i = ;

cells=FillingTransform@Binarize[i,{0,.7}]

markers=MaxDetect[DistanceTransform[cells],.25]

segments=SelectComponents[WatershedComponents[GradientFilter[cells,2],markers],"Area",200<#<2000&];
Colorize[segments]

circles=ComponentMeasurements[segments,{"Centroid","EquivalentDiskRadius"}];
Show[i,Graphics[{Red,Thick,Circle@@#&/@circles[[All,2]]}]]