290 PATTERNS AND PROBLEMS OF DEVELOPMENT 



regions in different directions. In many hydroids and planarians there 

 is no progressive limitation of potency except in the hydranth or the 

 head. 



At present it appears that developmental fields are not sharply and 

 definitely bounded in their earlier stages, and it has often been suggested 

 that different fields may overlap so that a certain region may be part of 

 two fields at the same time, but direct proof is lacking. If fields are pri- 

 marily gradient systems, clearly defined boundaries are not to be ex- 

 pected, and extent of a field may increase or decrease with the activity 

 in it. If such changes occur, certain regions may be parts of different 

 fields at different times. Within the field, as in development in gen- 

 eral, stable structure and differentiation appear to be secondary and to 

 result from a pattern primarily nonspecific and labile with a graded meta- 

 bolic differential. That a field in its most general form is anything more 

 than a quantitative gradient system of a particular kind remains to be 

 proved and in the light of experimental evidence appears improbable. 



HARMONIOUS-EQUIPOTENTIAL SYSTEMS 



Driesch introduced the concept of harmonious-equipotential systems 

 and in one of his papers characterized them as follows: "Each part can 

 give rise to any part ['jedes Element kann jedes'] and each effect [i.e., 

 developmental result] occurs only once or a definite number of times and 

 in a fixed relation to all other effects" (Driesch, 1899a, pp. 73-74)- He 

 regarded the Tiihularia stem and cleavage stages of the sea urchin as such 

 systems and later discovered what he beheved to be other systems of the 

 same sort. The term has since been applied to various developmental 

 systems, among them the amphibian limb field. The existence of these 

 harmonious-equipotential systems was regarded by Driesch as evidence 

 for "autonomy of vital processes," that is, for so-called "vitaUsm." It is a 

 fact amply confirmed by many investigators that any level of the Tubu- 

 laria stem can develop into any body-level of Tuhularia. This is also 

 true for Corymorpha and many other hydroids and for the postcephalic 

 regions of various planarians, nemerteans, and annelids. However, the 

 various body-levels, even the levels of the Tuhularia and Corymorpha 

 stems, are equipotential only specifically, not quantitatively, at any given 

 moment, that is, any of them can give rise to the various parts of the 

 body but scale of organization and rate of development decrease from 

 distal to proximal or from anterior to posterior levels. That is, individuals 

 developing from different levels are primarily different. The amphibian 



