The kinetics of the growth mode transition from two-dimensional to islanding growth during Ge/ Si(001) heteroepitaxy have been investigated by a combination of in situ reflection high-energy electron diffraction, atomic force microscopy, and photoluminescence spectroscopy. It is found that the two-dimensional wetting layers undergo a morphological instability well before reaching the critical thickness. The layer instability appears to be strain driven and gives rise locally to the formation of intermediate clusters between the wetting layers and macroscopic islands. We provide evidence that such intermediate clusters are metastable both in view of structural and optical properties.