The role of the actomyosin network is investigated in the elongation of C. elegans during embryonic morphogenesis. We present a model of active elongating matter that combines prestress and passive stress in nonlinear elasticity. Using this model we revisit recently published data from laser ablation experiments to account for why cells under contraction can lead to an opening fracture. By taking into account the specific embryo geometry, we obtain quantitative predictions for the contractile forces exerted by the molecular motors myosin II for an elongation up to 70% of the initial length. This study demonstrates the importance of active processes in embryonic morphogenesis and the interplay between geometry and nonlinear mechanics during morphological events. In particular, it outlines the role of each connected layer of the epidermis compressed by an apical extracellular matrix that distributes the stresses during elongation.