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 
