Fig. 1. Pre-implantation development of the mouse embryo from the zygote to the blastocyst stage. (A) Fertilised egg with visible female (pink star) and male (blue star) pronuclei and polar body marking the animal pole (red asterisk). (B) Two-cell stage embryo. (C) Eight-cell stage embryo. (D) Early blastocyst showing its animal-vegetal (yellow) and embryonic-abembryonic (blue) axes. (E-G) Expanding blastocyst seen from three different perspectives to show its bilateral symmetry. (E) View from the side, focused on plane of bilateral symmetry to show the long axis (yellow) of the ICM. (F) View from the animal pole to show the short axis (green) of the ICM. (G) View from the embryonic pole to show both long and short axes of the ICM. Scale bar: 25 µm.