192 LIFE: ITS NATURE AND ORIGIN 



tinguishable in early stages, these cells tend to breed true in tissue 

 culture. 



(B) Morphogenetic fields or gradients. The field is an ordered 

 pattern of instabilities set in a cellular matrix. 20 The field effect 

 results from an equilibrium position, and the term gradient 

 stresses the increase or decrease of field potency from one point or 

 pole to another. 



(C) Individuation, the field forces which render asymmetrical 

 the neural axis (arising from the chemical stimulation of the evo- 

 cator) by differentiating head end and tail end. 



Needham 9a says that "individuation is one of the most difficult 

 problems confronting embryology today . . . the possibility is 

 by no means excluded that other chemical substances may be at 

 work in the formation of the normal neural axis besides the pri- 

 mary evocator. The existence of second grade inductor sub- 

 stances is, of course, beyond doubt. . . . But we may also find it 

 worth while to distinguish between substances which stimulate 

 tissues to form an organ (inductors) and substances which modify 

 in various ways the shape of organs so induced. If such sub- 

 stances exist, it is at any rate clear that the position in which they 

 are normally liberated is so exactly controlled by the individua- 

 tion field of the inducing tissues that the regional correspondence 

 of inducer and induced is secured. Waddington 21 has discussed 

 the possible existence of these substances or Eidogens in a short 

 review. The important thing which head-organizer does as op- 

 posed to tail-organizer is to modify the cross section of the neural 

 tube to form a brain rather than spinal chord. Inductors and 

 eidogens would thus be the mechanism by which the individua- 

 tion field of the inducer controls that of the induced." 



This last statement follows the very common but erroneous notion 

 that the demonstration of chemical compounds, and even their isola- 

 tion, complete analysis and synthesis (though all this is of primary and 

 vital importance) make clear the mechanism by which they act. For 

 example, the "bios" problem was under attack for many years before 

 Professor F. Kogl (Utrecht) in 1936 isolated biotin, which noticeably 

 affects the growth of yeast in dilutions of one part in 400 billion. Its 

 structure was unravelled by K. Folkers and V. du Vigneaud, 22 and 

 biotin itself was successfully synthesized in the laboratories of Merck & 

 Co. 23 But this admirable work of itself does not demonstrate the 

 mechanism by which biotin acts. It seems, however, that individua- 



