Movie 1.

Early gastruloid elongation. Gastruloids were embedded in 50% Matrigel/50% N2B27 in a 35 mm imaging dish at 95 h and imaged during the beginning of elongation (96-104 h). The sample was maintained at 37°C with 5% CO2 for the duration of the timelapse and imaged using differential interference contrast (DIC) microscopy with a 20x air objective. Images were acquired every two minutes. Posterior is oriented towards the bottom of the frame, as confirmed by endpoint Bra^GFP imaging.