UROGENITAL SYSTEM. 315 



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 metacoele. As the devel- 

 opment of the embryo proceeds the number of tubules increases by 

 budding in a manner not readily described (fig. 320). These tubules 

 unite with the distal ends of those first formed, so that the distal part 

 of these form collecting tubules. Each of these secondary tubules 

 forms its own Malpighian body and all of the tubules elongate, be- 

 come convoluted, and the mesonephros loses its primitive metameric 

 character. 



FIG. 320. Reconstruction of three somites of the Wolffian body (mesonephros) of 

 Hypogeophis, after Brauer. a, aorta; m l -m 2 , primary and secondary Malpighian bodies; 

 n 1 -n 2 , corresponding nephrostomes; s, tertiary segments of mesonephros; t l -t 2 , primary 

 and secondary mesonephric tubules; w, Wolffian duct. 



At the same time changes are introduced into the mesonephric 

 circulation. The veins emerging from the renal corpuscles extend 

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

 system of capillaries which envelop the tubules before returning the 

 blood to the postcardinal vein. The subcardinal vein (p. 279) 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 details of the modifications of the mesonephric circu- 

 lation see pages 290-292.) 



The Mesonephric Ducts. The conditions in the elasmobranchs 

 have been regarded as very primitive. In them (and to some extent 

 in some of the amphibia), when the mesonephros develops, the pro- 

 nephric duct divides longitudinally from its hinder end as far forward 

 as the anterior end of the Wolffian body. Of the two ducts thus 

 formed (fig. 321, A), one, the Wolffian (Leydig's) duct, remains 

 connected with the tubules of the mesonephros and forms its excretory 

 canal. The other, the Mullerian duct, is similarly related to the 



