These diagrams provide an explanatory key for the simulation videos and illustrate the principal signals, cell behaviours and the major tissues involved in gastrulation. Three time points are shown: stage XI, stage 2 and stage 3+. The upper 7 rows are dorsal views onto the epiblast; the lower 3 rows are oblique views. Colours are additive when a cell is positive for more than one displayed state (see e.g., the row labelled ‘combined’, which symbolises the sum of all features in the rows above it for the forming primitive streak). Nodal(+) cells are shown in red (top row), Wnt-PCP(+) cells in yellow (second row). Cells positive for both Nodal and Wnt-PCP appear orange (third row). At Stage XI all cells in the future streak-forming region are Nodal and Wnt-PCP positive. Later, most continue to have both activities but some cells are only positive for Nodal (red). Cells undergoing EMT are shown in blue and ‘mesendodermal’ cells in aquamarine (fourth row). For combinations of Nodal, Wnt-PCP, EMT and mesendoderm, note that Nodal(+)-EMT cells appear purple (red + blue); if also Wnt-PCP(+) then approximately violet (red + yellow + blue) (‘combined’). The hypoblast is shown chocolate-coloured and the endoblast greenish-slate (rows 6 and 8). Hypoblast displacement by the endoblast (at stage XIV; between stages XI and 2 in the Figure) disinhibits Nodal in the overlying epiblast (see text). Sequential cell positions are integrated by remembering all previous time points to form ‘trails’, as shown in row 7. For clarity, trails made from 15% of the cells are shown. The last three rows depict the embryo viewed from an oblique angle. In row 8 (‘hypoblast and endoblast’), the position of the lower layer can be seen (also see above, lower layer). Initially this consists only of hypoblast (chocolate). At later stages, endoblast (greenish-slate) partially displaces the hypoblast. The epiblast is also seen from below (‘epiblast ventral view’, row 9), allowing clear visualization of EMT (blue/purple/violet) and emerging and emerging middle layer (aquamarine) cells. The final row, ‘epiblast dorsal view’ (row 10), displays the epiblast from above with a pseudo-surface applied, simulating indentations caused by ingressing cells. These indentations sum as cells approach the posterior midline, generating a midline groove at the PS. The pseudo-surface is created by tessellating points representing the top of each epithelial cell (using the cell body for cells undergoing EMT).