Fig. 2. (A-D) Apical-basal position of invagination sites in Glomeris. Transverse sections of untreated embryos (A,B) and embryos stained for a digoxigenin (DIG)-labelled GmASH probe (C,D). Basal is towards the top. (A) After formation of the first invagination sites, groups of up to 11 cells are visible on the basal side (asterisks) that are attached to the apical surface (arrow). (B) Groups of invaginating cells are located over and above each other (arrowheads) after formation of the third wave of neural precursors (asterisks and crosses). The cells are still attached to the apical surface (arrow). (C,D) GmASH is transiently expressed in the invaginating cell groups, which are located at different apical-basal positions (arrows). Some of the invaginating cell groups form stacks (arrowhead). Scale bar: 10 µm in A-D.