THE ADDITION OF DETAILS 



79 



touch the skin, they act as organization centres and 

 induce the skin to form a lens (Fig. 21). There are 

 other secondary organization centres which induce 

 various parts of the embryo to become legs or gills, 

 etc. We do not always know exactly which sorts of 

 tissue carry the organizing power, but we find that 



BRAIN 



SKIN ^'^-~— ' ^=^-^ " SI^'N 



Fig. 21. — ^The development of the eye in vertebrates. A dia- 

 grammatic section through the head; the left side shows an 

 early stage and the right side an older one 



if we graft non-presumptive leg material into the 

 region where tl>e leg will appear, there is something 

 there which succeeds in turning this tissue into a leg. 



Special Factories Making Special Parts 



Once the various parts of the embryo have been 

 started off in definite directions by the organization 

 centres, they can continue developing on their own. 

 They are, in fact, capable of self -differentiating. This 

 does not mean that every single part of the leg-, for 

 instance, has its fate absolutely fixed and can only 



