Any cooperative savings and loan association or any building and loan association organized and existing under the laws of this territory, shall have the power to make loans upon the security of first mortgages on real estate, without the requirement that shares of such association be pledged as additional security for such loans. Such loans shall be repayable in monthly installments, beginning not later than sixty days after the date of the advance of the loan, sufficient to retire the debt, interest and principal, within twenty-five years; provided that in case of construction loans the first payment shall not be later than four months after the date of the first advance. The monthly payments shall be applied first to interest on the debt and the remainder to the reduction of the debt until the same is paid in full.