216 G. C. PRICE, 



was at first thought they were parts of the original segmented coelome, 

 that is, nephrotomes ; but the manner of formation, as illustrated in 

 Figs. 9 a — 9 f, together with the fact above mentioned, that they are 

 present in stage B in segments where they are absent in stage A, 

 seems to render this interpretation untenable. 



An important difference, and the only important difference, between 

 the older and younger embryo of stage B, is the fact that in the 

 older embryo a part of the anterior end of the system has entirely 

 disappeared. On account of an injury, which however does not effect 

 the excretory system itself, it is not possible to tell with absolute 

 certainty the position of the anterior end of the system , but it can 

 not be far from the twenty-second segment, that is, sixteen segments 

 posterior to the position of the anterior end in the younger embryo, 

 and nine or ten segments in front of the position of the pronephros 

 of the adult. In the part remaining, there are no indications of 

 degeneration. 



It was stated that in the younger embryo the posterior end of 

 the system was wanting, here it is present, and is very distinct. 

 Throughout the posterior nineteen segments there are no tubules; as 

 in stage A there were no tubules in the two posterior segments, 

 seventeen tubules must have degenerated. The extreme posterior end 

 of the duct is a rod of cells without any indication of a lumen. It 

 comes into close contact with the entoblast, but here the embryo lies 

 flat upon the yolk, and there are no indications of a folding off of 

 the alimentary canal. 



What has already been said in regard to the excretory system 

 in the adult, applies in a large measure to stage C. There are two 

 parts, pronephros and mesonephros. The posterior part of the seg- 

 mental duct is free of tubules , as in the adult , and in the older 

 embryo of stage B, although here the tubules are wanting through 

 twenty segments instead of nineteen as in stage B. This is a matter 

 of no consequence, however, as the adults show that a variable number 

 of tubules may degenerate. As in the adult, so here, the ducts open 

 beside each other, on a papilla, into the urino-genital sinus. 



The anterior end of the system lies in the thirty-first segment, 

 that is, twenty-five segments back of the anterior end in the younger 

 embryos of stage B. It was in this region that the system developed 

 most slowly, or rather in the more anterior part of this region, and 

 it was here also that the irregularities in segmental symmetry were 



