When thresholds are to be determined in physiological experiments, the most common method used today is to increase or to decrease the stimuli gradually. In view of the aftereffects of stimulation, it is obvious that existing procedures require modification. Recently, Ueda 1) reported that even in the generation of impulses of the mechanoreceptors the threshold determined by increasing mechanical stimuli differs from that determined by decreasing stimuli. Moreover current procedures are especially inadequate for experiments in psychology and physiology of sensations. For this reason "the method of constant stimuli" has been generally used in experimental psychology. The same phenomena apply equally in the case of electric stimulation. Accordingly, an electronic device was constructed capable of sustaining a predetermined duration of rectangular pulses in random sequences of equal-step strengths. The present paper will describe the simplest but basic set as well as the principles involved in this device. According to the table of random sampling numbers, pulses from 0 to 9 are recorded on a sound recording tape digitally. These pulses change the position of the glow-discharging cathodes of a dekatron which activates one of the relay-switches of the output-dividing network of an ordinary electric stimulator.
In order to procure thorough knowledge upon the development of an organ, besides the histogenesis and the development of the external form, the growth pattern of the organ should also be taken into account. Up to the present time, most investigators 1) have dealt with the occluso-apical growth of dental germs and that only in regard to the hard tissues, i.e. enamel and dentine. Although the mesio-distal extension of calcified tissue has been measured 2), that of the epithelial part has never been considered. The aim of the present study is, therefore, to decide whether, (in the growth pattern of the incisor and canine germs), the mesio-distal growth of the epithelial part of the dental germs takes place evenly during the course of the fetal period or whether it occurs at certain developmental stages.