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. 



ar. 



pro. 



OS. 



cw. 



ov. 



FIG. 55. 



Diagrams of the urinogenit.il 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 (Lepidostens and most Teleostei). a.d, 

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

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

 ridge; m, mesentery; m.d, Miillerian duct; in.f, Mullerian fold (developing duct); mn.i; 

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

 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 



