Regression of the PSM domain in the chick tail bud. Lateral view of whole-mount chick tail buds at (A, F, K) HH stage 20, (B, G, L) stage 22, (C, H, M) stage 24, (D, I, N) stage 26 and (E, J, O) stage 27 analysed by in situ hybridisation with (A-J) c-Tbx6 and (K-O) c-Dact2/c-MyoD showing the position of the PSM or the first somite, respectively. (F-J) Sections of tail buds stained with the c-Tbx6 probe; the borders of the PSM domain (p) and the last somite (s) are demarcated. (P) Bar chart showing the size of the PSM (green bars) and the size of the last somite (red bars) at HH stages 10-25. (Q) Bar chart showing the PSM/last somite ratio during the same window of development calculated separately for each embryo and then the average ratio was calculated for each stage. Error bars represent standard deviation.