Studies of experimentally manipulated embryos have led to the long-held conclusion that the polarity of the mouse embryo remains undetermined until the blastocyst stage. However, recent studies reporting that the embryonic-abembryonic axis of the blastocyst arises perpendicular to the first cleavage plane, and hence to the animal-vegetal axis of the zygote, have led to the claim that the axis of the mouse embryo is already specified in the egg. Here we show that there is no specification of the axis in the egg. Time-lapse recordings show that the second polar body does not mark a stationary animal pole, but instead, in half of the embryos, moves towards a first cleavage plane. The first cleavage plane coincides with the plane defined by the two apposing pronuclei once they have moved to the centre of the egg. Pronuclear transfer experiments confirm that the first cleavage plane is not determined in early interphase but rather is specified by the newly formed topology of the two pronuclei. The microtubule networks that allow mixing of parental chromosomes before dividing into two may be involved in these processes
