268 



PRINCIPLES OF EMBRYOLOGY 



he concludes that that organ is quite unnecessary for the production of the 

 mesonephros. However, it is not at all clear that even in the Amphibia 

 the original production of the rudiment of the duct is quite independent 

 of the presence of the pronephros, and in birds the two components are 

 so intimately associated that it is difficult to imagine the duct being pro- 

 duced without at least some transitory appearance of the tubules of the 





1 



W.D 



M. 



Figure 12.8 



Figure 1 shows, on the left a diagram of the normal urogenital system, com- 

 prising the adrenal {A), gonad (G), mesonephros (M), MuUerian duct {MD), 

 WohFian (or mesonephric) duct {WD), ureter {U) and definitive kidney 

 (metanephros) (iC). The parts which develop independently of others are 

 shown in solid black, those resulting from inductive actions in outline. On 

 the right is the result of preventing the mesonephric duct from growing 

 posteriorly beyond the arrow; the posterior mesonephros and other organs 

 fail to appear. (From Gruenwald 1952.) Figure 2 is a section through a chick 

 embryo in which the posterior growth of the pronephric duct was prevented 

 on the right side. There is not only no sign of the duct [WD] but the meso- 

 nephros {M) has also failed. (From Waddington 1952^.) 



nephros. There seems, therefore, no good reason to doubt that we have, 

 in the appearance of the pronephros in birds, an example of the retention 

 during evolution of an organ for the sake of its function as a component 

 of the epigenetic system, rather than for any contribution it makes to the 

 physiological functioning of the embryo as a metabohsing organism. 



Although one may probably conclude that the pronephric duct is an 

 important factor in the epigenetic system, it is by no means the only 

 actor on the stage. The competence of the intermediate mesoderm plays 



