In our attempts to interrogate Nature about the development of the nervous system, we ask such questions as "How do the nerve cells originate and how do the correct types of cells differentiate at their correct positions; how do the neurons link together to form circuits whose functions are properly coordinated; and how are the functions of nerve cells related to behavior, to thought, and to conscious ness?" Those problems are intellectually challenging, not only because solving them would give us practical advantages but also because while they remain unsolved they stimulate the imagination and challenge the intelligence. It is precisely because they are difficult and controversial and have defied complete solution that such problems continue to attract subtle minds. The understanding that we now have of neural ontogeny seems to me to be farther from complete knowledge than from total ignorance. Nonetheless, it gives us a slightly elevated position from which to survey the vicissitudes of the past, to appraise our present understanding, and to consider ways in which our knowl edge might develop in the future. The history of this subject affords a particularly piquant illustration of Arthur Lovejoy''s comment that the "adequate record of even the confusions of our forebears may help, not only to clarify those confu sions, but to engender a salutary doubt whether we are wholly immune from different but equally great confusions.
