222 



ESSENTIALS OF ZOOLOGY 



just below the body myotomes 3 to 5. A thickening 

 appears on the outer wall about the period of hatching, 

 and it is converted into a longitudinal groove which differ- 

 entiates into an outer pronephric duct and three tubules 

 which open into the body cavity, each of the three openings 

 being a nephrostome. The pronephric duct grows posteriorly 

 and establishes a communication with the cloaca. The three 

 tubules and the duct joining them become convoluted and 

 they are surrounded by the posterior cardinal vein. The 



aorta also, opposite the pro- 

 nephros, sends out a short 

 pronephric artery which 

 breaks up into a glomerulus 

 distending the peritoneum 

 underneath the nephro- 

 stomes, thus independently 

 of the tubules. It would 

 appear, therefore, that water 

 derived from the glomerulus 

 and coelomic fluid is drawn 

 in by the nephrostomes, and 

 the excretory products are 

 removed by the tubules 

 from the blood of the 

 cardinal vein. 



The mesonephros appears about the end of the larval 

 period. Segmental masses of mesoderm left after the separa- 

 tion of the myocoels from the splanchnocoel posteriorly to 

 the pronephros, in the region therefore previously occupied 

 by the nephrotomes, become hollow and form tubules. The 

 tubules are put into communication with the pronephric duct. 

 Each tubule grows and becomes convoluted, and on its medial 

 side forms a Malpighian body consisting of a Bowman's capsule 

 and an arterial glomerulus, and the tubule extends to form 

 a nephrostomal communication with the splanchnocoel. As 

 the mesonephros increases in size by growth of its tubules 

 and the establishment of accessory tubules, the pronephros 

 degenerates. The degeneration is followed by the atrophy 

 of the anterior tubules of the mesonephros. The remaining 



Nephrostome 

 Glomerulus 

 Proneph. d. 



Mesoneph. 

 tubules 



Cloaca 



FIG. 111. Pronephros and developing 

 mesonephros of tadpole. 



