264 EMBEYOLOGY OF THE LOWEE VEETEBEATES CH. 



a tendency for functional activity to become specially marked 

 in certain tubules these becoming enlarged in comparison with 

 the others. The increase in size of pronephric ' tubules was 

 accompanied by increase in the size of their glomeruli, which con- 

 sequently came into contact and fused together. 



As the pronephric tubules drained the whole splanchnocoele 

 the peritoneal canal leading to their nephrocoeles became wider and 

 wider until at last they ceased to be marked off from the rest of the 

 splanchnocoele. 



The opisthonephric tubules — the renal functions being still for a 

 time undertaken- by the pronephros — developed in regular sequence 

 from before backwards. With the acquisition of new outlets for 

 fluid in the splanchnocoele, such as abdominal pores, or connexions 

 with lymphatic or blood vessels, the peritoneal canals leading from 

 it into the nephrocoeles (Malpighian bodies), in which the secretion 

 of coelomic fluid was specially concentrated, became gradually 

 reduced and finally disappeared, there being no longer any physio- 

 logical need for them. 



Within the series of opisthonephric tubules, the excretory 

 function became more and more concentrated in the segments nearest 

 the cloacal'" opening. In these segments the opisthonephros 

 increased in bulk owing to the specially active budding processes 

 which gave rise to successive generations of subsequent (secondary, 

 tertiary, quaternary and so on) tubules. 



The final stage in this process was reached in the Birds, where 

 renal activity became concentrated in a single segment close to 

 the cloacal opening. In this segment an immense hypertrophy of 

 the opisthonephric elements took place, successive generations of 

 tubules being added on in front. Thus thei opisthonephric mass 

 belonging to this segment came to extend headwards dorsal to the 

 anterior portion of the opisthonephros (mesonephros) and became 

 the definitive kidney or metanephros. 



Origin of the Nephridial Ducts. — As already pointed out 

 the nephridial tubes in craniate Vertebrates open primitively 

 into a longitudinal archinephric duct — the presence of this duct 

 being the most conspicuous feature which differentiates the renal 

 system in Vertebrates from the presumably ancestral condition as 

 exemplified by Annelids, where the tubules open separately upon 

 the external surface. 



Two possible ways in which this duct may have originated in 

 evolution have already been indicated and it has also been indicated 

 that on the whole the balance of probability seems to be in favour of 

 the view that it came into being through the backward shifting of 

 the external opening of each tubule till it became coincident with 

 the next behind it. 



Those who take this view usually assume that the archinephric 

 duct originally opened posteriorly upon the outer surface of the 

 body and that its opening' became secondarily shifted into the 



