CHAPTER V 

 THE ADDITION OF DETAILS 



Secondary Organization Centres 



After the organization centre has acted in the newt 

 or chicken embryo, the main outHnes of the future 

 animal are laid down. But the embryo is still very 

 undeveloped and the outlines have to be filled in 

 with all the multifarious details of the adult ; none 

 of the minor organs, such as eyes, ears, lungs, and 

 legs have yet appeared. Many of these minor organs 

 are also developed as responses to organizing 

 stimuli. The organization centres which have been 

 described so far are, in fact, only the primary 

 organization centres, and they are succeeded by 

 secondary ones, and these again by tertiary ones, and 

 so on. One example of this succession of organization 

 centres has already been mentioned; in the chick 

 there is first an organization centre in the endoderm 

 which induces the primitive streak, which then 

 becomes an organization centre itself and induces the 

 neural plate. Many similar examples are known. In 

 the newt, for instance, the neural plate closes up to 

 form the neural groove, and the two sides of the 

 groove eventually join together at the top and cut 

 off the lower part as a tube sunk below the surface. 

 From the front end of this tube, two hollow pro- 

 cesses grow out, which are the beginnings of the two 

 eyes. When these processes reach the surface and 



