SUMMING UP 127 



localization of the evocator in the newt's tgg. But 

 how shall we account for the localization inside the 

 organization centre of the factors which make part 

 of the neural plate become brain and another part 

 spinal column? We do not know. Probably we shall 

 have to invent quite new methods of attacking the 

 problem before we shall solve it. It may be that the 

 structure and orientation of the minute particles of 

 the cytoplasm are important. But it remains a 

 problem for the future. The discovery of organiza- 

 tion centres, to which so much of this book has been 

 devoted, only clears the air and makes it possible 

 to formulate more definitely the problem of what 

 causes the patterns in which animals develop and on 

 which their adult functioning depends. 



