Embryogenesis![Figure][1] Loss of cleavage symmetry in the early embryo PHOTO: E. KOROTKEVICH ET AL., DEV. CELL 40 , 235 (6 FEBRUARY 2017) © ELSEVIER For the first few cleavages, cells of the mammalian embryo are considered identical. Subsequently, symmetry is broken to generate the inner cell mass, which specifies the embryo proper, or the extraembryonic trophectoderm. Korotkevich et al. used live imaging to visualize miniblastocysts generated from single 8-cell–stage blastomeres. The mitotic spindles of mini-blastocysts aligned along the apico-basal axis. The polar cell containing the apical domain, which expresses Cdx2, enveloped the apolar sister to adopt a trophectoderm fate. Transplantation of the apical domain showed that it is required and sufficient for the first lineage segregation. It is not yet known whether surface adhesion molecules or mechanical differences affect the assembly of the apical domain for subsequent lineage segregation. Dev. Cell. 40 , 235 (2017). [1]: pending:yes