Fig. 3. Cleavage of blastomeres in culture from wild-type and it143 mutant embryos. (A) The regular helical pattern of cleavages exhibited by AB cells separated from a cultured four-cell de-vitellinized embryo (egg shell removed) (Edgar and Wood, 1993). (B,C) DIC images of such an embryo (arrows indicate AB cells) at the four- and six-cell stages, respectively. The EMS and P₂ (arrowhead) cells have been left in place to provide positional markers. In C, images at two focal planes have been superimposed to show positions of the four AB blastomeres. (D) Example of the more random cleavage patterns seen in cultured it143 mutant AB cells. (E) DIC image of AB cells from a mutant embryo. Two AB cells isolated from a de-vitellinized four-cell embryo have divided with their spindles oriented in approximately the same direction as those in the previous cleavage, to give an almost linear arrangement of four AB cells. Similar results were obtained in 10 independent experiments (see text).