ipS PRINCIPLES OF EMBRYOLOGY 



is at present unanswerable. The problem of whether the evocator stimulus 

 is specific or not in this sense is therefore, although an interesting one, not 

 profitable to discuss further at the present moment (Fig, 10.12), 



There is however rather a different sense in which the terms specific and 

 unspecific can be used. If the evocating stimulus is wholly unspecific, then 

 when used on one and the same type of tissue it can only produce one 

 result. Now this is not the case. We know that in normal development 

 ectoderm differentiates into different parts of the nervous system (e.g. 



>-*Organl««r* 



Figure 10.12 



The activation of the evocator. At the blastopore, something occurs which 

 converts the inactive tissue into an active evocator (change of (£) to E); this 

 can then act on the competent tissue C to produce neural differentiation N. 

 If a foreign substance S is placed in the blastocoel, and produces an induc- 

 tion, it might do so either (i) by acting on C (direct evocation), or (ii) by 

 acting on (£) and converting it into E (indirect evocation). 



brain, spinal cord, etc.). Further, when it is acted upon in a particular way 

 by the living organiser (e.g, by being grafted into the centre of presump- 

 tive mesoderm) presumptive gastrula ectoderm can be caused to develop 

 into notochord, somites, etc. Quite a number of evocators or evocating 

 conditions have now been investigated and it has become clear that they 

 do not all result in the same one out of this gamut of possibilities ; they 

 are therefore not unspecific in this sense. 



The methylene blue inductions did not live long enough for their 

 detailed characteristics to become clear. Some years later, however, 



