The results of three experiments with 172 pupils of the first to the ninth grade, with the application of psychological and physiological methods, show that the ability of children to differentiate stimuli of different signalling significance (which can be regarded as the physiological basis of the capacity to comprehend) passes during the perioď of compulsory school attendance through two periods of sudden rise and two periods of relative calm. A simple coefficient derived for this purpose (whose zero and negative values testify to a dialectical development, values significantly higher than zero to a gradual development of the given function) confirms that the mental development of the child is governed by dialectical laws.