94 HOW ANIMALS DEVELOP 



cess ; but as we have seen, it acquires other com- 

 petencies and reacts to other organizers by forming 

 other organs. 



Akhough we know so little about competence, or 

 what makes a tissue reactive, we have found out 

 something about the stimulus which the organizer 

 provides. It was first found, both in the newt and 

 the chicken, that the organizer was still effective 

 after it had been killed, so the stimulus cannot 

 depend on any property peculiar to living cells. 

 Quite recently it has been shown that the stimulus 

 is really due to a chemical substance, but the sub- 

 stance has not yet been purified and analysed. 

 When material containing this substance is im- 

 planted into an embryo, it stimulates the competent 

 tissue to form an extra, induced, neural plate. But 

 this neural plate seems to differ in a very interesting 

 way from those which are induced by lining 

 organizers. The usual induced neural plate is part 

 of an embryo, it forms the nervous system of a head 

 or a tail or part of the trunk (for instance, see the 

 induced head in Fig. i8). The neural plates induced 

 by the chemical substance on the other hand seem 

 to belong to no particular part of the embryo ; they 

 are just neural plates with no definite shape to show 

 which part of the embryo they are. If we think about 

 it, this is what we should expect. Because it is clearly 

 impossible for a single chemical substance acting on 

 the competent ectoderm to produce all the different 

 parts of the neural plate, unless, indeed, the various 

 parts of the ectoderm differ from one another in 



