Fig. 2. Isolated spherical blastomeres divide differently from cells in the embryo and often bisect a small apical surface. (A) Example of a colour-coded frame from a time-lapse movie that shows the three possible spindle orientations in the embryo, quantified for the 128- to 256-cell division in 10 embryos (n=447 divisions). Par., parallel; Per., perpendicular; Obl., oblique, as defined in the Materials and methods. (B) These three types of spindle orientation occur also in isolated blastomeres at 128-cell stage, as example frames from Movies 1-7 in the supplementary material show; however, the proportion of each type of division differs. (C) Comparison of the proportions of the three division types between the embryo (white bars) and isolated blastomeres, n=151 (black bars). Oblique and perpendicular divisions have significantly different proportions in isolated cells when compared with the embryo (-square goodness of fit, P=0.001). (D) In control embryos, cells with a very small apical surface (less than or equal to one quarter of its sister) will always divide perpendicularly and the apical pigment is not divided. Arrows in D indicate cells with a small apical domain. (E,F) In isolated blastomeres (E) and in embryos that were raised in Ca²⁺/Mg²⁺-free medium (D), such small apical surfaces do become divided. (F) Comparison of frequencies with which the cleavage plane divides a small apical surface. In control embryos: 0% (n=42 divisions in time-lapse movies of 24 embryos). In embryos raised in Ca²⁺/Mg²⁺-free medium, with rounded cells: 32% (n=60 cells with small apical surface were analysed in 12 different embryos).