a Lateral view of the embryo showing movement of a subset of cells around shield region (4.5–7 hpf; selected subset of tracks shown in inset from animal pole). Arrows indicate direction of epiboly and internalization movement towards the animal pole. Color code shows normalized level of mezzo expression along each track. b Scatterplot showing the average change in radius and latitude for each track during 4.5–7 hpf time interval. Gray region indicates cells moving towards animal pole. Color code shows normalized mezzo expression. c Lateral view of long-term cell flows around shield region. Same region-of-interest as in a. Color code indicates normalized mezzo expression along track. d Lateral view of three clustered cell tracks undergoing internalization and movement towards animal pole. Thick lines show the representative cluster centroid trajectories. Magnified view of region indicated by box outline in a. e Dorsal view of long-term cell flows, same color code and region as in a. f Lateral view of the same clustered tracks and cluster centroids as in d.