GRADIENTS, FIELDS, AND DETERMINATION 277 



as applied to development, has only a formal value. Identification of a 

 hydranth field, a limb field, an eye field, or, in general, reference of certain 

 developmental phenomena to a field indicates merely presence and a cer- 

 tain order of capacities or potencies for these phenomena in a certain 

 region but gives us no information concerning the physiological character 

 of the order or the potencies, or the conditions determining realization of 

 potencies in a particular part of the field. In other words, reference to a 

 field merely states experimental data in terms of an unknown, of a con- 

 cept without definite content, and the field often becomes little more 

 than a verbalistic refuge. 



The developmental field concept implies an ordering or controlling fac- 

 tor or factors of some sort. Evidently, however, many developmental 

 fields include differences in actual orders and patterns. For example, in 

 the case of the amphibian limb the area capable, or becoming capable 

 under experimental conditions, of developing a limb is considerably more 

 extensive than the field of actual limb development in any particular case, 

 and the same is true for various other organs and organ systems. Obvi- 

 ously, the potency field and the field of actual differentiation differ in 

 some way and must be distinguished. The individuation field of Wad- 

 dington and Schmidt (1933) does not throw any light on individuation; 

 and, as the authors apply it to vertebrate development, it seems to sug- 

 gest that individuation results from action of the chorda-mesoderm as 

 inductor, but actually individuation is present before induction and the 

 chorda-mesoderm is itself a part of the individual. 



If the physiological gradients are operative factors in development, the 

 question of the relation of gradients and dominance to developmental 

 fields is important. According to Huxley and De Beer (1934, P- 274), 

 it is found that the original control of differentiation in all cases appears to be exerted 

 in relation to what may be called a biological or morphogenetic field. Within these 

 fields various processes concerned with morphogenesis appear to be quantitatively 

 graded so that the most suitable name for them is field-gradient systems or simply 

 gradient-fields. 



This statement raises several questions, for it seems to imply that the 

 field is the factor determining differentiation; yet the field is regarded as 

 a gradient field — in other words, as a gradient system. What is the field, 

 as distinguished from the gradient or gradient system in it? Does the 

 field determine the gradients, or do gradients constitute the field? How 

 does the field determine or control differentiation? What part does domi- 

 nance play in the field? 



