The Senators shall be chosen by the qualified voters for the term of four years; but a new Senate shall be chosen after every apportionment, and the Senators elected after each apportionment shall be divided by lot into two classes. The seats of the Senators of the first class shall be vacated at the expiration of the first two years, and those of the second class at the expiration of four years, so that one half of the Senators shall be chosen biennially thereafter. Senators shall take office following their election, on the day set by law for the convening of the Regular Session of the Legislature, and shall serve thereafter for the full term of years to which elected.