310 FIELDS AND GRADIENTS 



two will interact — e.g. that the morphogenetic effect of a head of 

 given activity will differ according to the tail-gradient and the 

 effects which this exerts on the old tissues, just as in regeneration 

 from a posterior cut surface, with a given tail-gradient, the morpho- 

 genetic results will vary according to the size and activity of the 

 head. 



This leads on to a point which may prove to be of great theoretical 

 importance, although so far only limited discussion of it has taken 

 place. ^ It concerns the classification of gradient-fields into two 

 types. The first constitutes what Waddington refers to as an in- 

 dividuation- field, in which there exists some form of dynamic 

 equilibrium controlling morphogenetic processes. Removal of one 

 part of the system will, if growth is still possible, lead to the re- 

 generation of what is missing, as above pointed out (p. 276). 

 Further, the induction effect of a dominant region is exerted not 

 by contact as with the amphibian organiser, but apparently at a 

 distance, as with regenerating Sabella or Planarm; this is because 

 the essential effect of the dominant region is to establish a total 

 field. Another term for these would be gradient-fields of direct 

 effect. 



In contradistinction to this we find what may be called gradient- 

 fields of secondary effect. A gradient-system exists, and exerts its 

 effects, not directly, but by giving rise to a graded concentration of 

 some chemical substance which is then responsible for certain 

 morphogenetic effects. It appears that in amphibian eggs the dorso- 

 ventral gradient with the organiser at its high point is of this type. 

 The reasons for this assertion are in the first place that induction 

 is exerted mainly by contact (see p. 135) ; secondly, that dead organ- 

 isers may continue to exert their inductive effect (p. 153); thirdly, 

 that there is no evidence of equilibrium or saturation being ob- 

 tained in the organiser region. This last point requires elucidation. 

 In the bird, a complete "organiser-field", i.e. a sheet of epiblast 

 (ectomesoderm) containing the whole primitive streak and an ex- 

 tensive area around it, is still capable of inducing a neural tube and 

 other organs in a sheet of epiblast from another embryo.^ The 

 organising capacities of the primitive streak have not been ' ' satur- 

 ated" in the formation of its own field, as would be expected if 



^ Waddington and Schmidt, 1933. 



