CHAPTER VI 



THE DEVELOPMENT OF PATTERN 



How the Organizer Works 



The discovery of organization centres is only a 

 beginning of the analysis of why an embryo develops, 

 but it is a very promising beginning. It provides some 

 sort of an answer to the fundamental questions of 

 embryology, why does a certain part of the embryo 

 develop into this organ and another part into that? 

 and how are the parts fitted together? We can already 

 say why part of the gastrula becomes the neural 

 plate. It does so because the organization centre 

 stimulated it to develop in that way. But there are 

 two reasons why that is not quite the whole of the 

 story. For one thing, the embryonic tissues can only 

 react to the stimulus of the organizer when they are 

 ready to do so. The organizer does not seem to be 

 able to induce anything in tissue younger than the 

 early gastrula stage. We do not know why this is, 

 but probably various chemical processes go on in 

 the cleavage cells, so that by the time gastrulation 

 begins they have become reactive and can be 

 affected by the organizer. When they are reactive, 

 they are said to be "competent," but the competence 

 does not last long. After the end of gastrulation the 

 ectoderm loses it power to form neural plates under 

 the influence of the organizer. That is to say, it 

 becomes non-competent again as regards this pro- 



