Snail and Twist are required for epithelial tearing. (A) Images of myosin-GFP in live embryos injected with the indicated dsRNA. snaRNAi inhibits the formation of myosin II rings and the loss of cell–cell adhesion normally observed in armRNAi embryos. twiRNAi inhibits tearing, but myosin contractions still occur. (B) Images of cell outlines (Spider-GFP) in armRNAi and arm-twiRNAi embryos. Knockdown of Twist suppresses the formation of discrete epithelial tears. (C) Epithelial tears and loss of cell–cell adhesion occur within the Snail expression domain. Images of a fixed arm^M/Z embryo stained for Snail (green) and neurotactin (purple, cell outlines). An apical image of neurotactin is included to illustrate the position of the tear. The ventral midline is offset to visualize the lateral border of the mesoderm where two Snail-expressing nuclei do not initially lose adhesion (white bracket). Bars, 10 µm.