Associations may become members of the Federal Home Loan Bank system, may subscribe for and invest in stock of the Federal Home Loan Bank, and may secure insurance of its thrift accounts and creditor liabilities from the Federal Savings and Loan Insurance Corporation, and may insure their mortgages through such federal agency or agencies as may be available. They may borrow money from any source to meet withdrawals or extraordinary demands, to make real estate loans to their members or for other corporate purposes and may pledge its securities for such loans, but the aggregate amount of the outstanding indebtedness shall not at any time exceed 25 percent of the outstanding assets of an association not a member of the Federal Home Loan Bank and not to exceed 50 percent of the assets of an association which is a member of the Federal Home Loan Bank and the Federal Savings and Loan Insurance Corporation. A real estate loan shall be deemed to include a loan on unimproved real estate.