340 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



cut off from the ccelom, so that the whole resembles a renal corpuscle, 

 but is different in origin. In either case the exuding fluid passes into 

 the metaccele, from which it is drawn by the cilia of the nephrostomes 

 and passed into the tubules. 



The blood is brought to the glomus or glomeruli by short seg- 

 mental arteries arising from the dorsal aorta (fig. 364) and, after pass- 

 through the capillaries, it is carried away by the postcardinal veins 

 of the corresponding side to the heart, these veins keeping pace in 

 their backward development with the development of the nephridial 

 tubules. 



There is much that goes to show that the pronephros formerly had a 

 much greater extension than at present, including a larger number of somites. 



It has, however, been replaced in the adults of all 

 vertebrates (with the possible exception of Bdello- 

 stoma) by the mesonephros, and later, in the amniotes, 

 by the metanephros as described below. It persists, 

 however, with slight modification in the cyclostomes 

 and a few teleosts. " 



Mesonephros. — The mesonephros or 

 Wolfiian body is the second excretory organ 

 to arise. It arises after the pronephros, and 

 its ducts are formed, by the development_of 

 a series of mesonephric tubules which grow 

 out from the nephrotomes behind those con- 

 cerned in the formation of the pronephros. 

 These tubules extend laterally until they 

 meet and fuse with the pronephric duct, 

 which now acts as the excretory canal of the 

 new gland. In some cases the point of origin 

 of the mesonephric tubules is clearly dorsal to that of the pronephric 

 tubules (fig. 364), and in some cases (birds, cascilians) pro- and 

 mesonephric tubules have been described as arising from the same 

 nephrotome, one above the other. In most ichthyopsida the open- 

 ing of the nephrotome into the metaccele forms the nephrostome, 

 but in the amniotes this opening is closed before the tubules are 

 formed and consequently nephrostomes are lacking in the latter 

 group in all stages of development, and the tubules have no connexion 

 with the peritoneal cavity. 



Usually several metameres intervene between the last pronephric and the 

 first mesonephric tubule, there being seventeen such metameres in Amia. On 



Fig. 363. — Reconstruc- 

 tion of the pronephros of 

 Salamandra, after Seraon. 

 c, aorta; c, connexion of 

 nephrostomal with the main 

 ccelom; d, pronephric duct; 

 g, glomus; I, 2, 3, nephro- 

 stomes of pronephric tu- 

 bules. 



