The vertebral column exhibits segmentation and regionalization along the antero-posterior axis. During embryogenesis, the rhythmic production of the precursors of the vertebrae, the somites, imposes a segmented aspect to the spine, whereas the spine's regional differentiation is controlled byHoxgenes1,2. Here we show that in the paraxial mesoderm,Hoxbgenes are first activated in a temporal collinear fashion in precursors located in the epiblast lateral to the primitive streak. Our data suggest that collinear activation ofHoxbgenes regulates the flux of cells from the epiblast to the streak and thus directly controls the establishment of the genes' characteristic nested expression domains in the somites. This suggests that establishment of the spatial co-linearity in the embryo is directly controlled by theHoxgenes themselves.