COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



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

 tubules (fig. 318), and in some cases (birds, cascilians) pro- and meso- 

 nephric tubules have been described as arising from the same nephro- 

 tome, one above the other. In most ichthyopsida the opening 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. 



FIG. 319. Stereogram of mesonephros. a, aorta; cv, postcardinal vein; g, genital 

 ridge; gl, glomerulus; m, mesentery; me, Malpighian corpuscle; mt, mesonephric tubules; 

 my, myotome; n, nephrostome; nc, notochord; p, peritoneal lining; w, Wolffian duct. 



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. 319). The glomerulus thus 

 formed presses the wall before it, while the rest of the nephrotome 

 closes around it as a Bowman's capsule, the whole forming a Mal- 

 pighian body (in some rodents the glomeruli are rudimentary or absent). 

 In most ichthyopsida the Malpighian body is connected on one side 

 with the metaccele by the nephrostome, and on the other with the 

 mesonephric tubule. 



