To clarify the relation between the physical growth and the motor development, 1230 infants between the ages of 1 and 20 months were taken measurements of their height and weight, and tested for the development of their activity. Followings are the results obtained.
(1) The percentage of infants who had acquired the activity arised higher with the growth of weight and height, showing a rapid increase in percentage near such a value of physical growth at which 50% of the infants had acquired the activity
(2) The higher became the stage of motor development, the larger was the difference in the values of physical growth during which the percentage of the infants who had acquired the activity came to 100% from 50% than that in the values during which the percentage came to 50% from 0%.
(3) The higher became the stage of motor development, the larger became the number of the infants who failed to acquire the activity in spite of their physical growth being in higher level, suggesting more complicated mechanism of the acquisition of activity
