The growth of platinum shell crystals on the heterogeneous facets of ruthenium metal nanocrystals was investigated via in situ small angle X-ray scattering analysis. Our results illustrated that the pathway of shell crystal growth is mainly affected by controlling the incubation time prior to the thermal reduction of Pt ions by polyol radicals. For short incubation times, the shell crystal grows on the <111> radial facets and the <200> face facets of the nano-cylinder core via heterogeneous nucleation and crystal growth with and without assistance by transmetallation, respectively. On the other hand, severe chemical etching was observed prior to shell crystal growth after incubation for 2 hours. Consequently, the shell structure is formed from the nucleation of a PtRu alloy followed by the growth of Pt crystals. In both cases, dynamic restructuring at the core-shell interface was observed upon deposition of the shell crystal. These results enable the development of core-shell crystallites in expected configurations and with tuneable atomic structures.