262 PHYSIOLOGICAL GENETICS 



9. Pattern by differential growth. This is not contained in 

 Henke's review, because nol involved in color pattern, but most 

 important for embryological pattern formation. All of the fore- 

 going types may combine in the embryonic patterning, though 

 some of them may be more frequent, viz., 4, 5, 6, 9. 



There can be no doubt that the formation of patterns, i.e., 

 the decisive process of development, may, even in the present 

 stage of our knowledge, be reduced to the action of a gene- 

 controlled system of properly timed reaction velocities. These 

 reactions set in motion the different types of stratification 

 processes discussed and simultaneously provide the details by 

 proper distribution of onset, length of action, and threshold of all 

 the other incidental processes. A simple event at a definite 

 time and place will automatically set in action the whole series 

 of consecutive events. But each partial event may be shifted 

 by mutant genes or the action of modifiers or the environment. 

 There is no reason to believe that other developmental patterns 

 are controlled in a different w r ay from those which have been 

 used as models. 



