V. FACTORS AFFECTING THE GROWTH OF HAIR Although complete agreement as to the exact nature of the histologic changes that take place in the follicle during growth and replacement of hair is still lacking, the general character of these processes is more or less evident. But when it comes to the factors that control, or even influence, production of hair, the available information is far less satisfactory. Even in the apparently simpler phases of the problem, those relating to the normal duration of life for individual hairs and periods of activity and quiescence in the follicle, there is a deficiency of convincing data. This is not because of lack of work in these lines, but is due rather to certain inherent difficulties in the problem and occasionally, perhaps, to faulty premises as to the character of hair growth in general. The evidences of endocrine and other indirect influences are abundant,