IV. VERTEBRATA. 



551 



rise to two canals, the Wolffian (mesonephric), and the Miillerian 

 ducts, the latter retaining its relation to the pronephros. 



The pronephros is usually functional only in embryonic life 

 and then only in early stages, possibly in some cases not at all. 

 Its relations to the other parts are yet in question. In most 



m/(0d) 



FIG. 581. Scheme of urodele urogenital system based on Triton. (From Wieders- 

 heim, after Spengel.) A., male; B, female, a, excretory ducts; gn, sexual part 

 of mesonephros; Ho, testis; Zo, Leydig's duct (ureter i; wgr, Miillerian duct 

 (oviduct) ; mg\ its vestigial end in male ; JV, functional part of mesonephros ; 

 Or, ovary; Ot, ostium tubse; Ve, vasa efferentia ; *, collecting duct of vasa effer- 

 entia (rudimentary in B). 



teleosts the mesonephros is equally developed in nearly the whole 

 length of the body cavity, but in the Amphibia (fig. 581) and 

 many elasmobranchs its anterior part is smaller than the rest, a 

 condition which has its explanation in its relations to the sexual 

 apparatus. 



