Cell fusion is a fundamental biological process that can be artificially induced by different methods. Although femtosecond (fs) lasers have been successfully employed for cell fusion over the past few years, the underlying mechanisms are still unknown. In our experimental study, we investigated the correlation between fs laser-induced cell fusion and membrane perforation, and the influence of laser parameters on the fusion efficiency of nonadherent HL-60 cells. We found that shorter exposure times resulted in higher fusion efficiencies with a maximum of 21\% at 10 ms and 100 mJ/cm2 (190 mW). Successful cell fusion was indicated by the formation of a long-lasting vapor bubble in the irradiated area with an average diameter much larger than in cell perforation experiments. With this knowledge, we demonstrated, for the first time, the fusion of very large parthenogenetic two-cell porcine embryos with high efficiencies of 55\% at 20 ms and 360 mJ/cm2 (670 mW). Long-term viability of fused embryos was proven by successful development up to the blastocyst stage in 70\% of cases with no significant difference to controls. In contrast to previous studies, our results indicate that fs laser-induced cell fusion occurs when the membrane pore size exceeds a critical value, preventing immediate membrane resealing.