http://agathe.gr/democracy/the_council_and_magistrates.html
The Council and the Magistrates Like selection for military service, allotment to the Council was organized according to the division by tribes; 50 members from each tribe acted as a unit in the Council ... The Council also supervised the election of the generals, one from each of the 10 tribes, who were the chief executive officers of the administration, replacing in this function the archons, whose power decreased in the early fifth century when they began to be chosen by lot.