IV 



PKONEPHEOS 



237 



pn\. 



lumen. The pronephric swelling extends backwards into the archi- 

 nephric duct. No glomerulus develops but segmentally arranged 

 branches of the dorsal aorta appear on the right side corresponding 

 in number and degree of development with the pronephric tubules. 

 These give rise either one of them (Riickert, van Wijhe) or by fusion 

 together (Eabl) to the root of the vitelline artery but are termed by 

 Rabl pronephric arteries. 



The pronephros undergoes rapid degeneration and eventually 

 nothing is left of it but the coelomic funnel of the Mlillerian duct 

 (see below). 



SAUROPSIDA. In the fowl pronephric tubule rudiments develop 

 to the number of about 12, in the 

 form of solid outgrowths of the somatic 

 mesoderm at the level of the nephro- 

 tomes although, except in the case of 

 the most anterior, the mesoderm is not 

 yet segmented at this level at the time 

 when the rudiments appear. In at least 

 some cases segmental dilatations of the 

 otherwise split-like coelome occur op- 

 posite the tubule rudiments and are no 

 doubt to be interpreted as the nephro- 

 coeles of the corresponding tubules. 

 The first tubule rudiment makes its 

 appearance in embryos with 8 or 9 

 segments, its position showing consider- 

 able variation (usually segment 4, 5 

 or 6). 



The successive tubules appear in 

 rapid succession-almost synchronously. 



At about the 19 -Segment Stage the 



inyotomes become separated from the 



nephrotomes, the latter remaining in 



continuity with the lateral mesoderm 



and their cavities (nephrocoeles) with 



the splanchnocoele. About the same stage the backwardly projecting 



tip of each tubule rudiment undergoes fusion with its successor in 



the series and thus gives rise to a continuous longitudinal rod-like 



structure the rudiment of the archinephric duct (Felix, 1904). 



The archinephric duct in its anterior portion thus would appear to 



develop in a manner essentially the same as that found in the 



Gymnophiona and so many other of the lower vertebrates. 



The hinder end of each tubule rudiment, as well as the archi- 

 nephric duct itself, is at first solid. The definitive lumen makes its 

 appearance (about 20-segment stage) secondarily in the form of dis- 

 continuous chinks which gradually become continuous and spread 

 backwards. 



Pronephric gloineruli develop in the Bird though at a late stage 



Torpedo. (After Riickert, 1888.) 



a .n.d, archinephric duct; &., biood- 

 pn.i, etc., pronephric tubules. 



" lth 



