We have followed by scanning tunneling microscopy (STM) the Stranski-Krastanov (SK) growth of thin Ge films obtained by reactive deposition epitaxy on a Si(111) surface. For Ge thickness smaller than 1.2 ML, STM images show large flat regions (reconstructed 7 x 7) without protrusions. With increasing thickness Ge-Si 2D islands (reconstructed 5 x 5) start nucleating, while the Si substrate retains its original 7 x 7 reconstruction. The islands are flat and have a triangular shape with a lateral size which increases progressively with deposition. We have studied the growth law of the average dimensions of the islands and of the average number of islands per unit surface as a function of coverage. STM images suggest a sizeable process of intermixing at the Ge/Si interface, which has been directly confirmed by XAFS (X-ray absorption fine structure) spectra on Ge/Si(111) samples. We have followed the evolution of the wetting layer up to its completion, both after and during deposition. The appearance of a percolated structure is observed when a critical fraction of the surface (about 70%) is covered.