ABSTRACT The polar lobe of the egg of Bithynia, with a volume of less than 1 % of that of the egg, was surgically removed. Lobeless embryos form no mesentoblast cell nor mesoderm bands. They fail to establish bilateral symmetry and to form eyes, foot, operculum, shell, etc. They always differentiate larval head cells, ganglia, larval liver and mesenchyme and sometimes columnar epithelium, stomodeal entrance cells. endodermal tubes and muscles. These results are discussed in relation to the structure of the lobe, the analysis of development of Bithynia and to similar experiments in other species.