We investigated the initial stages of Si and Ge epitaxy on Si(111) surfaces using in situ ultrahigh vacuum reflection electron microscopy (UHV REM). The characteristic growth length involving descriptions of the transition between step-how and nucleation modes of growth was considered. Two methods of measurement of this characteristic length were proposed : measurements of the minimal distances between atomic steps for two-dimensional nucleation using, first, dispersion in the interstep distances and, second, surface morphology containing step bands along with anti-bands. The activation energy barrier for diffusion of Ce adatoms on the Si(111) surface was estimated from the dependence on temperature of the characteristic growth length.