Electromyograms (EMG's ) of developing chick embryos were recorded with an electrolytically sharpened tungsten electrode and with a concentric unipolar electrode.
Earlier observations of the occurrence of movements on the sixth day were confirmed. Before the twelfth day, only small irregular potentials could be obtained from the muscles (amplitude less than 50 pV). After the twelfth day, the amplitude of the action potentials (potentials) increased rapidly. Up to the fifteenth day, the potentials were mainly monophasic (amplitude 150 p V ) , On the seventeenth day, the amplitude reached about 500 p V and the potential configuration was similar to that of the adult. The duration of the potentials did not change significantly during this period.
The EMG activity occurred in bursts. The duration of the bursts from the wing muscles tended to be less than one second whereas in the leg muscles it was generally 2-5 seconds. This difference in burst duration was systematically studied on six embryos aged 14-17 days and its functional significance was discussed.
To obtain a rough estimate of the maximal firing frequency of the motor units, the '*overall frequency" of the potentials was measured. A steep increase in frequency was found between the twelfth and fifteenth day and a n interference pattern of the adult type was obtained on the seventeenth day. The temporal correlation between these changes in the EMG pattern and other properties of the developing muscles was discussed.