Expression of cxcr4 and cxcr7 in the migrating primordium. Double in situ hybridization with a cxcr4b probe (green) and a cxcr7 probe (red). cxcr7 is exclusively expressed by cells in the trailing region and in cells that are being deposited, while cxcr4 b is more ubiquitously expressed (albeit at a higher level by the cells in the leading region). White dots outline the migrating primordium, primI. The direction of migration is shown by the white arrow. Picture provided by C Dambly-Chaudière and M Rossel.