Injecting embryos at different stages. (A) To completely label an embryo, inject (asterisks) both blastomeres of the two-cell embryo or (B) all four cells of the four-cell embryo. (C) Position embryos in a depression in the injection dish so that the blastomere to be injected (red arrows) is facing the micropipette (not shown, coming in from the right at a ∼45° angle). (D) The micropipette (dashes) should cause only a small puncture hole that darkens into a spot as the membrane heals (red arrow).