lateral line primordium [@venkiteswaran2013generation] show tightly-controlled Sdf1 signaling, as measured by Cxcr4b internalization, suggesting that CCV may be small enough to allow for accurate gradient sensing.

We also show that there is a direct link between fluidity and chemotaxis as shown by [Eq. ]{}\[eq:snrt\]. Verifying this expression in experiments requires simultaneous measurement of several quantities, including cluster size, cluster re-arrangement, and signal gradient. Therefore, care has to be taken when modifying experimental conditions as these might change several of these quantities simultaneously. Altering adhesion, for example, changes both cluster fluidity and spreading as shown in a recent study using neural crest clusters [@kuriyama2014vivo], creating a confounding factor. Nevertheless, these types of experiments may be successful in setting bounds on possible time-averaging and the link between fluidity and chemotaxis.

Our results suggest that many recent experiments may need reinterpretation. Measured chemotactic accuracies can depend on cluster size [@theveneau2010collective; @malet2015collective; @cai2016modeling]; these results have been modeled without time averaging or CCV [@camley2016emergent; @malet2015collective; @cai2016modeling; @camley2016collective]. Our results show that rearrangement times $\tau_r$ also influence chemotaxis – and that $\tau_r$ depends on cluster size. Cluster relaxation dynamics are therefore an unexplored potential issue for interpreting collective gradient sensing experiments.

Essential in the reduction 