Section 21. There shall be in the department of the state secretary, but in no way subject to his control, a commission on interstate co-operation, in this and the four following sections called the commission. The commission shall consist of nine members of the general court, of whom three shall be members of the senate designated by the president thereof and six shall be members of the house of representatives designated by the speaker thereof, one of the commissioners on uniform state laws designated by said commissioners, the director of the division of planning of the department of commerce, and four persons, who may be state officials, appointed by the governor. Members appointed by the governor shall serve at his pleasure. Each member of the senate or house of representatives designated as a member of the commission shall hold office for the term of four years, unless, prior to the expiration of such term, he shall cease to be a member of said senate or house of representatives; and, in any such event, the resulting vacancy shall be filled by designation, for the remainder of the unexpired term, made in the same manner as an original designation. The chairman and the vice-chairman of the commission shall be elected by the commission. The members of the commission shall serve without compensation but shall be paid their necessary expenses incurred in the performance of their duties. The written approval by the chairman or vice-chairman and four other members of the commission of bills or vouchers for necessary expenses incurred by the commission shall be sufficient to authorize the comptroller to certify such bills or vouchers. The commission may employ a secretary and such other assistants, including technical experts, as may be required in the performance of its duties.