238 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



when the pronephros is already degenerating. They were discovered 

 first by Balfour and Sedgwick (1878) in the Fowl where they vary in 

 number from about 3 to about 7. They may, as so commonly occurs 

 in pronephric glomeruli, undergo a less or greater amount of fusion 

 with one another and also with the anterior glomeruli of the opistho- 

 nephros. The whole pronephros in the Bird undergoes rapid atrophy 

 and by the sixth day of incubation has usually in the Fowl com- 

 pletely disappeared except the glomeruli which may still be detected 

 lor a day or two longer. 



In the Reptiles also a rudimentary pronephros makes its appear- 

 ance but degenerates without becoming functional. The nephro- 

 tomes or protovertebral stalks, at first solid, develop a patent cavity 

 or nephrocoele. In a varying number of segments (in Lizards 6-8, 

 commencing with segment V) pronephric tubule rudiments develop 

 as outgrowths of the somatic wall of the nephrotome after the 

 ordinary fashion and fuse together at their outer ends to form the 

 archinephric duct. 



The Archinephric Duct. — As has already been indicated, it is 

 characteristic of the Vertebrate that its nephridial tubes no longer 

 open directly to the exterior, but that, on the contrary, they open 

 into a longitudinal duct on each side — the archinephric duct — which 

 in turn opens into the alimentary canal towards its hinder end. The 

 first steps in the evolution of the archinephric duct have passed 

 beyond our ken and to decide as to how they came about we have to 

 balance probabilities on a basis of somewhat scanty embryological 

 and anatomical data. Two obvious possibilities present themselves 

 — (1) that the row of segmentally arranged nephridial openings came 

 to be sunk beneath the general surface in a longitudinal groove and 

 that this groove became covered in to form a longitudinal duct, and 

 (2) that the external opening of each tubule became shifted back- 

 wards so as to open into its successor in the series and so give rise 

 first to a common opening with it and later to a common longi- 

 tudinal duct (Fig. 131) in the way exemplified by the posterior 

 kidney collecting tubes of male Elasmobranchs. On which side the 

 balance of probability lies will be apparent on considering the 

 developmental facts so far as they are known to us at present. 



It will be recalled that in Kypogeophis, according to Brauer, the 

 anterior portion of the archinephric duct arises by a number of 

 pronephric tubule rudiments bending tailwards at their outer ends 

 and undergoing fusion together. The fused portion forms the duct 

 rudiment and it proceeds to extend backwards by independent 

 growth until eventually it reaches and fuses with the wall of the 

 cloaca. It is only a small portion of the duct close to its anterior 

 end which is formed by the direct fusion of tubule rudiments — the 

 tubules farther back growing out and fusing secondarily with the 

 already formed duct. 



If we turn to other Vertebrates we find considerable evidence 

 for believing that Eypogeophis presents to us a mode of development 



