Revising a textbook is a fascinating exercise that allows one to see quite starkly the changes that have occurred in one's discipline through the subsequent editions. As I revise a textbook that was originally published in 1985, I can see the numerous advances that have transformed The discipline of developmental biology. But even more Important and much rarer than the advances are the true Breakthroughs. A breakthrough is more than just an Advance. For something to be a breakthrough, it must Have encountered resistance-it must have broken through Something. In thinking about breakthroughs in develop- Mental biology, let me first construct three categories Of breakthroughs.
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