Using an experimental setup with a three-phase intersecting boundary of PTFE/PE/iPP, the nucleation power of PTFE compared to iPP on the PE was studied by TEM. It was found that the nucleation of the PE on the PTFE interface started at a higher temperature than epitaxial nucleation of the PE onto the iPP interface. During cooling of the melt, the growth direction of the PE crystalline lamellae changes in a continuous manner from the transcrystallization direction of the PTFE/PE interface into the heteroepitaxial "crosshatched" orientation of the iPP/PE interface. A (still highly speculative) self-assembly of the PE macromolecules at the respective interface just in front of the actual crystallization edge is used to explain this observed phenomenon.