8 4 



KIDNEYS 



Now such a complete tubule is rarely found in the pronephros 

 (Figs. 55, A; 56, A). It occurs, however, in Ganoids, Apoda (Gymno- 



A. 



or. 



prc. 



cw. 



ov. 



FIG. 55. 



Diagrams of the urinogenital organs of the Craniata, seen in transverse sections of the trunk. 

 A, pronephros ; B, mesonephros ; C, young stage in development of male ; D, adult male ; E, 

 female with ' free ' ovary ; F, female with closed ovisac (Lepidosteus and most Teleostei). a.of, 

 archinephric duct (which becomes the mesonephric duct in all except A) ; ao, aorta ; ar, artery 

 to glomerulus ; c, main tubule ; c.o, cavity of ovary closed off; c.w, wall of coelom ; g, genital 

 ridge; m, mesentery; m.d, Mullerian duct; m.f, Miillerian fold (developing duct); ms.c, 

 mesonephric capsule ; ms.d, mesonephric duct ; mx.f, mesonephric funnel ; nt, notochord ; os, 

 ostium of oviduct ; ov, ovary ; p.f, pronephric funnel ; v.c, cardinal vein. 



phiona), and less typically in Myxinoids. As a rule, the pronephric 

 tubules gradually dwindle at the anterior and posterior end of the 

 organ, and even the middle tubules are often incomplete. For 



