The income of the mutual trust investment company shall be computed on the accrual basis and the apportionment of income shall be determined at each valuation date. The income shall be distributed to shareholders not less frequently than quarter-annually, either on the basis of income accrued or on the basis of income actually received. To facilitate the distribution of accrued but uncollected income, the cash principal of the mutual trust investment company may be used to the extent necessary to purchase income accrued.