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 metaccele. 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. 
Fic. 320.—Reconstruction of three somites of the Wolffian body (mesonephros) of 
Hypogeophis, after Brauer. a, aorta; m'—m?, primary and secondary Malpighian bodies; 
n'-n?, corresponding nephrostomes; s, tertiary segments of mesonephros; ¢'~i?, 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 Miillerian duct, is similarly related to the 
