The empire was divided into provinces (kuni) of four classes--great, superior, medium, and inferior,--and each province was subdivided into districts (kori) of five classes--great, superior, medium, inferior, and small. The term "province" had existed from remote antiquity, but it represented at the outset a comparatively small area, for in the time of the Emperor Keitai (A.D. 507-531), there were 144 kuni. This number was largely reduced in the sequel of surveys and re-adjustments of boundaries during the Daika era (645-650), and after the Daiho reforms (701-704) it stood at fifty-eight, but subsequently, at an uncertain date, it grew to sixty-six and remained permanently thus. The kori (district) of the Daika and Daiho reforms had originally been called agata (literally "arable land"), and had been subdivided into inaki (granary) and mura (village). A miyatsuko had administered the affairs of the kuni, holding the office by hereditary right, and the agata of which there were about 590, a frequently changing total as well as the inaki and the mura had been under officials called nushi. But according to the Daika and Daiho systems, each kuni was placed under a governor (kokushi), chosen on account of competence and appointed for a term of four years; each district (kori) was administered by a cho (chief). MILITARY INSTITUTIONS In the capital there were three bodies of guards; namely, the emon-fu (gate guards); the sa-eji-fu and the u-eji-fu (Left and Right watches). There was also the sa-ma-ryo and the u-ma-ryo (cavalry of the Left and of the Right), and the sa-hyogo-ryo and the u-hyogo-ryo (Left and Right Departments of Supply). These divisions into "left" and "right," and the precedence given to the left, were derived from China, but it has to be observed in Japan's case that the metropolis itself was similarly divided into left and right quarters. Outside the capital each province had an army corps (gundan), and one-third of all the able-bodied men (seitei), from the age of twenty to that of sixty, were required to serve with the colours of an army corps for a fixed period each year. From these provincial troops drafts were taken every year for a twelve-month's duty as palace guards (eji) in the metropolis, and others were detached for three-years' service as frontier guards (saki-mori) in the provinces lying along the western sea board.