FACTORS IN EMBRYOGENESIS 33 



see exemplified in the distribution of the vascular tissues. How these 

 several components of what we describe as pattern are induced will 

 evidently call for investigations of a most searching kind. 



In considering the distinctive organisation of any species, it is 

 evident, as already indicated in Chapter II, that genie action is involved 

 in all the underlying metabolic processes. The characteristic differential 

 or allometric growth of the embryo, i.e. the distribution of growth in 

 different directions which results in the production of a characteristic 

 shape, is also held to be under genie control. Furthermore, at certain 

 stages of development, particular genes appear to be invoked and these 

 in some way bring about the inception and development of particular 

 organs. While it may be accepted that the genes are the primary 

 factors in all morphogenetic processes, an essential scientific task is to 

 ascertain the nature of the underlying mechanism. Even if we assume 

 that genes are biochemical agents, exercising their effects through the 

 control or direction of metabolism, we are still very far from an adequate 

 account of the assumption of form or the differentiation of tissues. 

 Other factors and relationships of an essentially extrinsic character are 

 also involved. In short, the phenomenon of organisation in the embryo 

 is essentially a multi-aspect and integrative one. Some of the things 

 that are involved in this conception are indicated in the descriptive 

 table on p. 332. 



THE ORGANISM AS A REACTION SYSTEM 



In this book certain assumptions are made as a basis for explaining 

 features of the embryogeny in different classes. These include the 

 following: 



(i) A developing zygote is a very complex, specific, diffusion 

 reaction system, which functions in conformity with the laws of physical 

 chemistry. In considering how complex and unique the reaction system 

 of a particular species is, it is well to remember that the constitution of 

 its protoplasm is the result of a long evolutionary process. 



(ii) A biochemical pattern, i.e. a patternised distribution of meta- 

 bolites, always underlies and precedes the visible morphological or 

 histological pattern. 



(iii) The mechanism which brings about the patternised distribution 

 of metabolites — which may be described as one of the most important, 

 enigmatic and challenging phenomena in biology — is to be sought in 

 the laws of physical chemistry as applied to the metabolic systems 

 present in embryonic tissues. 



(iv) A contemporary diffusion reaction theory of morphogenesis 

 (Turing, 1952; Wardlaw, 1953) postulates that in a zygote or embryonic 

 tissue, in which the metabolic substances may initially be distributed in 



