EXCRETORY ORGANS. 



703 



in the adult, which probably implies the atrophy of the prone- 

 phros ; in other instances the cephalic portion of the kidneys is 

 the only part developed. Its relation to the embryonic pronc- 

 phros requires however further elucidation. 



In the adult the ducts in the lower part of the 'kidneys lie as 

 a rule on their outer borders, and almost invariably open into a 



pr n 



FIG. 395- SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT 

 PARTS TEN DAYS BEFORE HATCHING. 



pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body 

 cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; jr. 

 subnotochordal rod ; al. alimentary tract. 



urinary bladder, which usually opens in its turn on the urino- 

 genital 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 con- 

 tinuous 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 

 Mullerian ducts of Elasmobranchii, 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 



