embryos were prepared for ec ( early chick ) cultures and then electroporated and/or ablated followed by culture on the microscope stage for time - lapse recordings . rusty landford ( caltech ) . a monoclonal antibody to avian fibronectin , b3d6 ( developmental studies hybridoma bank , university of iowa , iowa city , ia ) was directly coupled to a fluorochrome alexa-555 ( molecular probes , eugene , or ) and microinjected into the psm and lateral mesoderm 1 h before the start of a time - lapse experiment . time - lapse microscopy and image treatment were accomplished as described and in the supplementary methods . a fragment corresponding to the xenopus fgfrdn ( xfd ) ( gift from dr patrick lemaire as described ) was cloned into pbi - egfp ( clontech ) . for the inducible constructs , pbi - egfp or pbi - fgfrdn was coelectroporated with pcirx - rtta - advanced ( gift from dr tadahiro iimura ) . a version of the pbi vectors devoid of gfp and rtta - advanced without red fluorescent protein was also used in order to facilitate tracking of co - electroporated h2b - gfp or fibronectin antibody . full methods and any associated references are available in the online version of this paper at www.nature.com/nature . embryos were prepared for ec ( early chick ) cultures and then electroporated and/or ablated followed by culture on the microscope stage for time - lapse recordings . rusty landford ( caltech ) . a monoclonal antibody to avian fibronectin , b3d6 ( developmental studies hybridoma bank , university of iowa , iowa city , ia ) was directly coupled to a fluorochrome alexa-555 ( molecular probes , eugene , or ) and microinjected into the psm and lateral mesoderm 1 h before the start of a time - lapse experiment . time - lapse microscopy and image treatment were accomplished as described and in the supplementary methods . a fragment corresponding to the xenopus fgfrdn ( xfd ) ( gift from dr patrick lemaire as described ) was cloned into pbi - egfp ( clontech ) . for the inducible constructs , pbi - egfp or pbi - fgfrdn was coelectroporated with pcirx - rtta - advanced ( gift from dr tadahiro iimura ) . a version of the pbi vectors devoid of gfp and rtta - advanced without red fluorescent protein was also used in order to facilitate tracking of co - electroporated h2b - gfp or fibronectin antibody . full methods and any associated references are available in the online version of this paper at www.nature.com/nature .
vertebrate embryos are characterized by an elongated antero - posterior ( ap ) body axis , which forms by progressive cell deposition from a posterior growth zone in the embryo . here , we used tissue ablation in the chicken embryo to demonstrate that the caudal presomitic mesoderm ( psm ) plays a key role in axis elongation . using time - lapse microscopy , we analysed the movements of fluorescently labelled cells in the psm during embryo elongation which revealed a clear posterior - to - anterior gradient of cell motility and directionality in the psm . 
 we tracked the movement of the psm extracellular matrix in parallel with the labelled cells and subtracted the extracellular matrix movement from the global motion of cells . 
 after subtraction , cell motility remained graded but lacked directionality , indicating that the posterior cell movements associated with axis elongation in the psm are not intrinsic but reflect tissue deformation . 
 the gradient of cell motion along the psm parallels the fibroblast growth factor ( fgf)/mitogen - activated protein kinase ( mapk ) gradient 1 , which has been implicated in the control of cell motility in this tissue2 . 
 both fgf signalling gain- and loss - of - function experiments lead to disruption of the motility gradient and a slowing down of axis elongation . 
 furthermore , embryos treated with cell movement inhibitors ( blebbistatin or rhok inhibitor ) , but not cell cycle inhibitors , show a slower axis elongation rate . 
 we propose that the gradient of random cell motility downstream of fgf signalling in the psm controls posterior elongation in the amniote embryo . 
 our data suggest that tissue elongation is an emergent property that arises from the collective regulation of graded , random cell motion rather than by the regulation of directionality of individual cellular movements .