All vacancies in the office of the commissioner that occur while the Legislature is not in session shall be filled by appointment by the Governor, which appointment shall expire at the end of thirty days after the date of which the Legislature next convenes. Prior to the expiration of the thirty days the Governor shall transmit to the Senate for its approval an appointment for the unexpired portion of the regular term. Vacancies occurring during a session of the Legislature shall be filled as regular appointments before the end of the session and for the unexpired portion of the regular term.