  An inelastic light (Raman) scattering study of nematicity and critical
fluctuations in ${\rm Ba(Fe_{1-x}Co_x)_2As_2}$ ($0\le x \le 0.051$) is
presented. It is shown that the response from fluctuations appears only in
$B_{1g}$ (${x^2-y^2}$) symmetry. The scattering amplitude increases towards the
structural transition at $T_s$ but vanishes only below the magnetic ordering
transition at $T_{\rm SDW} < T_s$, suggesting a magnetic origin of the
fluctuations. The theoretical analysis explains the selection rules and the
temperature dependence of the fluctuation response. These results make
magnetism the favorite candidate for driving the series of transitions.%Below
$T_{\rm SDW}$ the gap of the magnetically ordered phase opens up.
