The complex process of growth is still surrounded by an aura of mystery. The efforts of biologists the world over in their search to solve the puzzle of growth have provided us with blocks of knowledge to be placed here and there within a framework, but the picture is far from complete. The important role played by the pituitary-secreted growth hormone as well as by the hormones secreted by the thyroid, gonads, and pancreas has long been recognized. To these have now been added recent discoveries of other growth-promoting substances, which suggest a wide field for investigation by both biologists and clinicians.
Hormones, strictly speaking, are substances that act at a distance from the site of their synthesis. Since some of these recently discovered growth-promoting substances do not always comply with this criterion, many investigators refer to them as growth factors, although many use the term "factors" interchangeably with "hormones."