In contrast to stress induced post grown radial cracks or low angle grain boundary arrays another category of grown-in type macrodefects was found. They can be described as slits with a linear extension up to some millimetres and a width in the micrometer range. Their traces are visible in the basal plane, extended preferentially in <1<(1)over bar>00> respectively <11<(2)over bar>0> directions. The slits may penetrate the whole crystal along the growth direction. It could be shown that they are strongly correlated to the presence of polytype changes. The deterioration of the crystal perfection at the polytype borders generates bundles of dislocation walls, preferably at steps, mainly consisting of basal dislocations which are assumed to be involved with the slit generation. The induced dislocation walls may contribute to separate the material locally resulting in a macroscopic slit. The generation of slits is especially observed during the growth of 4H-crystals.