1 68 whitman: [Vol. I. 



coils so closely packed that the linear arrangement of the cells 

 is obscured. I think this is a fair inference from the appearance 

 of the nephric plate. {^Vide No. i, Fig. 92, PI. XV.) 



The larval nephridia of other annelids, so far as known, con- 

 sist at most of only a few cells ; and in some cases, e.g., 

 Eupomatus, the duct of the fully developed organ is formed 

 within a single elongated cell, stretching from the oesophagus 

 back to the mesoblast of the same side. A number of spherical 

 cells are found around the anterior end of this elongated cell, 

 and these are regarded by Hatschek (No. 35, p. 143) as be- 

 longing to the excretory organ. The entire organ arises from 

 tzvo cells, one of which forms the duct, while the other splits up 

 into the spherical "end-cells" (p. 134). Whether the two cells 

 arise by successive divisions of the mesoblast, or by division of 

 a primary nephroblast, we are not informed by Hatschek. Both 

 cells are regarded as mesoblastic, but this interpretation would 

 be perfectly consistent with the second mode of origin. 



Hatschek finds a pair of primary mesoblasts (" Urmesoderm- 

 zellen"). Each of these divides into two unequal parts, a 

 large " pole-cell " and a small " daughter-cell." The " pole- 

 cells " evidently correspond to the two " mesoblasts " of Clepsine ; 

 and the " daughter-cells " appear to me to represent nephro- 

 blasts. But, if Hatschek is right in regard to the origin of these 

 cells, there is one difficulty in the way of identifying the 

 "daughter-cells" with the nephroblasts; for the former are 

 mesoblastic, while the latter are ectoblastic. If, however, we 

 examine the facts a little more closely, the objection appears 

 less formidable than at first sight. In Clepsine we have seen 

 one cell give rise by division to the mesoblasts, the nephro- 

 blasts, and the neuroblasts. The first division separates the cell 

 into a "primary mesoblast" and a " neuro-nephroblast." The 

 point of fundamental importance for our comparison is the twin 

 origin of these cells. If we call one cell mesoblastic and the 

 other ectoblastic, that is a matter of inter-pretation, which may 

 be justified by appearances in the one case, and contradicted 

 by them in the other. The fact remains, that the genetic rela- 

 tion between mesoblast and nephroblast is equally close in both 



(35.) Hatschek, B. Entwicklung der Trochophora von Eupomatus uncinatus 

 Philippi (Serpula uncinata). Arbeit, a. d. zool. Inst. z. Wien. VI. H. i., p. 121. 

 188 ■;. 



