At d2 in Glass-3D, prior to the addition of BMP, a polarized pluripotent (SOX2+) cyst structure has formed, and a small central lumen is visible (Figure 1C – 0 min); this structure is similar to that of the developing epiblast immediately prior to the initiation of amniogenesis (Shahbazi and Zernicka-Goetz, 2018; Taniguchi et al., 2019). This evidence concerns the gene SOX2 and cyst.