THE DEVELOPMENT OF PATTERN 95 



some way, which does not seem to be the case. It 

 seems, then, that the chemical substance simply gives 

 the stimulus for the neural plate to be formed and 

 that something else must be present in the living 

 organizer to determine which part or parts of the 

 neural plate it shall be. The chemical stimulus is 

 called the evocator and the something else which 

 determines which part of the neural plate shall be 

 induced is called the individuation field. 



The fact that an organization centre contains an 

 individuation field is the second of the two reasons 

 why it is not sufficient to say that Spemann's dis- 

 covery merely tells us why some particular part of 

 the embryo becomes neural plate. The full statement 

 is that some of the ectoderm in the gastrula becomes 

 neural plate because, while it is in a competent 

 state with respect to this process, it is acted on by the 

 evocator in the organization centre; and that it 

 becomes a particular part of the normal neural plate 

 fitted into the whole embryo because it is within 

 the individuation field of the organization centre. 

 The idea of the evocator expresses the way in which 

 the organizer is active as a determiner, and the idea 

 of the individuation field emphasises the way in 

 which it is active as a centre of the organization of 

 the embryo. 



Particular Causes and the General Pattern 



It is the individuation field which fits the different 

 parts of the embryo together to make up one single 

 complete animal. It determines what part of the 



