Re-analysis of publicly available time-lapse imaging datasets and proposal for a new model of first zygotic cleavage. (A) Histogram shows the angle (°) of departure of the first cleavage axis from the shorter diameter versus A–V axis of zygotes in three mammalian species. Angle (ordinate) versus species (abscissa) is shown for each scored oocyte of each study (note the symbols to the right side of the histogram). The angle of first cleavage is closer to the shorter diameter than to the A–V axis (mouse, ****P < 0.0001; human, *P = 0.0136; bovine, ns, P = 0.2391; Wilcoxon test). (B) Summary of the previous models of first zygotic cleavage (based on post-zygotic observations) compared to our proposed model (based on pre-zygotic observations). A–V, animal-vegetal; PB2, second polar body; MII, metaphase II.