Extended Data Figure 8. The onset of gastrulation is normal in the acellular embryos.

(a) Immunostaining of mesoderm determinant Snail in the acellular and control embryos fixed at early cellularization, late cellularization or early gastrulation. The pattern of Snail expression in the acellular embryos closely resembles that in the wild type embryos. At early cycle 14, the Snail proteins are clearly detectable in the prospective mesoderm. The staining appears graded towards the mesoderm/ectoderm boundary at this stage. At mid-cycle 14 and early gastrulation, the staining becomes uniform across the entire prospective mesoderm. Scale bar: 50 μm. (b) Quantification of duration between beginning of cycle 14 and the onset of gastrulation. On each box, the central mark (red) is the median, the edges of the box are the 25th and 75th percentiles, and the whiskers extend to the most extreme data points not considered outliers. (c) Apical myosin dynamics visualized using Sqh-GFP after the onset of gastrulation (t = 0 min). Scale bar: 30 μm. (d) Scanning EM images showing the ventral surface of the wild type and acellular embryos. Bottom panels show the enlarged view of the boxed regions in the top panels. Membrane blebs are formed in the ventral surface of the acellular embryos, indicating that apical constriction still gathers surface membrane into blebs despite the lack of cells. Scale bar: 50 μm (top); 10 μm (bottom).