The first cell division in C. elegans is asymmetric. Asymmetric cell division requires correct positioning of the mitotic spindle. Prior to metaphase, the nuclear-centrosome complex, the precursor of the mitotic spindle, is positioned in the cell center. During anaphase, the spindle is displaced towards the posterior so that bisection of the spindle during cytokinesis leads to daughter cells of unequal sizes. Forces that center and position the spindle come from cortical force generators that pull on astral microtubules. In order to generate force, the cortex needs to provide a stiff anchoring platform. However, a role for the cortex in C. elegans has only been described with respect to polarity establishment. We perturbed the acto-myosin cortex by RNAi of non-muscle-myosin II (nmy-2) using conditions that allowed us to avoid disturbing polarity. Strikingly, in nmy-2(RNAi), membrane tubes are pulled from the plasma membrane into the cell. They were seen after RNAi against other actin cytoskeleton proteins and members of force generation complex, suggesting that the cortical force generators pull the invaginations, and a weakening of the cortex. As expected, we observed an increase in the variance of spindle position and orientation in nmy-2(RNAi). We used the oscillations of the centrosomes during anaphase as a reporter of spindle mechanics, and measured an increase in oscillations frequency but only a marginal decrease in amplitude. In order to understand this phenotype, we used our previously published model to analyze the results. Only by including the cortex into the model, we were able to fully describe the role of NMY-2. In summary, the occurrence of tubes after nmy-2(RNAi) strongly points towards a weakening of the cortex and the analysis of the spindle positioning suggests that the cortex provides a rigid platform for anchoring the force generators.