THE EPIGENETICS OF THE EMBRYONIC AXIS I97 



for some time to the view that the unnatural evocators acted mainly 

 through the cytolytic mechanism, but he eventually (1948/?) found himself 

 driven to speak of a 'sub-lethal cytolysis', a somewhat question-begging 

 term which, in effect, admits that the unnatural evocators alter the meta- 

 bolism of the cells on which they act without actually leading to cell 

 death. 



One may take it then that the active substances cause some change in 

 the cell metabolism in the competent ectoderm. We have therefore to 

 conclude that all the factors necessary for development into nervous 

 tissue (and also into organiser derivatives such as chorda, somites, etc.) are 

 already present in the gastrula ectoderm, but require activation before 

 they can be effective. 



b. The specificity of the evocator 



A certain amount of discussion has gone on in the literature as to 

 whether the unnatural evocators can be considered as 'specific' or 'un- 

 specific' stimuli. It is rarely that very defmite meanings have been at- 

 tached to these two terms. Perhaps the situation should be envisaged as 

 follows. Let us suppose that, in normal development, a substance, a, 

 diffuses from the archenteron roof into the competent ectoderm and sets 

 going a process, b, wliich in turn gives rise to process c and d and so on, 

 until neural tissue is fully differentiated. The hypothesis originally put 

 forward by Waddington, Needham and Brachet, and still supported on 

 the whole by Waddington (1940^), Needham (1942), was that substance a 

 already exists within the ectoderm but inactivated in some way, perhaps 

 by being combined with some other substance, x, to form a complex ax. 

 Then the abnormal evocator was envisaged as causing the breakdown of 

 ax and the liberation of the active a. According to this scheme the se- 

 quence of processes b, c, d, etc. can only be set in motion by one specific 

 substance, namely a. Alternatively we might suppose that the various 

 unnatural evocators can act immediately on process h, setting it in motion 

 and thus leading to c, d and so on. This would be called an unspccific 

 stimulus because b is supposed to react, not only to a, but to all the other 

 possible unnatural evocators. It must not be forgotten however that even 

 in the first case, although b requires a specific stimulus to set it off, the 

 inactive complex ax is supposed to react unspecifically to any of the 

 abnormal evocating substances. Thus a critical point as regards these two 

 alternatives is whether, when abnormal evocators act on competent ecto- 

 derm, a substance appears which is the same as that which normally 

 diffuses from the mesoderm into the ectoderm in normal development. 

 Since, as we shall see, this substance cannot yet be identified, the question 



