In the beginning of the study, there were only three periods, the early period, the middle period, and the end period; however, the earlier period and the last period were added due to the increase in available materials and the progress of research and the incipient period was added lastly.
研究当初は、前・中・後の三期区分だったが、資料の増加や研究の進展によって早期、晩期が加わり、最後に草創期が加えられた。