FIGURE 1:

Cleavage pattern in a bispermic frog egg. Metaphase spindles in Xenopus are ∼50–60 μm long in a 1.2-mm egg. After anaphase onset, the small asters at the spindle poles grow steadily until they fill the cell. Asters from the two poles of the same spindle (sister asters) contact each other soon after anaphase onset at the plane previously occupied by the metaphase plate. Asters from the poles of two different spindles (nonsister asters) contact each other later. In both cases, aster growth halts where the asters contact each other, and a zone of lower microtubule density is observed at the aster–aster interaction zone (gray shading). When asters grow to touch the cortex, furrows initiate between sister aster pairs but not between nonsister pairs. Adapted from Brachet (1910), Wühr et al. (2009), Snook et al. (2011), and Mitchison et al. (2012).