VI FOREWORD 



about the energy sources for the growth processes. What starts and 

 maintains the chemical reactions that bring about the increase in mass, 

 the elaboration of new protoplasm? Up to this point we are in the posi- 

 tion of a man who has the plans for a building, all the many types of 

 material necessary to construct it, and many willing hands to do the 

 labor, but has failed to provide the foreman whose job it is to guide, 

 to direct, to speed up or slow down as occasion demands, the action of 

 those hands on the material available. In many respects the enzymes of 

 living organisms are analogous to these foremen. In the case of enzymes 

 the action is on the chemical reactions going on within cells. "Cellular 

 Metabolism and Growth" (Chapter VI) brings out many important 

 aspects of this problem. 



The first six chapters of the book appraise many of the basic physio- 

 logical mechanisms and chemical reactions that confer on the individual 

 cell its form and function in growth. In the development of the multi- 

 cellular organism, however, these units of form and function appear as 

 part of the diverse types of structure that compose the adult organism. 

 The remaining chapters of the book suggest how to deal with these 

 problems. 



Among the central problems of growth are those concerned with 

 differentiation of cells. How is it that in embryonic growth, and during 

 the various types of growth which occur in an adult organism, cells of 

 similar origin come to possess widely divergent characteristics? What 

 controls different modes and rates of growth? What are the factors 

 which govern the establishment of biochemical and morphological 

 differences among cells? These are the problems of "Differential 

 Growth" (Chapter VII). 



The differentiation of the developing egg as it goes through embryonic 

 stages to the adult condition presents many "Problems of Organization" 

 (Chapter VIII). In some studies it has been shown that there are 

 localized regions of the egg, in some cases visibly different, which 

 determine the pattern of development. If these areas are disoriented, 

 visible alterations in the normal development of the embryo occur. In 

 other studies on the early stages of development of the embryo specific 

 groups of cells influence the activities of neighboring cells in such a way 

 that they determine the fate of their neighbors. 



Growth does not always proceed in accordance with the functional 

 needs of the growing or adult organism. All too commonly the cells of 

 a particular tissue or organ will go on a growing spree of their own. 

 Such abnormal growths, neoplasms, are a part of the overall problem 



