THE ADDITION OF DETAILS 85 



saying that they have been induced by organizers, 

 because, if so, where does the organizer come from? 

 It is rather the other way about. The ordinary newt 

 organizer should be regarded as itself very like an 

 organ-forming substance. Wherever it is, there the 

 mesoderm develops out of it and the rest of the 

 embryo is induced ; if it is removed altogether, no 

 embryo appears. In the frog's egg we can actually 

 see the organizer, coloured grey and collected on one 

 side of the egg where the blastopore will appear later, 

 just like an organ-forming substance in an Ascidian's 

 egg. The organizer must be prepared by chemical 

 processes going on in the egg and then localized in 

 one region by the external agents, which were 

 mentioned earlier (p. 62). We know very little about 

 the preparatory processes, but we have a very few 

 hints about one kind of process which must be 

 involved in the newt. 



As we shall see in the next chapter, one of the 

 important things about the organizer is that it con- 

 tains an active chemical substance which gives the 

 ectoderm the necessary stimulus to make it develop 

 into neural plate. We do not yet know exactly what 

 the substance is, though we are rapidly finding out. 

 It is provisionally called the evocator, because it 

 evokes the formation of the neural plate. Now it has 

 been shown that an inactive form of the evocator is 

 present in all adult tissues and, further, that the same 

 inactive form is distributed throughout the whole 

 newt's egg. In the organization centre this inactive 

 form is converted, much earlier than elsewhere, into 



