156 RALPH S. LILLIE. 



As Child has expressed it, one chief problem in the physiology 

 of development is to determine how metabolism produces struc- 

 ture. 1 The converse problem of how structure influences meta- 

 bolism is equally important. The course of structural modifica- 

 tion as development proceeds is orderly ; to this must correspond 

 a similarly orderly alteration in the character of the chemical 

 transformations which furnish at the same time both the energy 

 and the material for development. Another consideration is 

 important here. Corresponding to the increasing structural 

 diversity with advance in development there is an increasing 

 chemical diversity; of this we have already ample evidence; 

 hence the need for special mechanisms of integration and coordi- 

 nation, both structural and chemical, must become greater with 

 each advance in development. Presumably the development of 

 these mechanisms runs parallel with the development of the 

 organism as a whole. The continued and stable existence of the 

 latter at any stage in its life-history of the embryo as well as 

 of the adult is in fact contingent upon the uninterrupted and 

 adequate working of these regulatory processes. In this respect 

 also the developing germ differs in no essential manner from the 

 adult organism. The physiological problems presented by the 

 organism at different developmental stages differ in degree 

 rather than in kind. From the egg to the embryo and from the 

 embryo to the adult the transitions are continuous; the physiology 

 of maintenance and the physiology of growth and inheritance are 

 at bottom the same. 



1 Journal of Morphology, loc. cit., p. 193. 



