228 RALPH S. LILLIE AND EARL N. JOHNSTON. 



which more structure is formed. 1 At each stage of development 

 the structure already present determines the nature of the struc- 

 ture which is being formed at the time; i. e., the formative meta- 

 bolism, like metabolism in general, is controlled by the structural 

 conditions present in the growing regions, at the site of the syn- 

 theses and other chemical reactions concerned in the constructive 

 process. 2 In considering the general physiology of growth the 

 first question to be asked relates to the manner in which struc- 

 ture of a certain kind determines the production of further struc- 

 ture of the same or similar kind. Differentiation in ontogeny 

 presents certain additional problems, but all developmental 

 processes presuppose growth, or increase in organized material 

 of a type already present. 3 



It is to be assumed that the chemical reactions constituting 

 the specific metabolism of any cell or other living system are 

 under the control of structural conditions of a highly definite 

 kind. Protoplasm is polyphasic in its physico-chemical con- 

 stitution, and both the nature and the velocity of the chemical 

 reactions in such a system must be largely influenced by boundary 

 conditions (adsorption, capillarity, interfacial potentials, etc.); 

 hence in a system of a definite chemical composition constancy 

 in the arrangement, character and extent of the phase-boundaries, 

 i. e., in the structural conditions present in the reacting system, 



1 The general features of this relation are very clearly discussed in Child's 

 "Senescence and Rejuvenescence," University of Chicago Press, 1915, Chapter i, 

 pp. 26 seq. ; cf. also the special papers there cited. 



- It should be noted that the site of the structure-producing chemical changes 

 may in some cases be extracellular; materials produced by normal metabolism 

 within the cell may pass to the exterior (be secreted, etc.) and later undergo chemical 

 change giving rise to definite structure; supporting or skeletal structures are often 

 thus formed (matrix of bone, cartilage, connective tissue, etc.); other instances 

 are formation of spiders' webs and similar structures, clotting of blood, etc. 



3 In differentiation, i. e., the general process by which groups of descendants 

 of the same germ-cell become structurally and physiologically different and func- 

 tionally specialized in later development, factors seem to enter which are absent in 

 simple growth. The phenomena of Mendelian inheritance plainly suggest that 

 differentiation is controlled by the distribution of a limited number of structure- 

 determining factors embodied in chemically stable material particles which can be 

 transferred with properties intact from cell to cell. The chromosomes answer to 

 this conception, and apparently form an essential part of the special physiological 

 mechanism controlling differentiation; of course this control must be exercised by 

 modifying metabolism, in a manner and by means which are as yet unknown. 



