EXCRETORY SYSTEM 



Gi-' 



is impossible to separate one part rigidly from another. A pro- 

 nephros, in the sense of an anterior set of tubules which can be dis- 

 tinguished from the rest, is found only in those vertebrates with 

 a larval stage, namely. Dipnoi, Crossopterygii, Actinopterygii and 

 Amphibia. In all these it is the larval excretory organ, developed 

 early in life ; it may be seen in a careful dissection of the tadpole 

 (Fig. 474). It generally atrophies, but is functional in the adults 

 of a few teleosts (e.g. Fierasfer). In amniotes it is represented, 

 if at all, only by a few anterior rudimentary tubules, but its 

 duct persists in the female of all vertebrates as the oviduct. 



7nP3 



A. 



B 



Fig. 474. — Diagrams of the development of the excretory system of the frog.— 



From Bourne. 



A The system of a tadpole about 12 mm. long, showing the pronephros and origin of the mesonephric 

 ' tubules ; B, the system at the end of metamorphosis. The broken line represents approximately the 



position'of the strip of peritoneal epithelium which gives rise to the oviduct. 

 cl., Cloaca ; d.ao., dorsal aorta ; f.b., fat body ; gl., glomus ; g.r., genital ridge ; »u-s., mesoncphros : mst., 



mesonephric tubules; od., oviduct; ovf., position of oviducal openmg ;^n./., pronephnc funnels; 



pnp., pronephros ; sg., segmental duct (the hne points to the part which becomes the Wolffian duct). 



Behind the region of the pronephros the nephrogenic part of 

 the mesoderm is often condensed or not segmented, but the 

 general development of the tubules is basically the same, except 

 that it is common for secondary and tertiary tubules to bud off 

 from the primary ones. This part of the kidney, the mesonephros, 

 is the functional kidney of nearly all adult anamniotes, including 

 the elasmobranchs. Its duct, the mesonephric or Wolffian duct, 

 is formed in the same way as the pronephric or holonophric 

 duct. It is also active in the embryos of reptiles, birds and 

 mammals, and in reptiles, monotremes and marsupials for a short 

 time after birth. Two related tendencies are to be noted m it : 

 towards separation into anterior and posterior portions, and 

 towards association with the gonads. In all gnathostomes except 



