The formation of dislocation clusters in cast mono ingots is investigated using etch pit density measurement and X-ray diffraction. The majority of detrimental dislocation clusters in the ingot originate in the seeding region. Beside the well-described dislocation source at the seed joints, a second origin of dislocation clusters is described in this work. Initial dislocation clusters appear in the seed by a rearrangement of dislocation cells to dislocation domains. It is demonstrated that a cell size below ca. 50 mym is required for this process. The dislocation density in these domains is about one order of magnitudes higher than in the cells. The high local dislocation density gains a very high shear stress. In consequence, the domain is able to penetrate the phase boundary between the remaining seed and the seeded material. Thus, the cluster is spreading unhindered into the ingot under the influence of thermo-mechanical stress during crystallization. X-ray diffraction measurements prove that the plastic deformation of the seeds leads in particular to such small cell sizes.