This episode's headline: Mad Cow Disease; Alien Nation?; Finally - In Memoriam. ANCHOR: JIM LEHRER; GUESTS: DR. WILL HUESTON, U.S. Department of Agriculture; MARK SHIELDS, Syndicated Columnist; GEORGE MITCHELL, Former Senate Majority Leader; DR. LAURA MANUELIDIS, Yale School of Medicine; CORRESPONDENTS: MARGARET WARNER; ELIZABETH FARNSWORTH; KWAME HOLMAN; CHARLAYNE HUNTER-GAULT;