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Fig. 4 Random Walk Is Sufficient for Endodermal Cells to Colonize the Yolk Surface (A) Cell migration was mathematically simulated to test how the observed pattern of endodermal cells at midgastrulation can be achieved. At the beginning of gastrulation, 100 cells were localized at the margin of the blastoderm (50% epiboly). Depending on their position along the dorsoventral axis, cells were marked in red (dorsal), blue (lateral), or green (ventral). An oriented motion toward the animal pole cannot account for the pattern observed in vivo at 75% epiboly. In contrast, a random walk efficiently spreads cells over the yolk, with the expansion of the layer being limited both by cell speed and by the margin. At midgastrulation (75% epiboly), cells have reached a position similar to the expression pattern of endodermal markers and have partially mixed. (B) Shown is the distribution of endodermal cells at the onset of gastrulation and at midgastrulation as assayed by sox17 expression.