702 



TELEOSTEI. 



funnel there is formed a glomerulus, projecting into the body 



cavity ; and at the same time that 



this is being formed the anterior end 



of the canal becomes elongated and 



convoluted. The above structures 



constitute a pronephros, while the 



posterior part of the primitive canal 



forms the segmental duct. 



The portion of the body cavity 

 with the glomerulus and peritoneal 

 funnel of the pronephros (fig. 395, 

 po) soon becomes completely iso- 

 lated from the remainder, so as to 

 form a closed cavity (gl). The 

 development of the mesonephros 

 does not take place till long after 

 that of the pronephros. The seg- 

 mental tubes which form it are 

 stated by Fiirbringer to arise from 

 solid ingrowths of peritoneal epi- 

 thelium, developed successively from 

 before backwards, but Sedgwick 

 informs me that they arise as dif- 

 ferentiations of the mesoblastic cells 

 near the peritoneal 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 convoluted, but the details of this 

 process have not been followed out. 



The foremost segmental tubes are situated close behind the 

 pronephros, while the hindermost are in many cases developed 

 in the post-anal continuations of the body cavity. The prone- 

 phros 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 post- 

 anal segmental tubes. 



In some cases the cephalic portion of the kidneys is absent 



FIG. 394. PORTIONS OF THE 

 MESONEPHROS OF MYXINE. (From 

 Gegenbaur; after J. Miiller.) 



a. segmental duct ; b. segmen- 

 tal tube; c. glomerulus ; d. afferent, 

 e. efferent artery. 



B represents a portion of A 

 highly magnified. 



