THE EXCRETORY ORGANS 



341 



the other hand, in crossopterygian ganoids, turtles and crocodiles the last 

 pronephric and the first mesonephric tubules arise from the same somite. 



Segmental arteries grow out from the aorta to the splanchnic 

 wall of each nephrotome, forming there a network of capillaries at a 

 higher level than the pronephric glomeruli (fig. 365). The glom- 

 erulus thus formed presses the wall before it, while the rest of the 

 nephrotome closes around it as a Bowman's capsule, the whole form- 

 ing a !Malpighian body (in some rodents the glomeruli are rudimen- 

 tary or absent). In most ichthyopsida the Malpighian body is 

 connected on one side with the metacoele by the nephrostome, and 

 on the other with the mesonephric tubule. 



Fic. 364. — Stereogram of developing pro- and mesonephros. a, aorta; g, glomus or 

 glomerulus; m, mesentery; mt, mesonephric tubule; n, notochord; nc, cavity of {rU) 

 nephrotome; ns, nephrostome; pc, postcardinal vein; pd, pronephric duct; pt, pronephric 

 tubule; pint, peritoneal membrane. 



Thus at first the mesonephros is a metameric structure, extending 

 over a much larger number of somites than does the pronephros and 

 reaching nearly to the posterior limits of the metaccele. As the 

 development of the embryo proceeds, the number of tubules, in all 

 vertebrates except the myxinoids, increases by budding in a manner 

 not readily described (fig. 367). These tubules unite with those 

 first formed, so that the distal part of these become collecting tubules. 

 Each of these secondary tubules forms its own Malpighian body and 

 all of the tubules elongate, become convoluted (fig. 366) and the 

 mesonephros loses its primitive metameric character. 



At the same time changes are introduced into the mesonephric 



