Methods\ Each superovulated female of 615 mice was placed individually with a stud male and was checked the next morning.

• 方法采用经超量排卵的615雌鼠与正常雄鼠交配,取受精卵,再经手术分别将单胚卵、双胚卵移入发情的假孕雌鼠的输卵管中,妊娠产仔。