276 FIELDS AND GRADIENTS 



In general, the term field implies a region throughout which some 

 agency is at work in a co-ordinated way, resulting in the establish- 

 ment of an equilibrium within the area of the field. A quantitative 

 alteration in the intensity of operations of the agency in any one part 

 of the field will alter the equilibrium as a whole. A field is thus a 

 unitary system, which can be altered or deformed as a whole ; it is 

 not a mosaic in which single portions can be removed or substituted 

 by others without exerting any eflFect on the rest of the system. 



The agencies operative within biological field-systems have not 

 yet been identified with certainty. In many cases, as in the re- 

 generation of hydroids and worms, it has been suggested with a 

 good deal of probability (on the basis largely of experiments on the 

 differential susceptibility of the regions of the system to toxic and 

 narcotic agents) that they concern a gradient in the rate of some 

 fundamental metabolic process (see p. 301). However, the precise 

 nature of the processes in question is irrelevant to the general dis- 

 cussion, and for the time being we shall refer to them under the 

 non-committal term of activity -gradients. In other cases, such as 

 the limb-producing capacities of the Urodele limb-field (p. 222) 

 which concern the morphogenesis of a single restricted region, the 

 simplest assumption is that there exists a graded concentration of 

 the specific chemical substances responsible for limb-production 

 and laid down by chemo-diflFerentiation. 



In all examples so far studied, the agencies in question appear to 

 be graded quantitatively in somewhat simple patterns, frequently 

 (Hydroids, Planarians, many eggs) in the form of a single gradient 

 with high point at one end and low point at the other, the direction 

 of the gradient coinciding with the long axis of the organism. It 



Fig. 131 

 Axial susceptibility- gradients of various oligochaete worms. The abscissae 

 represent the ordinal number of the segments of the worm, the ordinates the time 

 in minutes elapsing before death when exposed to weakly toxic solutions of KCN 

 (Njioo to NI500). Above, left, susceptibility-gradient of a mature Aeolosonia in 

 which secondary zooid formation has not begun. Above, right, the same for an 

 Aeolosoma in which the shape of the gradient indicates that the processes leading 

 to the formation of a posterior zooid have been initiated (compare also fig. 130, 

 in which a posterior zooid is visibly differentiated). Centre, susceptibility-gradient 

 of an individual of Dero without visible fission-planes. The posterior rise in 

 susceptibility, characteristic of most oligochaetes and associated with the sub- 

 terminal growth-zone, is well shown. Below, the same in Lumbriculus. The 

 posterior rise is more marked, and concerns a larger proportion of the body- 

 length. (From Hyman, Jfourn. Exp. Zool. xx, 1916.) 



