There are three ways in which the great problem of the relation between morphology and biochemistry -- the problem to which this book is devoted -- can be approached. In the first place we may make a direct attack upon that difficult region lying between the largest chemical particles and the smallest morphological structures which we know. This realm includes the study of paracrystalline aggregates, colloidal micelles, fibrous macro-molecules, protein structure, etc. In recent times the physico-chemical study of the viruses has played a part of cardinal importance in exploration along these lines. The second way in which we may attempt to bridge the gulf between morphology and biochemistry is by studying the chemical changes which go on during embryonic development, a time during which the morphological change is the most obvious variable.
Up to 1931, in which year the author's former book Chemical Embryology was published, this was perhaps the only contact between biochemistry and embryology, and the chemist could have relatively little to say about real morphogenesis itself. But it was in that very year that successful embryonic inductions were first obtained after the destruction of the living integrity of the primary organisation-centre of amphibia. Hence there opened the possibility of a third method of approach; the biochemical investigation of what may truly be called the morphogenetic hormones. The study of these chemical substances, of fundamental importance for all animal and plant development, including its hereditary aspect, occupies the greater part of the present book. The establishment of the existence of such substances, the essential elements in the processes of dependent differentiation, known since the time of Wilhelm Roux, took approximately the first twenty years of the present century and will always be associated with the names of Ross G. Harrison, Warren Lewis, Hans Spemann and Otto and Hilde Mangold. The second twenty years of the century, which may come to be spoken of as the period between the two great European wars, were characterised by numerous attempts to isolate the inductor substances and to understand their action. Although much has been achieved, as will be evident from the contents of the present book, we have come to see that the problems are often far more complex than had at first been thought, and that after all the larger part of the mystery remains in that we can as yet form little idea of what constitutes reactivity -- the competence to react to the morphogenetic inductor.
In accordance with this distribution of interest, therefore, the present book is divided into three parts. In Part 1 the morphogenetic substratum is considered, i.e. the chemical raw material of development and all the manifold nutritional problems which embryos have solved. Part 2 is devoted to the morphogenetic stimuli themselves, and Part 3 to the morphogenetic mechanisms. Under this head, besides the special metabolism of embryonic life, such general questions as dissociability, heterauxesis, the basis of polarity, etc. are discussed.