We use low-energy electron microscopy (LEEM) to study Si growth on {105} faceted Ge islands on Si (100). A quantitative analysis of LEEM growth sequences suggests, that for low Si coverage no growth occurs at the island apex, and that adatoms in the apex region are collected onto the (105) side facets. This ＇＇partial wetting" growth mode leads to a characteristic formation of a large (100) plane bounding the top of the islands. We find Ge surface segregation on the {105} facets to play an important role in the overgrowth process : Ge/Si intermixing leads to a stabilization of the facet structure over a large range of Si coverage and results in a shape of buried islands mapping their morphology during overgrowth : a truncated pyramid, bounded on top by a (100) plane.