Analysis of the localization and number of NMPs. Chicken embryos from stage 4 to 20 HH were harvested and fixed in 4% PFA at 4C overnight. Embryos were immunostained in whole mount using T/BRACHYURY, SOX2 and Alexa Fluor™ 647 Phalloidin (1/500 Thermo Fisher: A22287) combinations and imaged by confocal laser microscopy (LSM 780, Zeiss). SOX2/T double positive cells were segmented and counted using image J software. To identify the double positive cells, cells in the T and in the SOX2 channel were thresholded. We used the image calculator tool from ImageJ, to generate a new image by performing the image operation T AND SOX2 and identify the double-positive cells. Embryos were grouped by stage and **data** is shown in Figure 4a. n=7 at stage 4HH; n=11 at stage 5HH; n=6 at 5-10 somites; n=5 at 15-20 somites; n=5 at 30 somites; n=4 at 35 somites and older. Total n=38 embryos.