230 EMBRYOLOGY OF THE LOWEE VEBTEBEATES ci-i. 



op. 



10 



15 



20 



25 



30 



35 



J) 39 



Pig. 128. — Renal organs of the right 

 side of a Protopterus larva of stage 

 34. (From a reconstruction by 

 M. Robertson.) 



a-n.d, archinephric duct; op, opistho- 

 nephric tubules ; pn, pronephros. The 

 capital letters indicate nephrostomes and 

 the figures metotie * mesoderm segments. 



* ' ' Metotie " = posterior to the otocyst. 



In its later functional stages the pro- 

 nephros reaches relatively enormous 

 bulk, occupying the whole thickness of 

 the body-wall, but in these later stages 

 the two tubules become much elongated 

 and coiled as well as the duet itself. 



The nephrocoeles belonging to the 

 various nephrotomes which develop 

 tubules form a series of closed cavities 

 lying in a row one behind the other (Fig. 

 127, ne.B, nc.F). They are for a long 

 time, in Polypterus, the only coelomic 

 spaces which are widely open (Fig. 125, 

 B). As development goes on the 

 nephrocoeles connected with the func- 

 tional tubules (B and E) become more 

 and more dilated, their wall becoming 

 thinner as they do so, and the floor 

 bulging into the cavity to form the 

 glomerulus. Eventually the cavity of 

 the nephrocoele becomes continued ven- 

 trally, a split spreading downwards to 

 form the splanchnocoele, into which the 

 nephrocoele opens freely. The portions 

 of splanchnic mesoderm to which the 

 glomeruli are attached, i.e. the floors 

 of the original nephrocoeles, become 

 folded in towards one another, as the 

 splanchnoeoelic cavity dilates, to form 

 the dorsal mesentery so that the glo- 

 meruli are eventually borne by the 

 mesentery one on each side. 



Meanwhile the nephrocoeles belong- 

 ing to the tubules which atrophy 

 gradually shrink up and disappear, and 

 as they do so the two large functional 

 nephrocoeles increasing still more in 

 size meet and their cavities as well as 

 their glomeruli become continuous. No 

 definite constrictions (peritoneal canals) 

 are formed between nephrocoeles and 

 splanchnocoele, unless possibly during late 

 stages, but the dorsal portion of the more 

 posterior nephrocoele becomes cut off 

 from the splanchnocoele by another 

 method — the free edge of the glomerulus 

 coming to fuse with the somatopleure 

 so as to form a floor to the nephrocoele. 



