Genie Control of Development 409 



diflFerentiation of tissues, are correlated with the presence of growth 

 regulating substances. But how, in any one instance, the specific sub- 

 stance acts as an organ-forming, or as a tissue-differentiating, agent 

 does not emerge from this survey. It may indeed be questioned 

 whether the relationship is one of cause and effect . . . But what can 

 equally be said is, that if the specific substance is not present, the 

 formation of certain organs, or the differentiation of certain tissues, 

 will not (or may not) take place. Viewed in this more general way, 

 the activating substance may be said to be morphogenetic. But we 

 have no definite knowledge of specific leaf-, shoot-, root-, or flower- 

 forming substances . . . The morphogenetic action, insofar as it can 

 be so described, of a substance such as indoleacetic acid is very varied 

 indeed. It is primarily concerned with growth, not with the inception 

 of particular forms or patterns. Hence, Avery [1940] has said that the 

 specificity of response resides, not in the substance applied, but in the 

 tissues of the species being treated [compare with sex hormones in 

 vertebrates: R.B.G.] and Hammer [1938] has remarked that no 

 single tissue, or tissue system, responds either quantitatively or quali- 

 tatively in the same manner in the various species investigated. We 

 have also seen that, within a single species, some tissues are con- 

 siderably more sensitive and responsive to stimulation than others. So, 

 however essential and important it may be, a growth-regulating sub- 

 stance is only one factor in morphogenetic processes . . . 



"Whether we are concerned with organs or tissues, the main con- 

 clusion from our survey is that biochemical factors must act in 

 conjunction with other factors to produce the morphogenetic effect. 

 But it may well be that the biochemical factor is the master factor in 

 the situation . . . Growth-regulating substances and other important 

 metabolites are probably gene-controlled. Thus, although the sup- 

 porting evidence is still far from adequate, and the situation to be 

 explained seldom of a simple kind, we begin to see that the new 

 investigations of morphogenesis are likely to be fruitful. This effect 

 ... is essentially a matter of controlled biochemical activity, acti- 

 vating or growth-regulating substances being specially important in 

 the inception or inhibition of growth in particular regions of the 

 organisms. But such actions alone cannot be said to account for the 

 actual form or pattern which is assumed . . ." 



e. Mutational changes of determination; regulation and integration 



One of the most interesting chapters of genie interaction relates 

 to the genically controlled changes in primary determination processes. 



