The trust institution shall invest the moneys of an estate (whether the estate is administered by such trust institution or by an affiliated trust institution) in such common trust fund by adding the same thereto and by apportioning a participation therein to such estate in the proportion that the moneys of the estate added thereto bears to the aggregate value of all the securities of such fund at the time of such investment, including in such securities the moneys of the estate so added. The withdrawal of a participation of such common trust fund shall be on the basis of its proportionate interest in the aggregate value of all the securities of such fund at the time of such withdrawal, as hereinafter provided. The participating interest of any estate in such common trust funds may, from time to time, be withdrawn in whole or in part by the trust institution administering the estate and shall be withdrawn within a period of three months following the written request to do so of any person acting with the trust institution administering the estate in a fiduciary capacity. Funds for the purpose of any withdrawal shall be made promptly available by the trust institution through sale of securities of the common trust fund. Upon such withdrawals, the trust institution may make distribution in cash or ratably in kind or partly in cash and partly in kind; provided, that all such distribution as of any one time shall be made on the same basis.