. Text-book of embryology. Embryology. 230 EMBRYOLOGY OF THE LOWEE VEBTEBEATES 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.) , 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
. Text-book of embryology. Embryology. 230 EMBRYOLOGY OF THE LOWEE VEBTEBEATES 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.) , 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, , ). 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, 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 splanchnocoel
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