Periodic motility of normal and spinal chick embryos between 8 and 17 days of incubation

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Abstract
Previous studies of the spontaneous cyclic motility of the chick embryo were extended to incubation day 17. The activity and inactivity phases of the cycle were recorded on a polygraph, for a 15‐minute observation period. The percentage of time spent in activity rises steadily from less than 10% at the beginning of motility at three and one‐half days to 80% at day 13. This peak is maintained up to day 17; subsequently, motility declines. The mean duration of activity phases increases and that of the inactivity phases decreases until the high plateau is reached, though both values change independently of each other. In order to study the effect of the brain on spontaneous motility, spinal embryos were obtained by extirpation of part of the cervical cord in two‐day embryos. The spinal embryos retain their capacity for cyclic motility. However, the activity is reduced by approximately 10–20% at all stages between the eighth and the seventeenth day. The brain influence asserts itself in a lengthening of the duration of the activity phases and in a shortening of the inactivity phases. An inhibitory effect was not observed at any stage.