582 HAROLD HEATH, 



Regarding the second quartette cells little remains to be said at 

 this point; all are of essentially the same size, the one giving rise 

 to the first somatoblast being no larger than the rest. Their history 

 will be more fully considered in special sections. At present it may 

 be said that they are of great importance in the development of the 

 embryo and also that their destinies are most diverse, entering as 

 they do into the formation of the subumbrella ectoderm, the stomo- 

 daeum, the shell, foot and possibly the nervous system. 



3. Formation of Apicals and theThird and last Quartette 

 ofEctomeres, 24 cells (Figs. 9—12). 



In this cleavage also there are eight new cells formed, four from 

 a cleavage of the large first quartette micromeres wherein the newly 

 formed products lie radially disposed about the animal pole (Figs. 9, 

 13) and four from a division of the macromeres giving rise to the 

 third quartette (Figs. 10, 11). The first cells mentioned arise in a 

 dexiotropic division and constitute the apicals. These are relatively 

 smaller than the corresponding cells in Annelids and Molluscs, and 

 their parent cells, the intermediate girdle cells of Annelids, are much 

 larger than in any other form. In certain specimens the apical series 

 sinks below the general surface, and in some cases when greatly 

 crowded by the large surrounding parent cells they disappear almost 

 entirely from view, but at or before their next division they commence 

 to emerge again, and ultimately become level with the other cells of 

 the upper hemisphere. It reminds one of similar movements in the 

 rosette series of Annelids, and of the so-called invagination at the 

 animal pole of Neritina. 



As just said, the cells corresponding to the intermediate girdle 

 cells are relatively very large, while in Annelids they are of the same 

 size or smaller than the apicals to which they give rise, and as will 

 appear later, it is partly owing to this fact that the cross as it ap- 

 pears in Gastropods is not forthcoming in the same cells in Annelids. 



Up to this time three quartettes have separated from the macro- 

 meres ; the cells of the first and second are of about the same size, 

 those of the third considerably larger than either, and the macromeres 



left hand side of the first quartette cells. The eggs were too opaque 

 to allow the spindles to be seen and therefore this bulging was con- 

 sidered to be an indication of division. Consequently it seems to me 

 quite probable that these cells in their formation do not form an ex- 

 ception to the alternation of cleavages. 



