Heat induced expression induces embryonic apical flattening, flattening occurs very soon after heat shocked embryos have cellularised: lateral cell apices are flattened and appear stretched, the cells have lost their regular hexagonal outlines as seen in wild type. Induced cell shape changes disrupt the morphogenetic movements that rely on cell shape changes. The ventral furrow is wider than normal (all twi-expressing cells have undergone apical construction), mesodermal cells ultimately invaginate into the interior, and invagination of the posterior midgut is well advanced at a stage when the ventral furrow is still open, in contrast to wild type embryos.

fog^hs.PM is a suppressor of ventral furrow phenotype of twi¹