Fig. 2.

VFM as applied to Drosophila cross-sections. A shows a transverse cross-section through a Drosophila embryo. The tissue spanning an angle of approximately ± 40° in this figure will eventually form part of the ventral furrow (B), and it is denoted mesoderm, whereas the balance of the epithelium is called ectoderm. B shows the associated VFM mesh several minutes later, when the ventral furrow is becoming evident. The mesh is “Lagrangian,” following the tissue it represents. The thin gray edges that divide the cells on the ventral aspect of the embryo are assumed to carry zero load. C shows the cells in a volume of length d along the anterior–posterior axis of the embryo corresponding to three of the regions shown in cross-section in B. Note that even if the regions are made exactly one cell wide, they will always represent parts of cells and possibly multiple cells (depending on the value of d). The VFM edge forces reported in the cross-sections of Fig. 3 are those generated by the cells in the corresponding extruded volumes.