Mo. Rev. Stat. § 128.020

When any new apportionment shall be made of the members to be elected to the house of representatives of the United States, whereby the number of electors to which this state may be entitled shall be increased or diminished, it shall be the duty of the governor to lay off the state into as many districts as shall be equal to the number of electors to which this state shall then be entitled, so that the said districts contain, as near as may be, an equal number of inhabitants.