128 Excrotorv Or,uaiis in I'xlcllnstoiiin Stnnti 



help to make the matter clear. The jiostorior lialf of the excretory 

 organs in hotli these eniljrvos were reconstructed on millimeter paper. 

 It Avas found that thirtv segments of the older embryo occupied the 

 same space as thirty-one of the younger, so the length of a segment is 

 very nearly the same in the two embryos. It was found further that 

 there was no Inmen in the segmental duct of the younger embryo, but 

 in the older embryo in every segment there was such a lumen, extending 

 in both directions from the tnbule, but more in the posterior than the 

 anterior. The length of each division of the lumen was nearly the same 

 as the length of the nephroccel in the younger embryo, while the width 

 of the tubule was less. It was thus clear that the lumen of the segmental 

 duct had been differentiated from the nephroccel, and had not been 

 formed in the solid rod of cells forming the segmental duct of the younger 

 embryo. This accounts for the fact before noted, that the lumen may 

 extend in both directions from the tubule. 



cMc 



Fifl. 25. Vicr. 26. 



Fig. 25. — Section through a tubule in the region where the nephrocoels have dis- 

 appeared in an embryo corresponding to stage B of the earlier paper, t, tubule ; sc, 

 splanchnocoel ; Xj part corresponding to the part x of Fig. 26. 



Fig. 26. — Section of a tubule which has been cut off from its connection with the 

 splanchnoccel, and from the same embryo as Fig. 2.5. cmc, cavity of the future Mal- 

 pighian corpuscle ; t, tubule ; sc, splanchnocoel ; x, part that will form the Bowman's 

 capsule of the Malpighian corpuscle. 



In an embryo a little older some of the nephrocoels, or better tubules, 

 have lost their connection with the splanchnoccel, by the somatic mesoblast 

 at the point where the splanchnocoel and nephroccel come together, grow- 

 ing down and uniting with the splanchnic mesoblast. In general, the 

 process occurs first in the posterior region, but it does not begin at any 

 one particular segment and from there proceed in regular order, as may be 

 gathered from the fact that on one side in this embtyo the tubules 

 which are closed off are in segments thirty-nine, forty-three, fifty-eight, 

 sixty, sixty-four to sixty-six, and sixty-nine to seventy-one. Beyond 

 this the tubules are degenerate and have almost disappeared. On the 

 opposite side the tubules which are closed off are not quite so numerous, 

 nor do they always occur in the same segments. 



In the anterior region where the nephrocoels have disappeared the 



