(a) Sketches showing mouse embryo anatomical landmarks used for staging between Early Streak and Early Bud stages. (b) Quantification of embryonic and extra-embryonic mesoderm cell division events (mean ± SEM, n=4 embryos, p=0.0286, Figure 1—figure supplement 1—source data 1.). Cells were observed over an average of 360 min. Cells migrating close to the embryonic/extra-embryonic interface were not taken into consideration. (c) Ex vivo growth of an embryo dissected at E6.75 (Early Streak stage, left panel), after imaging for 12 hr using confocal imaging plus 12 hr of subsequent culture (Zero Bud stage, right panel). (d) Ex vivo growth of an embryo dissected at E7.25 (Late Streak stage, left panel), after 8 hr two-photon imaging (Early Bud stage, right panel). (e) Transverse section at primitive streak level of a E7.5 (Late Streak stage) T-Cre; mTmG embryo stained for F-actin (Phalloidin, red) and nuclei (DAPI, blue) (scale bar: 50 μm). (f) Quantification of mosaicism as the ratio of GFP + cells/all cells in the mesoderm layer (identified anatomically between the epiblast and the visceral endoderm) in anterior, posterior and lateral quadrants at different stages of gastrulation: E7 (Mid Streak stage) and E7.5 (Early Bud stage) (mean ± SEM, n=5 embryos for each stage). P values were calculated using the Mann–Whitney–Wilcoxon. A: Anterior, P: Posterior, Pr: Proximal, Di: Distal, mGFP: membrane GFP, in green.