342 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



circulation. The veins emerging from the renal corpuscles extend 

 out into the region of the tubules, each breaking up there into a sec- 



I'lci. 305." bLcrcugram of mesonepliros. a, aorta, n, jiusuardinal vein; g, genital 

 ridge; gl, glomerulus; m, mesentery; mc, Malpighian corpuscle; nit, mesonephric tubules; 

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



ond system of capillaries which envelop the tubules before returning 

 the blood to the postcardinal vein. The subcardinal vein (p. 311) 



brings the blood from the caudal region 

 (and usually from the hind limbs) to the 

 Wolffian body and this is also returned 

 via the postcardinals to the heart. (For 

 g. x^ -xis ^^^ details of the modifications of the meso- 

 nephric circulation see pp. 311-327.) 



The Mesonephric Ducts. — The con- 

 ditions in the elasmobranchs have been 

 regarded as very primitive. In them 



Fig. 366. — Reconstruction of , , . c ■> 



a mesonephric tubule of Ambly- (and to some extent m some ot the 

 «?nephrostomf ■ ^' ^^°°'^''''^"'= amphibia), when the mesonephros devel- 

 ops, the pronephric duct divides longi- 

 tudinally from its hinder end as far forward as the anterior end 

 of the Wolfiian body. Of the two ducts thus formed (fig. 368, A), 



