A revised version of the paper, originally announced on April 20 this year, has been placed on my web site (www.math.mcgill.ca/makkai). The major change is an expanded introduction. The paper is in a WinZip file that unzips to 11 POSTSCIPT files. Michael Makkai 18-May-2005 16:59:34 -0300,1476;000000000000-0000000c