FIELDS AND GRADIENTS 31I 



organisation were an affair of equilibrium between the morpho- 

 genetic capacities of the organiser and the neighbouring tissues, as 

 is clearly the case with a regenerating Planarian (see p. 287). 



These two types of gradient are of course not mutually exclusive. 

 The primary gradient-field of the amphibian egg is an individua- 

 tion-field ; but as a result of its existence, graded accumulations of 

 yolk and other substances occur, which then exert effects upon 

 development. In this case, the substances accumulated are mere 

 raw materials, but in other primary gradients, doubtless, true organ- 

 forming substances are formed in this way. It is, of course, also 

 possible to conceive of the graded formation within a gradient-field 

 of some substance which has no further effect on development, so 

 that there is no secondary action of the field as occurs with the 

 organiser. Such fields we may if we like distinguish as fields of in- 

 direct action but without secondary effect. Many cases of graded 

 distribution of pigment within organs are doubtless of this type. 



The gradient-field of the amphibian organiser appears to be 

 essentially one of secondary effect ; but it very possibly acts as a 

 weak individuation-field in the stages before gastrulation. In birds, 

 as already mentioned (p. 160), portions of the organiser (primitive 

 streak) when isolated regularly produce more than their presump- 

 tive fates, thus showing a tendency to individuation. 



Partial fields such as the limb-field in Amphibia appear to par- 

 take of both these aspects of field-action. They seem undoubtedly 

 to be areas in which there has resulted a graded concentration of a 

 specific chemical substance which is capable of producing limb- 

 formation : but they also have their own individuation-field, which 

 sees to it that what is produced is normally neither a partial nor a 

 multiple structure but one whole organ. 



As with any new concept, considerable analysis, both experi- 

 mental and theoretical, will be needed before the different roles of 

 field-systems in ontogeny can be properly understood. Meanwhile, 

 however, this distinction between fields of direct and indirect action 

 is a first important step, helping considerably to clarify amphibian 

 development (see Chap, ix, p. 318). 



