Fig. 9. Hypothesis to explain the difference in 1d division asymmetry when the polar lobe or the A/C cells are removed. Micromeres are shown with their apical pole towards the top; broken lines indicate the plane of their first division. (A) The normal 1a, 1b and 1c micromeres, which are presumably equivalent to the 1d micromere formed after polar lobe ablation. (B) In the normal 1d cell, a polar lobe-derived factor repositions the division plane with reference to the apical pole. (C) In partial embryos, the polar lobe-derived factor still positions the 1d division plane, but as the cell is bigger, division occurs proportionally higher on the axis of the cell. In this diagram, the resulting volume of the apical cell is intermediate between the normal volumes of 1d¹ and 1a¹/1b¹/1c¹.