Fig. 2. The early dividing two-cell stage blastomere contributes to the embryonic part of the blastocyst. Blastomeres of two-cell embryos were labelled with DiI or DiD (left hand micrographs; A,D). At the three-cell stage it was recorded whether the earlier dividing cell had been marked with the former (red, upper panel, B) or latter (blue, lower panel, E) dye. Embryos were then allowed to develop to the blastocyst stage (right hand micrograph; C,F). The blastomere that divided first to the four-cell stage gave rise to cells in the embryonic part in 49/56 (88%) embryos in this series of experiments. Scale bar: 25 µm.