(a) Stages of preimplantation mouse embryo development. Axes of polarity are marked on the blastocyst. XY is known as the embryonic-abembryonic axis, as it starts in the inner cell mass, from which the embryo derives, and finishes at the floor of the blastocoel, from which extra-embryonic tissues originate. AB is the 'axis of bilateral symmetry', and corresponds to the anterior-posterior axis of the later embryo. The second polar body (not shown) lies at one end of this axis, showing that it is aligned with the so-called animal-vegetal axis of the zygote.
(b) Oil-drop mark points in the zona pellucida of the two-cell stage.