Mr Kerr, The Genito-urinary Organs of Dipnoan Fishes. 329 



The Genito-urinary Organs of Dipnoan Fishes. By J. Graham 

 Kerr, M.A., Christ's College. 



[Read 20 January 1902.] 



Lepidosiren. 



Pronephros. 



The pronephros forms the functional kidney of Lepidosiren up 

 to about stage 35. 



The archinephric duct arises as a solid flattened rod which splits 

 off from the mesoblast just at the junction of myotome and lateral 

 plate. The splitting-off process gradually spreads backwards, and 

 the archinephric rudiment is separated completely from the under- 

 lying mesoblast except anteriorly, where through a region ex- 

 tending through two myotomes it remains continuous with the 

 main mass of mesoblast along its inner edge. This attached 

 region is the rudiment of the pronephros. The archinephric 

 rudiment lies close under the epiblast from which however it is 

 sharply marked off by its coarsely-yolked character. At its 

 posterior end it passes directly into mesoblast. At no time is 

 it continuous with the epiblast. 



The mesoblastic structures are at first completely solid. About 

 stage 23 the first traces of coelomic cavity appear as a split 

 traversing the region of the nephrotome, extending up slightly 

 into the myotome, and outwards slightly into the region of the 

 perivisceral cavity. The coelomic cavity so arising rapidly spreads 

 up into the pronephric rudiment as the cavity of the tubule. 

 There are on each side two nephrostomata as in Urodelan 

 Amphibians. Round the cavity of each nephrostome the mass of 

 mesoderm becomes as it were carved out to form the wall of the 

 funnel. From the nephrostome the cavity gradually extends 

 backwards along the rudiment of the archinephric duct. The 

 coelomic split opposite the nephrostomes widens out to form a 

 considerable pronephric chamber, and in stage 24 the splanchnic 

 wall of this forms a dome-shaped projection into the cavity oppo- 

 site each nephrostome. In the interior of this sinus-like cavities 

 are formed, rudimentary blood-vessels, containing round corpuscles. 

 The two dome-shaped structures in question upon each side be- 

 come continuous and give rise to the glomus. At first lying on 



