Nikkei tumbled Friday morning amid persistent fears about the debt problems in Europe. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 106.86 points, or 1.26 percent, from Thursday to 8, 372.77. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was down 9.09 points, or 1.25 percent, at 718.62.