GENERAL CONCLUSIONS 331 



A biochemical theory would thus afford the basis for both the general 

 and the specific aspects of embryogenesis. 



In attempting to refer embryogenesis to physical and chemical 

 factors, it has always to be borne in mind that even the simplest cell is 

 a very complex system, the product of a long hereditary process. Each 

 species, in fact, is a unique physical system. 



As an embryo enlarges, the underlying reaction system apparently 

 changes in a characteristic manner. The epigenetic development, which 

 is recognised as being characteristic of plant growth (each stage of 

 development being affected by the stages preceding it), could be referred 

 to the regulated metabolic changes in the morphogenetic regions ; and 

 the progressive organisation which becomes evident during develop- 

 ment could be ascribed to changes in the initial reaction system under 

 the impact of genetical and environmental factors. Changes in the 

 reaction system during development would also account for another 

 important phenomenon, namely, the competence of cells to react to 

 morphogenetic and other stimuli. Any contemporary biochemical 

 theory of embryogenesis will unavoidably be a vast over-simplification 

 of the processes actually involved in it. To know how different meta- 

 bolites may react and become distributed so as to constitute a particular 

 pattern is to describe only a part of the morphogenetic process : the 

 energy relations within the system, and the physical properties of the 

 reacting substances and of the final products, must also be known. 

 The evidence thus far available is consistent with the view that, in the 

 actual determination of form and structure, physical factors, such as the 

 forces of cohesion, surface tension, gravity and so on, are of primary 

 importance. The segmentation pattern in many embryos, for example, 

 is in close agreement with Errera's law, the position and orientation 

 of the new walls being largely determined by the forces developed in 

 the enlarging cells. 



It is difficult to be specific about a phenomenon such as organisation, 

 but, from the considerations now before us, we begin to see that an 

 activated organismal diffusion reaction system will behave in charac- 

 teristic ways, yielding a patternised distribution of substances; and that 

 in relation to the biophysical properties of these substances, to various 

 physical factors which become incident, to factors in the environment, 

 and to mathematical relationships, the specific form and structure of 

 the organism will begin to become manifest and will change in a regu- 

 lated and harmonious manner during growth to the adult state. These 

 regulated and specific developments are what we mean when we speak 

 of organisation. The writer's conception of the morphogenetic process, 

 culminating in the organisation of the adult, is indicated diagram- 

 matically and in a condensed form in the table on p. 332. From a 



