iv KENAL ORGANS 263 



tubules which rescinl.lc more closely those of the mesonephros and 

 similarly that in tin- region of the mesonephros, in addition to the 

 ordinary tubules, there occur vestiges of another set of tubules 

 resembling more closely those of the pronephros. Consequently, of 

 the set of potential tubules (primary, secondary etc.) which is repeated 

 in each segment, it is not the corresponding jnember which becomes 

 the functional or main tubule in the pronephric and opisthonephric 

 regions respectively. To the present writer the various observations 

 which have been brought to support this argument do not appear to 

 be anything like so convincing as the very clear evidence afforded by 

 Hypogeophis and he consequently holds that in the present state of 

 our knowledge there is no adequate reason to refuse to accept the 

 precise homology of the first-appearing (" primary ") tubules of the 

 opisthonephros with those of the pronephros. 



The idea of the primitive continuity between mesonephros and 

 metanephros is less open to attack than that between the pronephros 

 and the anterior (mesonephric) portion of the opisthonephros. Apart 

 from the evidence of embryology we find in various of the lower 

 vertebrates (Elasmobranchs, Urodeles) an elongated opisthonephros 

 in the adult which shows in the clearest possible manner an incipient 

 stage in the differentiation of the organ, into an anterior genital 

 region and a posterior renal region, of precisely the same kind as we 

 believe to have taken place in the Amniota. 



Further we have seen that in actual ontogeny the tubules of 

 mesonephros and metanephros arise from an at first perfectly con- 

 tinuous mass of nephrotomal mesenchyme. As regards the minor 

 problem whether one or more primary tubules still persist in the 

 metanephros among its immense mass of subsequent tubules there is, 

 as yet, no adequate evidence. 



Accepting then the idea of the archinephros as a sound theory 

 of the primitive condition of the renal system of Vertebrates we 

 may sketch out the probable course of the modifications which have 

 come about in its development somewhat as follows. 



Primitively its tubules developed in accordance with the develop- 

 ment of the body-segments generally in regular sequence from 

 before backwards. 



The disappearance of segmentation in the ventral portion of the 

 coelorne enabled the early-formed tubules those towards the head 

 end to drain the whole length of the splanchnocoele. Correlated 

 with this these tubules became greatly enlarged and their efficiency 

 greatly increased. 



This high development of the anterior tubules to drain the whole 

 splanchnocoele enabled them to cope with the entire excretory 

 needs of the developing animal for a prolonged period and the 

 tubules behind them in the series being unnecessary were either 

 delayed in their appearance or ceased entirely to develop. 



Thus a gap arose separating off the precociously developed 

 tubules as the pronephros. Within the pronephros itself there was 



