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This paper comprises the Oliver-Sharpey Lectures for 1936, delivered at the Royal College of Physicians, together with an address to the Section of Pathology of the Royal Society of Medicine, -delivered on the above date.OCT,-PATH. 1Proceedings of the Royal Society of Medicine 3£ an implanted organizer, that is to say, whether, invaginated early, it will come to lie under the presumptive head; or, invaginated late, it will come to lie under the presumptive tail. Spemann [51, specifically investigating this subject, found that head organizer would produce heads either in head or tail regions, while tail orgauizer would produce head in head region and tail in tail region. The organizer itself contains, therefore, some hidden regional quality, determining the regional character of what will be induced, as well as some morphogenetic stimulus which will simply account for the existence of what will be induced. The former type of determination has been called Individuation, and the second type Evocation (Needham, Waddington and Needham [7]) and these terms have become generally adopted. Thus we may say that in an ordinary induction, one of these determinations, evocation, is always performed by the graft, while the other, individuation, fig.81.