IT is well known that oocytes, in general condition, can be penetrated by sperm rather thanround spermatids. In 1993, Ogura injected round spermatid nuclei into hamster oocytes anddemonstrated that the spermatid nuclei could transform into pronuclei with DNA synthesis andthat their chromosomes mingled with oocyte chromosomes immediately. In order to avoid themechanical damage to the oocytes, an alternative method, cell-to-cell fusion, was used. Approximately 20%—40% and 30% of round spermatids could be fused with oocytes by electric...