IV 



ARCHINEPHRIC DUCT 



241 



2nd. 



above but making full allowance for this it seems impossible to escape 

 the admission that in many forms (Petromyzon, Lung-fishes, most 

 Amphibians, Teleosts and probably actinopterygian Ganoids) the duct 

 is prolonged backwards by a process of this kind. 



It being accepted that in a number of Anamnia a large part of 

 the archinephric duct arises in development by being split off from the 

 mesoderm, we are faced by the problem how this mode of develop- 

 ment is to be correlated with the mode of development by fusion of 

 the outer ends of tubule rudi- 

 ments. It may be suggested 

 that what has happened is that 

 the development has been 

 accelerated — as often happens 

 — by skipping over the early 



stages. The mode of develop- ■>, my- 



ment in question may have 

 been derived from the more 

 primitive mode by the omission 

 of the separate tubule stage 

 and the passage at once to the 

 stage in which the tubule 

 ends are fused into, a continu- 

 ous structure. 



In some cases however the 

 primitive mode of development 

 has undergone a further modi- 

 fication. This is exemplified 



by PolypterUS (Graham Kerr, PlQ- 132 ._ Transverse section through Polypterus 

 1907) where the hinder portion of stage 23 at level of cloacal opening. 



of the duct appears to be 



formed by bodily conversion 



of the series of nephrotomes. 



These are not segmented but 



form a continuous structure which becomes converted directly into 



the archinephric duct. 



In whichever way the archinephric duct completes its extension 

 backwards, it eventually comes to open into the cloaca. This is, in 

 the great majority of Vertebrates, described as coming about by 

 fusion of the previously freely-ending tip of the duct with the 

 cloacal wall. It is obvious that such a process cannot correspond 

 with what happened during evolution as the duct must have had 

 its posterior aperture throughout in order to perform its function. 



It is possible that a clue to the evolutionary origin of the com- 

 munication between archinephric duct and alimentary canal is given 

 by Polypterus. It has already been mentioned that in this animal 

 the hinder part of the archinephric duct arises by bodily conversion 

 of the row of fused nephrotomes. Fig. 132 shows that the opening 

 of archinephric duct into the alimentary canal presents a striking 



VOL. II R 



a.n.d, opening of archinephric duct into cloaca; cl, 

 opening of cloaca to exterior ; end, alimentary canal 

 rudiment ; my, myotome ; N, notochord ; s.e, spinal 

 cord. 



