Movie 3.

Late gastruloid elongation in the presence of a MEK inhibitor. Equivalent experimental setup to Movie 2, except here gastruloids were pre-treated in MEK inhibitor from 96 to 108 h, then were embedded in 50% Matrigel / 50% N2B27 at 108 h where they were again supplied with MEK inhibitor in the surrounding media. Gastruloids were live-imaged from 109-120 h, with images acquired every five minutes. Posterior is oriented towards the bottom of the frame. This timelapse was acquired simultaneously with Movie 2.