(xxviii)"policy of group insurance" means any instrument covering, not less than fifty or such smaller number as the Central Government may approve, either generally or with reference to any particular case, by which an insurer, in consideration of a premium paid by an employer or by an employer and his employees jointly, engages to cover with or without medical examination and for the sole benefit of persons other than the employer, the lives of all the employees or of any class of them, determined by conditions pertaining to the employment, for amounts of insurance based upon a plan which precludes individual selection;
