The board of directors of National Airlines continued to meet yesterday for the second straight day to debate a merger bid from Pan American World Airways.

In mid-evening, spokesmen for both airlines said there would be no announcements or official statements until today.

A Pan Am source said negotiations were continuing.

Tuesday night, after the two airlines' separate board meetings, Pan Am's Chairman William T. Seawell had predicted that the two carriers would reach a definitive merger agreement yesterday, but the National directors continued to meet into the night yesterday.

A Pan Am board meeting was much shorter.

National has been resisting a takeover attempt by Texas International Airlines, which already owns at least 18.2 percent of National's stock, but said it would consider Pan Am's proposal. Originally, Pan Am offered to boy National shares at $35 each, but last week said it would negotiate substantive terms of a merger that would result in its paying $41 a share. (National's stock, which has gone up and down, closed at 33 7/8 yesterday, up 5/8, on the New York stock Exchange.)

Even with the execution of a definitive merger agreement and the approval of the boards, the merger has to get the approval of National's stockholders, the Civil Aeronautics Board and the president.

In other merger news yesterday, officials of North Central Airlines and Southern Airways announced they had agreed on the financial terms of their proposed merger. Under the agreement, North Central would exchange 2.2 shares of its stock for each of Southern's 1.6 million common shares outstanding if Southern achieves certain pre-determined earnings for the year ending Dec. 31, 1978.

A sliding seale formula provides for adjusting the North Central shares that will be issued if Southern's 1978 earnings are less than $3.5 million. If Southern's earnings are between $2 million and $3.5 million, North Central will exchange 2.1 shares for each share of Southern.

The proposed merger is subject to approval of the stockholders of both companies, lenders, the CAB and the president.

North Central, based in Minneapolis, serves 93 cities in 17 states, largely in the Northern part of the country, and two Canadian provinces.

Southern, based in Atlanta, serves 70 cities in 19 states, with an emphasis on the south, and the Cayman Islands in the Caribbean.