iv PRONEPHROS 235 



The whole mass is at first solid, the cavity of duct, tubule, and 

 pronephric chamber, developing secondarily. 



The cavity of the pronephric chamber is for a time continuous 

 with the split-like splanchnocoele, but it soon becomes constricted 

 off from it and forms a completely closed cavity. Bearing in mind 

 the segmented condition of the pronephric rudiment in its first 

 stage of development and the process of fusion of successive nephro- 

 celes which takes place in Ganoids, we may conclude that the 

 pronephric chamber of the Teleost probably represents a number of 

 nephrocoeles fused together. The single pronephric tubule is very 

 possibly the same member of the series as that which occurs in 

 Amia although this has not yet been actually determined. 



A remarkable peculiarity found within the group Teleostei is that 

 in a few genera {e.g. Fierasfer, Zoarces, Zepadogaster) the pronephros 

 retains its renal function throughout life (cf. Guitel, 1901, 1902). 



Amphibia. — In Amphibians other than Gymnophiona the pro- 

 nephric rudiment first becomes visible as a solid swelling of the 

 somatic mesoderm at the level of the anterior mesoderm segments 

 (Sana segments 2-9, Triton alpestris 1-6, Mollier). Though at 

 first no segmentation is to be detected by the ordinary methods of 

 observation in this swelling it is reasonable to interpret it as 

 representing, morphologically a series of closely apposed or fused 

 nephrotomal projections like those of Hypogeophis. This pronephric 

 rudiment gradually becomes demarcated off from the rest of the 

 mesoderm by a split which becomes apparent on its ventral side at 

 first laterally and then spreads inwards. 



The rudiment now forms a thick flap (cf. Lung-fishes, Fig. 129, 

 A and B) hanging down on the outer side of the mesoderm, and 

 continuous with the somatic mesoderm along its dorsal and median 

 edge. Segmentally arranged coelomic splits make their appearance 

 along the line of attachment of the pronephric flap and these we 

 may interpret as incipient nephrocoeles. The split already mentioned 

 as demarcating the pronephric rudiment ventrally spreads round its 

 median edge, so as to detach it completely from the (nephrotomic) 

 mesoderm except at segmentally arranged points where a connect- 

 ing isthmus remains as the nephrostomal end of a tubule. The 

 pronephric rudiment now undergoes a kind of modelling process 

 similar to that occurring in Crossopterygians and Lung-fishes, its 

 outer portion being gradually cut off from behind forwards so as to 

 form the archinephric duct, while the part nearer the mesial plane 

 forms the recurrent portion of the duct with the tubules connected 

 with it. 



The whole rudiment is at first solid. The earliest coelomic 

 spaces to appear are the nephrocoeles and from these split -like 

 extensions spread outwards in each tubule rudiment, while towards 

 the outer margin of the rudiment the continuous longitudinal 

 cavity of the archinephric duct develops. 



Of the tubule rudiments, as usual, only a few become functional — 



