Researchers injected sea urchin sperm nuclei into eggs to study male pronucleus formation. They found that two steps are needed for this process. First, the nuclei need to partially decondense, which happens during egg maturation and can be blocked by a specific chemical. Second, the cytoplasm needs to become more alkaline, but not due to an increase in calcium or activation of a specific pathway. This study helps us understand the specific conditions required for male pronucleus formation in sea urchin eggs.