Fig. 2. Allocation to specific blastocyst regions of the progeny of blastomeres with vegetally marked membrane. (A) A lateral view of a two-cell stage embryo with one two-cell stage blastomere labelled with red dye and the other two-cell stage blastomere with a bead attached at its vegetal pole. pb, polar body. (B) Polar view of a three-cell stage embryo in which the blastomere labelled with dye has divided first through a meridional plane. (C) A four-cell stage embryo in which the dye labelled two-cell blastomere has divided meridionally and its sister two-cell blastomere undertakes an E-division. The bead remains in a `vegetal' position (on the e2 cell), indicating that membrane has not been displaced during division. The cell carrying the bead (which we term e2+) is then labelled with a blue dye (blue arrow) and allowed to develop to the blastocyst stage. (D) Blastocyst derived from the embryo in C showing contribution of the blue labelled cells to mural trophectoderm. (E) A similar four-cell stage embryo to that shown in C in which the bead is now in a position proximal to the polar body (on the e1 cell) indicating that vegetal membrane has been displaced during division. The e2 cell that does not carry the bead (that we term e2-) is then labelled with a blue dye and allowed to develop to the blastocyst stage (F) Blastocyst derived from the embryo in E, showing contribution of the blue labelled cells to the boundary zone. Scale bar: 10 µm.