Fig. 1. A schematic representation of the origins of blastocyst pattern in the first two cleavages of the mouse embryo. Orientation of the first cleavage, and the order and orientation of the second cleavage divisions in both two-cell stage blastomeres affect the spatial pattern of allocation of their progeny at the blastocyst stage. Two groups of embryos are presented: ME, in which the earlier second cleavage is an M-division the later an E-division; and EM, in which the earlier second cleavage is an E-division and the later one an M-division. Taken together, both of these groups account for 80% of all embryos in our studies (Piotrowska-Nitsche and Zernicka-Goetz, 2005). In both of these groups of embryos, there is a strong tendency for the one of the daughter cells to contribute most of its progeny to the future embryonic part and for the other to the future abembryonic part of the blastocyst. However, only in ME embryos does the early dividing blastomere preferentially contribute its progeny to the embryonic part. In EM embryos, the earlier dividing blastomere can equally often contribute to the embryonic or abembryonic part (hence two possible outcomes are presented).