THE MESOXEI'/Ilx'OS WV.y 



the functional organ. I )uring the enil)ryoloo;y of those forms having 

 the metane])hros, the other two types are formed successively and 

 may function tem])()rarily until the metane])hros takes ()V(>r the 

 excretory function. 



THE PRONEPHROS. 



The pronephros consists of short, paired segmental tubules located 

 well toward the head region, the tubules of each side connecting 

 with a longitudinal duct on that side, and this opens posteriorly into 

 the cloaca. Each pronephric tubule has a proximal, ciliated, funnel- 

 like end, the nephrostome, opening into the ctelom. As they 

 develop, the distal ])<)rtion of each tubule bends backward and unites 

 with the distal end of the next tubule to form the common longi- 

 tudinal collecting duct, the pronephric duct, for each sifle. As a 

 result of caudad growth this pair of excretory ducts extend pos- 

 teriorly until they open into the cloaca on its lateral wall. Near 

 the nephrostome, but entirely separated horn the prone])hric tubule, 

 a coiled portion of a small artery covered l)y peritoneum ])rojects 

 into the cielom from the body wall. These simi)le vascular net- 

 works, or glomeruli, filter wastes from the blood into the crelomic 

 fluid. Such wastes, together with cnelomic fluid, are drawn into 

 the pronephric tubules by the ciliary action of the nephrostomes 

 and are then carried down the collecting tubule to the cloaca for 

 expulsion. 



A urinary system of this type is the fimctional kidney of Amphi- 

 oxus and myxinoid cyclostomes. In other cyclostomes it persists, 

 but yields its i)lace as the functional kidney to the newly formed 

 mesonephros a])pearing caudad to it. In the embryological de^'el- 

 opment of other higher forms, the pronephric kidney is formed but 

 degenerates, and its functions are taken over tem])orarily, or per- 

 manently, by the mesonephros. The pronephric ducts, howe\'er, 

 become the main urinary ducts of the mesonephros. 



THE MESONEPHROS. 



The mesonephros, or Wolffian l)ody, forms caudad to the site of 

 the pronephros and is composed of more numerous tubules develop- 

 ing metamerically in pairs. These tubules fuse mesially with the 

 pronephric duct, which is now called the mesonephric or Wolffian 

 duct. With development, the mesonephric tubules become gath- 

 ered into a more or less compact mass, the kidney. In fishes, each 



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