Genie Control of Development 313 



time; but that they can take up their specific action (as enzymes or 

 producers of specific enzymes, coenzymes, "hormones," etc. ) only when 

 the cytoplasm of the cell has reached a certain biochemical and 

 biophysical condition in regard to substrate, hydration, acidity, co- 

 enzymes, and so on, to which the specific genie material is attuned. 

 It is the process of stratification which provides — in an extension of 

 Waddington's terminology — the "competent" substratum. It seems 

 hardly possible to form a theory of genically controlled patterning in 

 development without reasoning in general along this line, though the 

 details may be worked out in different ways when an attempt is made 

 to give specific chemical meaning to the whole, as we saw in the last 

 chapter dealing with one of the possibilities. 



When we first developed such ideas, we used as models for the 

 process of stratification gradients and especially diffusion patterns 

 like Liesegang rings (today paper chromatography and tube chro- 

 matography would serve as comparable models). The idea was that 

 any physicochemical agency like selective diffusion and precipitation 

 which can separate different chemical entities could be the means of 

 the stratification process involving chemically different, specific sub- 

 stances characterizing the locally differentiating substrates ( notice that 

 Brachet's microsomes also require such a sorting-out mechanism). 

 Those products of the genie material, always present but probably in 

 subthreshold quantities, for which one or the other stratified specific 

 substrate would be competent, would start reacting with this substrate 

 to produce the specific genically controlled chains of reaction leading 

 to the synthesis of the active determining substances of different kind 

 for which we used the general term "hormones," enzyme systems 

 being only one of the possibilities included. It is these chains of 

 reactions which offer by their rates and threshold conditions a chance 

 to understand the proper timing as well as more or less irreversibility 

 of step-by-step results in regard to determinative processes, a process 

 which Waddington has since called the canalization of development. 



Since the time when such ideas were developed, the progress of 

 our knowledge of the biochemistry of enzyme systems, proteins, and 

 nucleic acids, together with such biochemical facts as have already 

 been reported, as well as the facts underlying the ideas on so-called 

 plasmagenes, have led to a number of attempts to express more or less 

 the same type of model in more concrete terms, which are somewhat 

 different if proposed by embryologists than if presented by geneticists. 

 The ideas of one embryologist, Brachet, have been discussed because 

 of their specificity and relation to facts of biochemical cytology. We 



