Any physician or dentist who meets the qualifications for a reserve commission in the respective military departments shall, so long as there is a need for the services of such a physician or dentist, be afforded an opportunity to volunteer for a period of active duty of not less than twenty-four months. Any physician or dentist who so volunteers his service, and meets the qualifications for a reserve commission shall be ordered to active duty for not less than twenty-four months, notwithstanding the grade or rank to which such physician or dentist is entitled under the provisions of the Act of September 9, 1950, as amended.