580 



(UNO I DEI. 



papilla immediately behind the genital pore, but in a few instances 

 there is a common urinogenital pore. 



In most Osseous Fish there are true generative ducts continuous 

 with the investment of the generative organs. It appears to me 

 most probable, from the analogy of Lepidosteus, to be described in 

 the next section, that these ducts are split off from the primitive 

 segmental duct, and correspond with the Miillerian ducts of Elasmo- 

 branchii, etc. ; though on this point we have at present no positive 

 embryological evidence {vide general considerations at the end of the 

 Chapter). In the female Salmon and the male and female Eel the 

 generative products are carried to the exterior by abdominal pores. 

 It is possible that this may represent a primitive condition, though it 

 is more probably a case of degeneration, as is indicated by the pre- 

 sence of ducts in the male Salmon and in forms nearly allied to the 

 Salraonidse. 



The coexistence of abdominal pores and generative ducts in 

 Mormyrus appears to me to demonstrate that the generative ducts in 

 Teleostei cannot be derived from the coalescence of the investment of 

 the generative organs with the abdominal pores. 



Ganoidei. The true excretory gland of the adult Ganoidei resem- 

 bles on the whole that of Tele- 

 ostei, consisting of an elongated 

 band on each side — the meso- 

 nephros — an anterior dilatation 

 of which probably represents the 

 pronephros. 



There is in both sexes a 

 Miillerian duct, provided, except 

 in Lepidosteus, with an abdomi- 

 nal funnel, which is however 

 situated relatively very far back 

 in the abdominal cavity. The 

 Miillerian ducts appear to serve 

 as generative canals in both sexes. 

 In Lepidosteus they are contin- 

 uous with the investment of the 

 generative glands, and thus a re- 

 lation between the generative ducts 

 and glands, very similar to that in Teleostei, is brought about. 



Posteriorly the Miillerian ducts and the ducts of the mesonephros 

 remain united. The common duct so formed on each side is clearly 

 the primitive segmental duct. It receives the secretion of a certain 

 number of the posterior mesonephric tubules, and usually unites 

 with its fellow to form a kind of bladder, opening by a single pore 

 into the cloaca, behind the anus. The duct which receives the 

 secretion of the anterior mesonephric tubules is the true mesonephric 

 or Wolffian duct. 



The development of the excretory system, which has been 



Fio. 396. Section through the trunk 

 OF A Lepidosteus embryo on the sixth 



DAT AFTER IMPREGNATION. 



mc. medullary cord; ms. mesoblast; sfj, 

 segmental duct; cli. notochord; x. sub- 

 notochordal rod ; hy. hypoblast. 



