The costs of the blood or other tests required by the court and the compensation of each expert witness appointed by the court shall be fixed at a reasonable amount. It shall be paid as the court shall order. The court may order that it be paid by the parties in such proportions and at such times as it shall prescribe, and that, after payment by either of the parties or both, all or part or none of it be taxed as costs in the action. The fee of an expert witness called by a party but not appointed by the court shall be paid by the party calling him but shall not be taxed as costs in the action.