaced functionally by 

 the postcaval (compare p. 456) : the anterior part of one 

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

 or veins (e.g. rabbit, p. 525). 



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 or pronephros (Fig. 162, A, /. npti), the mid- 

 kidney or mesonephros (ins. npJi) and the hind-kidney or meta- 

 nephros (mt. npli). Each of these is provided with a duct 

 \hepro- (sg.d], meso- (msn.d\ and meta-nephric(mt. n.d) duct 

 respectively, opening into the cloaca. The gonads (gon) lie 

 in the ccelome suspended to its dorsal wall by a fold of perito- 

 neum : they are developed as ridges covered by coelomic 

 epithelium (compare pp. 194 196 and 345). 



The pronephros is nearly always functionless in the adult 

 and often even in the embryo, and it usually disappears 

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

 organ for some time. The mesonephros is the functional 

 kidney in the lower Craniata (Fishes and Amphibians), in 

 which no metanephros is developed, and the mesonephric 

 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. 



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. 162 A, p. 

 nph) originates as two or three coiled tubes formed from 



