126 HOW ANIMALS DEVELOP 



say why different parts of the face are differently 

 affected by an alteration in the hormones. There 

 must be a pattern of sensitivity, otherwise the face 

 would just get bigger or smaller as a whole. In 

 many cases it seems certain that the genes affect the 

 fundamental patterns of sensitivity and do not merely 

 cause one and the same pattern to be expressed in 

 different ways. 



SUMMING UP 



This question of the pattern of the individuation 

 field is the root of the whole matter. We have seen 

 how the mere description of the patterns which are 

 found led to the Recapitulation Hypothesis and the 

 discovery of the fundamental pattern of the three 

 Germ Layers. And we have seen in outline how the 

 elements of the pattern arise. Processes go on in the 

 egg which produce organ-forming substances, or 

 evocators and competent tissues which react to give 

 determined organ-rudiments. But how are the organ- 

 forming substances or organ-rudiments arranged to 

 form the pattern of a complete harmonious embryo? 

 Somehow they must be localized. It is not difficult 

 to imagine one or two substances being localized; 

 it might be done by the way the blood-vessels bring 

 substances up to the tgg while it is developing, or 

 it might be done (and, in fact, sometimes is done) 

 by the sperm when it enters the tgg at a definite 

 place. In such a way we can account, say, for the 



