No. 3.] THE CELL-LINEAGE OF NEREIS. 417 



left stomatoblasts respectively. I cannot state as positively that 

 b'^-'^ is the median stomatoblast, but I have no doubt that such 

 is the case. The first division of ^ and d^ is shown in Fig. 50, 

 and the connection between this and our starting-point (Fig. 55) 

 is given by Figs. 53 and 54. 



4. Closttre of the Blastopore. Formation of the StomodcEum. 



The blastopore now rapidly closes by a convergence of all 

 these cells towards a central point, which lies slightly anterior 

 to the lower pole, usually about the middle of the cross-furrow. 

 The convergence of the cells, which is rather sudden, is brought 

 about by a very interesting process, which has superficially the 

 appearance of an embolic gastrulation. A marked depression 

 appears around the lower pole, bounded in front and at the 

 sides by the stomatoblasts and the small cells between them, 

 and behind by the cells immediately covering the primary meso- 

 blasts. This is shown from the lower pole in Figs. 62, 63, 

 XVII, in side-view in Fig. JJ \ and the same embryo is shown 

 in optical section in Fig. 'j^. The depression now rapidly 

 disappears and the embryo again becomes perfectly spherical 

 (as shown in actual section in Fig. 88, XX). This is not 

 caused, however, by a bending out of the walls, but by a 

 drawing together or convergence of the cells towards a central 

 point, at which a distinct pore is for some time left (see Fig. 

 79). The stomatoblasts meanwhile divide radially, and by the 

 curious process just described they are drawn together so as 

 to form an arc of cells, just behind which appears the pore 

 (Fig. 79). The small cells that occupied the floor of the de- 

 pression are meanwhile crowded together and entirely change 

 their form. Hitherto they have formed a thin pavement-epi- 

 thelium over the floor of the depression. Now they are squeezed 

 together until they assume a narrow, prismatic form, and form 

 a kind of plug {cf. Figs. 78, %%) between the stomodaeal arc and 

 the primary mesoblast-cells (immediately internal to which lie 

 the entoblast-nuclei). The pigment appears in the cells of this 

 plug on either side tJie pore as the convergence takes place, and 

 a comparison of the figures shows from their position that they 

 tnnst be, in large part at any rate, the secondary mesoblast-cells. 



It is an extremely important question, but one well-nigh 



