414 WILSON. [Vol. VI. 



a relatively early period, before the inward migration of the 

 pigment-cells. For this and other reasons it is nearly certain 

 that they arise from the mesoblast-bands, i.e. from the progeny 

 of the primary mesoblasts after the pigment-cells have been 

 budded forth. They form, at first, a small loose bundle on 

 either side dorsal to the seta-sacs, whence they extend them- 

 selves over the dorsal region. The remaining differentiations 

 of the mesoblast I shall consider hereafter. 



3. History of the Second and Third Groups of Micronieres. 



The last points to be considered in the embryonic develop- 

 ment relate to the general history of the second and third 

 groups of micromeres, including the formation of the stomo- 

 daeum and the general relations of the blastopore. It will be 

 convenient to- begin with one of the middle stages, and to con- 

 sider the earlier history afterwards. 



At a stage shortly before the closure of the blastopore, three 

 cells are always found, symmetrically disposed about the lips of 

 the blastopore ; these cells I have called the stoinatoblasts {st., 

 Fig. 56, PI. XVII), since from them the stomodaeum is in large 

 part, perhaps wholly, derived. One of them [st.m.) lies in the 

 middle line in front, wedged into the cleavage-line between A 

 and B. The other two {st.r., st.l.) lie, one on either side, 

 opposite the cleavage-lines between B and C, and A and D 

 respectively. These cells are easily distinguishable from the 

 surrounding cells, not only on account of their size, but also 

 because they appear (perhaps on account of their thickness) 

 somewhat darker and more granular than the others. 



These cells have certain constant and characteristic relations 

 to the surrounding cells, which are shown in Diagram No. IV. 

 The space between the median and the left stomatoblasts is 

 occupied by a group of smaller cells, most of which, at any rate, 

 are the products of a^ (the left anterior micromere of the third 

 group). The corresponding space between the median and right 

 stomatoblasts is in like manner occupied by the progeny of b^ 

 (the right anterior micromere of the third generation). The 

 posterior lip of the blastopore, as has been shown, is formed by 

 the secondary mesoblast-cells, budded forth from the primary 

 mesoblasts. Wedsed in between them and the left stomatoblast 



