Development of between-limb movement synchronization in the chick embryo.

Spontaneous between-limb movement synchronization is described as the amount of concurrent limb movement observed in 7--19-day chick embryos. At early stages, a wing moved as often with the ipsilateral leg as with the contralateral wing. Later, between-girdle (ipsilateral wing-leg) synchronization progressively decreased and within-girdle (wing-wing) synchronization increased, especially after 15--17 days. Bilaterally synchronized movements of the wings as in flapping and of the legs as in walking appeared at embryonic stages. Both wing and leg motility increased between 7 and 13 days and declined until hatching. The wings and legs were equally active except at 17 and 18 days when the wings moved more frequently than the legs. The left and right wings were equally active. These behavioral events reflect developmental changes in the motor outflow of the central nervous system.