The idea is now widely accepted that active cell locomotion is an important mechanism in many kinds of animal morphogenesis. The history of the idea suggests, however, that there was a peculiarly strong resistance to its acceptance. The resistance was not overtly expressed, but localized cell multiplication was nearly always preferred in explanations of morphogenesis. It is argued that, by the end of the nineteenth century, the knowledge existed that should have encouraged interpretation of many embryonic events in terms of cell locomotion. It did not do so, and in the history of other branches of development biology a similar reluctance to entertain the idea is discernible. It can be illustrated from studies of tissue culture and of malignant invasion. Some speculations about the nature of this temporary blind spot are discussed.