After fifteen years of seminal work on the structure and function of DNA and on the genetic code, Crick decided to switch to a new field, embryology or developmental biology, as it came to be called. His overall goal was to understand how genes were expressed and controlled the growth and specialization of tissues and organs. He soon became intrigued by the fact that in two-dimensional sheets of cells, such as those that form the retina of developing embryos, each cell seemed to know its location relative to the other cells in the sheet, and could send out signals that directed further growth accordingly. This phenomenon was attributed to gradients, considered to be regular changes in the concentration of a chemical (Crick called it a morphogen) across the sheet. In this article, he offered a mathematical theory for how such gradients might form through diffusion of the morphogen from a source at one end of the sheet to a "sink" that destroys the chemical at the other. This theory was independent of the fact that the biochemistry of the proposed gradients--what molecules formed which gradient--was unknown. Nor could it be determined in the years to come, prompting Crick to leave behind this field of investigation.
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