The process by which fresh nuclei are caused to form in a supersaturated solution by the presence of a crystal of the solute has been given the title of “breeding” of nuclei. An experimental investigation has shown that breeding may occur by a number of distinct mechanisms. Initial breeding occurs once only when a crystal is first added to a supersaturated solution, and is possibly due to crystal dust which was already present on the surface of the parent crystal. Collision breeding occurs readily whenever crystals can collide with a solid object. If collision is prevented, breeding may still occur by two processes: in one, abnormal needle growth occurs, the needles subsequently breaking off; in the other, crystals under certain conditions appear to possess the property of causing the growth of other distinct crystals alongside of themselves (polycrystalline growth) and these may part company.