41 8 WILSON'. [Vol. VI. 



impossible to decide with certainty, what is the fate of the 

 small cells (the products of a^ and b"^) that lie between the 

 stomatoblasts, before their convergence. Do they pass into 

 the pigment plug with the secondary mesoblast-cells, and if 

 so, do they migrate inwards with them to form a part of 

 the mesoblast } Here we have, as I believe, the only possi- 

 bility in the development of Nereis that the mesoblast may 

 arise from the ectoblast in Salensky's and Kleinenberg's sense. 

 Unfortunately I cannot give a definite answer to this question. 

 But after studying a large number of favorable surface-views 

 (in which the history of the cells can be traced much better 

 than in section), it is my belief that they do not pass inwards, 

 for the following reasons : First, some of these cells certainly 

 become wedged in between the large stomodaeal cells as they 

 converge, while others remain as a group of unpigmented cells 

 that lie between the pore and the stomodseal arc (Figs. 79, 

 80, which accurately represent the cells as far as they are visi- 

 ble). This group of cells (as shown in Fig. 80) is enclosed by 

 the stomodaeal arc in later stages, and thus still more definitely 

 separated from the pigment-area, and it is certain that if any 

 contribution is made by these cells to the mesoblast, it must 

 be of minimal amount. 



The stomodaeal arc rapidly enlarges by radial divisions of 

 its cells (Fig. 80) and is soon converted into a circle with a 

 small opening in the middle which forms the mouth (Fig. 81). 

 The cells at the same time assume a columnar or wedge-shaped 

 form and are partially overgrown by the neighboring small cells, 

 so as to reach the surface only at the lips of the mouth-opening 

 {cf. Figs. 82, 84, 88). Meanwhile a group of cells (s.g}i arises 

 on either side the stomodaeum (undoubtedly derived from it), 

 which in later stages forms a pair of glandular masses (.'') 

 lying at the junction of the stomodaeum (pharynx) and the 

 mesenteron (Fig. 92). These may be called the stomodcsal 

 glands. 



I will finally call especial attention to the fact that upon the 

 demarcation of the pigment-area, which takes place at the same 

 time as the completion of the stomodaeum, it lies in immediate 

 juxtaposition to the latter (Figs. 81, 82). The pigment-area, 

 however, marks the posterior end of the adult body, and hence 



