In one of the most dramatic days in Wall Street’s history, Merrill Lynch agreed to sell itself to Bank of America for roughly $50 billion to avert a deepening financial crisis, while another prominent securities firm, Lehman Brothers, hurtled toward liquidation after it failed to find a buyer.

The humbling moves, which reshape the landscape of American finance, mark the latest chapter in a tumultuous year in which once-proud financial institutions have been brought to their knees as a result of hundreds of billions of dollars in losses because of bad mortgage finance and real estate investments.