88 KIDNEYS 



and so to develop at different times ; or they represent three 

 organs ; or again three separate longitudinal series of excretory 

 tubules one above the other, homodynamous, but not strictly 

 homologous. That the first view is, at all events in a general 

 sense, correct can hardly be doubted now that we know that there is 

 no essential difference between the pronephric and the mesonephric 

 tubules (Sedgwick [296], Brauer [45], Price [335], Kerens [258]). 

 When the pronephros and mesonephros appear to overlap, as seems 

 to be the case in birds, for instance (Felix), the 'mesonephric' 

 tubules of the pronephric segments are probably comparable to the 

 secondary generations of tubules further back. It also seems 

 clear that the metanephros represents merely the specialised and 

 retarded hinder end of the series (Semper [404], Balfour [29], 

 Sedgwick, Schreiner [390&]). The chief difficulty lies in the 

 relations of the ducts. 



We may assume, perhaps, with Kiickert [376] that there was 

 originally a metameric series of coelomostomes opening independently 

 to the exterior (p. 27). Subsequently these fused at their distal 

 ends to form a longitudinal duct opening behind. At this stage, 

 then, the ancestral Craniate possessed an archinephros (Lankester), 

 consisting of a continuous series of archinephric tubules opening 

 into a single archinephric duct on each side (Fig. 56, A). No living 

 Craniate presents such a structure, but obvious traces of it are seen 

 in the Cyclostomes (Bdellostoma, Price [335]), where a continuous 

 series of metam.eric tubule rudiments is formed, some of which 

 become the pronephros and others the mesonepliros. Some tubules 

 disappear between the pro- and mesonephros ; others disappear at 

 the hinder end of the series. Here, in the Myxinoids, the metameric 

 order is preserved in the abdominal region, no secondary tubules 

 are formed, and the single duct persists. 



In the Craniata, however, not only may several series of tubules 

 be produced in the abdominal region (meso- and metanephros), but 

 these only secondarily come into connection with the already 

 completely developed longitudinal duct of the pronephros. If, 

 as seems certain, this duct was originally formed by the fusion all 

 along its course of archinephric rudiments, it would appear that the 

 process of formation of the hinder region of the duct by the free 

 growth backwards of the anterior portion is due to secondary 

 modification, correlated with the late development of the hinder 

 tubules. The pronephric tubules, together with the first series 

 of mesonephric tubules (metamerically arranged and provided with 

 coelomic funnels), would thus represent the original archinephric 

 series. Possibly this series has been suppressed in the metanephric 

 region. The limit between the pro- and mesonephros is not fixed, 

 and differs considerably in the various classes ; it is quite indefinite 

 and has doubtless been gradually established. The front end of 



