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Fig. 3 Patterns of gene expression in migrating primordia. A: the expression of cxcr7 is strong in the trailing third of primI and absent in the leading half. B: neuromast deposition depletes the cells that express cxcr7 most strongly (left of the arrowheads) but cells at the trailing edge of the primordium also express cxcr7, albeit at a lower level. The expression of cxcr7 must be quickly up-regulated in these cells to re-establish the pre-deposition pattern (compare B and A). C: the expression of cxcr4 is high in most of the primordium but weaker in the trailing cells. D: a double in situ hybridization reveals that the patterns of expression of cxcr7 (red) and cxcr4b (green) are largely complementary but not exclusive. In all panels the large arrow shows the direction of primordium migration and the dots outline the migrating primordium. In panel B the arrowheads indicate the limit between the group of cells that are slowing down to form a neuromast, and those that will keep migrating. The fine arrows indicate the boundary of the region of cxcr7 expression.