Fig. 4. Construction of chimaeras of individual four-cell stage blastomere types. ME embryos were selected in which the later dividing two-cell blastomere had been labelled at its vegetal pole as described in the legend for Fig. 3. (A) One such embryo that has been subjected to limited digestion with pronase in order to thin the zona pellucida (red arrow) so that it continues to maintain the tetrahedral morphology of the embryo and yet does not provide any resistance for micromanipulation of the individual blastomeres. (B) The embryo is held in the left-hand pipette by a m-blastomere while the e2 blastomere is withdrawn into the pipette on the right. (C,D) The procedure is repeated to remove an e1 blastomere into the right-hand pipette. (E,F) One of the m blastomeres, which is attached to the polar body, is then withdrawn into the right hand pipette. (G) The completed dissection showing all four four-cell stage blastomeres, the second polar body is still attached to one of the m cells. Scale bar: 60 µm. (H) An aggregate of e2 blastomeres from three such dissections. (I) The same aggregate as shown in H viewed under fluorescence optics to show the fluorescent beads used to label the vegetal poles. Scale bar: 18 µm in A,H,I; 33 µm in B-F.