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The author has previously worked extensively in this field and has published a number of important papers. These are now brought together, synthesized, and enlarged into a book, which gives in detail every important stage of the cortical development of the primate brain from lemur to man. The fundamental pattern of cortical expansion is traced through all these primates, and there is found to be a constant and persistent arrangement of the sulci and gyri. This gives an idea of the dynamics of the progressive expansion of the cortex from the less convoluted to the more convoluted types. Normal brains show a regular sequence in the development of the gyri, which leads to the greater growth and expansion of the newer areas. Size alone does not determine the complexity of the cortical pattern, although usually the larger brains have the most fissures and convolutions. There are instances, however, in which