Bissected chick or mouse caudal explant pairs treated ‘−’ or ‘+’ inhibitor (A, C, E, G) or treated with inhibitor and subjected to the fix and culture assay (B, D, F, H) and then analysed by in situ hybridisation for Lfng mRNA expression : (A, C, E, G) Treatment of chick PSM explants in the presence (+) or absence (−) of XAV939 (A), Roscovitine (C), DRB (E) and PHA7667491 (G) for 3 hr reveals that ‘+’ explants have lagging expression of cLfng, often with one less somite formed than the ‘−’ explants. (B, D, F, H): After 3 hr treatment in the presence of XAV393 (B), Roscovitine (D), DRB (F) and PHA767491 (H), one chick PSM explant was fixed while the other was treated for another 45 min, showing that cLfng expression is still dynamic in the presence of these inhibitors. (I, K): Mouse PSM explants treated in the presence or absence of XAV939 (I) or Roscovitine (K) for 4 hr revealed a delay in the oscillations of mLfng expression. (J, L): Treatment of one mouse PSM explant for 4 hr, and the other for 5 hr in the presence of XAV939 (J) or Roscovitine (L) reveals that mLfng mRNA expression is still oscillating in the PSM. The red arrowheads identify the somites that have formed during the in vitro culture period of the assay. (M) Schematic representation of the expression domains of Lfng in the PSM in the three different phases of one oscillation cycle. S1, SII = the most recently formed somite. (P) = previous cycle.