Ferromagnetic Clusters and Magnetic Polarons as Evidenced by ESR in La1.35Sr1.65Mn2O7

${\mathrm{La}}_{1.35}{\mathrm{Sr}}_{1.65}{\mathrm{Mn}}_2{\mathrm{O}}_7$ is investigated in the regime where 3D magnetic order is lost but 2D correlations are strong. ESR shows that the localized $t_{2g}$ spins are coupled into intralayer ferromagnetic clusters of fixed size. The $e_g$ carriers form polarons with an effective spin growing from $1/2$ to 7. ESR and transport properties suggest that the 3D order originates from the onset of coherence of the ferromagnetic clusters mediated by the propagation of the polarons.