76 KARM NARAYAN BAHL 



like the testes and ovaries, or whether the mass arises by 

 multiphcation and growth of one or more cells lying between 

 the two adjoining sheets of a septum. Is the septal nephridium 

 intra-peritoneal or inter-peritoneal ; or, in other words, is it 

 mesodermal or ectodermal ? This is the fundamental morpho- 

 logical question to be answered. 



We have already noticed that during the course of develop- 

 ment of the primary integumentary nophridia the mass of 

 nephridial cells lying opposite and behind the intersegmental 

 septa, underneath the coelomic epithelium, segregates early 

 on into two groups — one forming the * retroperitoneal ' group 

 of cells and developing into an integumentary nephridium, 

 and the other consisting of a few cells that push their way into 

 the septum between its two layers of peritoneum. This second 

 group, which is directly traceable to the original nephric row 

 and has thus the same source as the integumentary nephridia, 

 is in fact the primordial rudiment of the septal nephridia. 



In a series of longitudinal sections of an embryo about 6 mm. 

 long, we can trace how a cell from this primordial group 

 travels through the septum to take up its final position in the 

 row of septal nephridia on each side of the dorsal vessel. If 

 we examine, in this series, a septal nephridium on one of the 

 anterior septa— say the twentieth — we find that it lies at a little 

 distance from the dorsal vessel immediately internal to the 

 commissural vessel (fig. 14 c). This incipient nephridium 

 and the commissural vessel are both situated between the two 

 sheets of the septum, one below the other (fig. 14). As we 

 trace this nephridial rudiment backwards we find that it 

 retains the same relative position with regard both to the 

 dorsal and the commissural vessels. We can thus trace the 

 nephridial rudiment, consisting of a few cells as it lies dorsally 

 on each side of the dorsal vessel on one of the anterior septa, 

 back through successive segments to the posterior end of the 

 worm, where the nephridial rudiment lies ventrally on each 

 side of the nerve-cord, and is just beginning to push its way 

 into the edge of a septum. On examining the sections showm 

 in figs. 13 and 14 two fundamentally important facts come out. 



