EXCRETORY ORGANS. 



579 



of the pronephros (fig. 395, ^o) soon becomes completely isolated from 

 the remainder, so as to form a closed cavity {gl). The development 



/^ 



pr n 



Fig. 395. Sectiox thbough the pronephros of a Trout and adjacent parts 

 ten days before hatching. 



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

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

 X. subuotoehordal rod; al. alimentary tract. 



of the mesouephros does not take place till long after that of the pro- 

 nephros. The segmental tubes which form it are stated by Furbringer 

 to arise from solid ingrowths of peritoneal epithelium, developed 

 successively from before backwards, but Sedgwick informs me that 

 they arise as differentiations of the mesoblastic cells near the peri- 

 toneal epithelium. They soon become hollow, and unite with the 

 segmental duct. Malpighian bodies are developed on their median 

 portions. They grow very greatly in length, and become much con- 

 voluted, but the details of this process have not been followed out. 



The foremost segmental tubes are situated close behind the pro- 

 nephros, while the hindermost are in many cases developed in the 

 postanal continuations of the body cavity. The pronephros appears 

 to form the swollen cephalic portion of the kidney of the adult, and 

 the mesonephros the remainder ; the so-called caudal portion, where 

 present, being derived (?) from the postanal segmental tubes. 



In some cases the cephalic portion of the kidneys is absent in the 

 adult, which probably implies the atrophy of the pronephros ; in other 

 instances the cephalic portion of the kidneys is the only part de- 

 veloped. Its relation to the embryonic pronephros 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 

 urinary bladder, which usually opens in its turn on the urinogenital 



37—2 



