There is a critical point in development when the size of an animal, arising from its previous plane of nutrition, determines its appetite thereafter, and hence its rate of growth and dimensions at maturity. A small size at this critical time, brought about by undernutrition, is not followed by "catch-up" growth, however liberal the diet. A full diet produces catch-up growth only if the undernutrition, whatever its cause, has occurred after this critical period is over. It can, moreover, only restore a young animal to its percentile channel of growth, and its ability to do this after long periods of undernutrition becomes progressively limited by the animals chronologic age when the catch-up growth became possible.