86 



KIDNEYS 



duct or oviduct (p. 90). The pronephric duct, however, persists, 

 although it may become much modified. 



There is no fundamental distinction between the pro- and the 

 mesonephros ; in the Myxinoids [335] and Gymnophiona [45] 

 the transition from one to the other is gradual. Such differences 

 as are found in the development appear to be chiefly due to the 



d. 



FIG. 50. 



Diagrams of the urinogenital system in the Craniata. A, hypothetical ancestral stage with 

 continuous archinephros. B, Cyclostome with anterior pronephros. C, female Gnathostome 

 (adult). D, male Gnathostome (adult), a.d, archinephric duct ; -ctr.t, anterior vestigial tubule ; 

 a.J, archinephric tubule ; c, Malpighian capsule ; cl, cloaca ; l.c, longitudinal canal ; m.d, 

 Miillerian duct; ms.d, mesonephric duct ; </, mesonephric funnel ; o.f, coelomic funnel ; ov, 

 ovary; pf, coelomostome (funnel); pr.d, pronephric duct; pr.f, pronephric funnel; pr.t, 

 posterior vestigial tubule ; r, vestigial network of vasa efferentia ; s.f, secondary funnel ; t?, 

 testis ; t.t, tertiary tubule ; v.e, vas efferens. The vestigial oviduct and the embryonic 

 pronephros are represented by dotted lines in C and D. 



fact that, as the mesonephros arises later, the mesoblastic somites 

 are by that time more completely differentiated. 



The mesonephric tubule, like the pronephric, arises from the 

 mesoblastic stalk (nephrotome, intermediate cell-mass, Urseg- 

 mentstiel) connecting the somite with the lateral mesoblastic 

 plate. These rudiments generally become completely separated 

 off from the somite, and sometimes also from the lateral plate, at an 



