278 PATTERNS AND PROBLEMS OF DEVELOPMENT 



The experimental evidence concerning physiological gradients seems to 

 indicate that developmental fields in their simplest, most general forms 

 are gradient systems, that is, the field is constituted by the gradient or 

 gradients present ; the gradients are the vectors of the field and determine 

 its extent and the orderly relations within it. Such a field may originate 

 in a region of localized activity which determines a gradient or gradients. 

 In the adventitious bud field of a plant (see Figs. 1-4) the primary region 

 of activity is the physiological center of the field and approximates, in 

 this case, the geometric center; and the gradient system or field is at 

 first radial and becomes axiate and polarized in consequence of differen- 

 tial growth. Such a field may be larger than the actual bud which develops 

 from its higher gradient levels. The earlier stages of these adventitious 

 buds of plants and many buds in animals, whether they develop into com- 

 plete individuals or parts, such as tentacles, appendages, etc., may be re- 

 garded as examples of simple developmental fields. The extent of the re- 

 gion of activation, that is, the extent of the radial gradient system, may 

 or may not be greater than the field of actual differentiation. These are 

 evidently gradient fields, and the field is the gradient system. 



Realization of field potencies in actual development is localized in the 

 high region of the gradient or gradient system constituting the field simply 

 because this region is the most intensely active. Experimental activation 

 of some other region of the field may bring about development there, pro- 

 vided it is not inhibited by the dominance of the physiological center. In 

 pieces of Corymorpha stem activation of the cells adjoining the level of 

 section determines a gradient or a system of parallel gradients extending 

 for a certain distance (see Fig. 31). This is a reconstitution field; and 

 within it parts of the new hydranth are localized, and alteration and 

 elongation of the stem occurs, so far as length of piece and scale of or- 

 ganization, as determined by intensity of activation and length and slope 

 of the resulting gradient, permit. A similar reconstitution field arises 

 following section in pieces of planarians, nemerteans, annelids, etc., in 

 relation to the activated region which becomes a head. These fields are 

 axiate and polar from the beginning. The dominant region is the high 

 end of the gradient system and is an inductor in the sense that it is the 

 primary factor in determining the gradient system, and consequently the 

 levels at which particular parts shall develop. Such fields as these are 

 fields of actual development, differentiation fields, and represent acti- 

 vated regions of the potency field, which in Corymorpha is the whole 



