208 WILSON. [Vol. IV. 



trochophore seems to consist at first of two layers only. The 

 mesoblast, like the neural foundations and those of the seta-sacs, 

 arises directly from a thickened bilobed ventral plate ; i.e. it 

 seems to arise from the ectoblast. But a further examination 

 of the matter gives it a different aspect. The cells of the ven- 

 tral plate differ markedly from the remaining cells of the outer 

 layer. They are larger, differently granulated, and upon treat- 

 ment with certain reagents (combinations of acetic acid, etc.), 

 assume a brownish color that differentiates them very sharply 

 from those of the upper pole. This renders it possible to trace 



II 



II 



Fig. I. — Eight-cell stage of Nereis viewed from the upper pole. I.-I., the first 

 cleavage-plane; II.-II., the second; A, B, C, D, the four macromeres; a, b, c, d, the 

 corresponding raicromeres.^ 



their origin, cell by cell, from the beginning of development, 

 and this origin is such as to show that the mesoblast is com- 

 pletely segregated in the anterior part of the plate, while the 

 posterior part alone gives rise to ectoblastic structures (neural 

 plates, seta^sacs). Moreover, eacJi of tJie two divisions of the 

 ventral plate may be traced back to a single cell {pro-te lob last), 

 which is obviously homologous to a corresponding cell in the early 

 embryo of Clepsine. A detailed account of the cleavage will be 

 given in a forthcoming paper, but the following synopsis will 

 suffice for the present purpose. 



1 AH of the cuts are from camera drawings, and are not schematized in outline, 

 though slightly simplified. 



