GRADIENTS, FIELDS, AND DETERMINATION 281 



and accomplishes nothing that can be regarded as organismic development. Even if 

 the organization of the egg originates in the orientation of elongate dipolar and sym- 

 metrical or asymmetrical protein molecules, as Harrison [1937] and others have sug- 

 gested, the question at once arises: to what do the molecules orient? Metabohsm is 

 going on continuously in the ovarian oocyte and in the parent organism to which it 

 is attached. Can the orientation possibly occur without relation to this metabolism? 

 In reconstitution of a piece of Corymorpha or of a planarian the new organization fol- 

 lows certain changes in metabolism in the region concerned and without them does not 

 develop. Doubtless changes in structure are also involved and affect metabolism, but 

 metabolism is the active and effective factor. At present there is no evidence that the 

 changes in metabolism characteristic of the new organization result from orientation 

 of protein molecules in all the thousands of cells concerned. The polarities of the indi- 

 vidual cells of the hydroid and planarian are apparently determined by local surface- 

 interior differences without relation to the axiate pattern of the whole, but these cells 

 become parts of the axiate pattern of the new individual and still retain their original 

 polarities. If their molecules are, or become oriented, are they oriented with respect 

 to the local surface-interior factors or to the axiate pattern of the whole individual? 

 If they are locally oriented then the new pattern is independent of them. If the new 

 pattern is determined by their reorientation, how are their original polarities main- 

 tained? 



Weiss, in his book [1939], has much to say of the field concept, of field energy, of 

 strong and weak fields and of decrease in field energy from a center, but he does not 

 tell us what a field is or may be as an active and effective factor in development, nor 

 does he say what makes a field strong or weak or what the source and character of its 

 energy may be. What is the basis for decrease of field energy from a center to periphery 

 of the field? New fields originate in the course of development: how do they arise? 

 Is their origin independent of metabolism? Is there some other source of energy in a 

 field than metabolism? Without information, or at least hypothesis, concerning these 

 points the field concept remains almost mystical in character. 



That organization with localized specific differences of substance and metabolism 

 does originate in some way and that dynamic, not merely structural factors, are 

 essential to its origin appears beyond question; that regional specificities originate 

 and increase during development is indicated by many lines of evidence. If metabolism 

 is merely incidental to, or a result of these changes, it would seem that we must 

 again postulate a specific vital energy: Driesch's entelechy wiU scarcely serve our pur- 

 pose, for that was conceived, not as a source of energy, but rather as controlling energy 

 transformation, and that in living protoplasm is in the final analysis, controlling 

 metabolism. 



The concept of physiological gradients in terms of dynamic factors effective in 

 bringing about development, rather than of purely structural factors, does not in any 

 way conflict with the concept of organization in terms of specific, regionally localized 

 material differences. It merely maintains on the basis of many lines of evidence 

 that such organization is not the primary pattern of development, but the result of 

 metabolic activity in a primarily quantitative pattern from which the regional specifi- 

 cities gradually developed. In short, this concept is an attempt to look beyond organi- 

 zation already present and to throw some light on the problem of its origin and develop- 

 ment [Child, 1940, "Lithium and echinoderm exogastrulation," Physiol. ZooL, 13]. 



