A dry explanation of cellular reproduction was considerably lightened Monday when Arnold Hager, 15, made a witty and cutting remark to a classmate during science class. 'I'd like to reproduce cellularly with Cindy Loomis,' Hager told his lab partner, Dennis Wender, 15. ''Cause Cindy's got, like, huge amoebas.' Reactions to the remark varied: The class' teacher, Mr. Scarpelli, 58, angrily assigned extra homework, while student Brett Bunn, 16, giggled uncontrollably. 