Proliferation of a single parasegment over the course of 22.4 h ( b ). Initially unorganized ectoderm at 69.5 h AEL coalesce into a PSPR by 71.6 h AEL. Cells comprising the PSPR then divide along the A–P axis as part of a mitotic wave, shown partially completed at 73.3 h AEL, which originates at the ventral midline and spreads outwards until there are two complete rows of cells (79.0 h AEL), denoted as ‘a/b’ and ‘c/d’. Each of these two rows then undergoes a second mitotic wave, shown partially complete at 85.6 h AEL, until there are four complete rows, denoted as ‘a’, ‘b’, ‘c’ and ‘d’, reflecting the origin of each row from its predecessor at the two-row stage. After the second mitotic wave, rapid differential cleavage sets in near the ventral midline, in which cells divide isotropically (91.9 h AEL). The insets (right) highlight the behaviour of a single cell and its progeny throughout the mitotic waves.
