Effect of acute hypoxia (5% O2 or 10% O2) for 20 or 40 min on the duration of the activity phases (open bars) and inactivity phases (solid bars) of chick embryonic motility and post-hypoxia 30 min recovery in air on days 10 (A, B), 12 (C, D), 14 (E, F) and 15 (G, H) of incubation. Data are presented as averages for 20 min prior to hypoxia, for 20 or 40 min during hypoxia and for 30 min during recovery in air. Results are expressed as means ± S.E.M. N is the number of embryos studied. (*) - significant difference compared with valaues in normoxia (control) (P < 0.05; Wilcoxon test); (#) - significant difference between valaues for 20 and 40 min of hypoxia (P < 0.05; Wilcoxon test).