The qualitative and quantitative changes in behavioral development in the prenatal and perinatal periods have been insufficiently studied. We observed the occurrence of spontaneous body movements in three strains of mice (ICR, C3H, and BALB) and the pulse counts measured by EMG activity gestation period normal for mice. During the prenatal period, fetal movements changed from simple movements to complex and coordinated movements. Differences in the timing of the development of these movements were not found in the three strains of mice. During the perinatal period, the three kinds of simple movement decreased. This decrease was affected by the onset of pulmonary respiration after delivery. The time span of spontaneous movement did not change during the perinatal period, while the pulse counts measured by EMG activity increased with age.

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