[Each such census of inhabitants required in section one shall likewise be the basis for determining the senatorial districts and also the councillor districts for the ten year period beginning with the first Wednesday in the fourth January following the taking of such census; provided that such districts as established based on the census in the year nineteen hundred and seventy-one shall terminate on the first Wednesday in January in the year nineteen hundred and seventy-nine.] The Senate shall consist of forty members. The General Court shall, at its first regular session after the year in which said census is taken, divide the Commonwealth into forty districts of contiguous territory, each district to contain, as nearly as may be, an equal number of inhabitants according to said census; and such districts shall be formed, as nearly as may be, without uniting two counties, or parts of two or more counties, into one district. The General Court may by law limit the time within which judicial proceedings may be instituted calling in question such division. Each district shall elect one senator, who shall have been an inhabitant of this Commonwealth five years at least immediately preceding his election and at the time of his election shall be an inhabitant of the district for which he is chosen; and he shall cease to represent such senatorial district when he shall cease to be an inhabitant of the Commonwealth. The manner of calling and conducting the elections for the choice of senators and councillors, and of ascertaining their election, shall be prescribed by law.