GRADIENTS, FIELDS, AND DETERMINATION 279 



stem. During the course of reconstitution in Corymorpha new fields — for 

 example, the tentacle fields — arise at certain levels of the gradient system. 

 These also appear to be primarily gradient systems like those of buds; 

 but since they determine particular organs rather than complete individ- 

 uals, the gradient activities in them doubtless differ in some way from 

 those in the original reconstitution field in which they arise. The appear- 

 ance of such secondary fields within the primary field indicates that the 

 latter becomes, sooner or later, something more than a simple quantita- 

 tive gradient system. Apparently specific differences of some sort appear 

 at certain levels, and some of these determine local activation with forma- 

 tion of organ fields at those levels. At present it is difficult to describe the 

 reconstitution of Corymorpha in any other terms than these. The gradi- 

 ents arise as the earliest distinguishable evidences of development and 

 can be made visible in the Hving animals. They may be determined at 

 any level and at either end of a stem piece by section, by a graft from 

 any stem-level, or by a lacerated wound; and their length may be altered 

 and controlled experimentally by environmental factors with correspond- 

 ing alteration of scale of organization (see chap. x). As already pointed 

 out, such forms of development come nearer providing a starting-point 

 for physiological analysis of development in general than does embryonic 

 development from an egg already more or less highly differentiated be- 

 yond the earlier stages of pattern. 



Symmetry or asymmetry of bud fields and organ fields arising in rela- 

 tion to pre-existing gradients in the body from which they develop may 

 be determined or influenced by these gradients. The fields of origin of 

 new cilia and cirri in the division and reconstitution of various ciliate 

 protozoa are evidently related in a definite way to the pattern of the 

 original individual;^ and some evidence concerning that pattern is given 

 by the ectoplasmic gradient in rate of dye reduction and susceptibility 

 in these forms. The "dorsiventral" or bilateral pattern of tentacle devel- 

 opment in the bud of Pelmatohydra is apparently determined by the gra- 

 dient and dominance in the parent body (Rulon and Child, 1937). The 

 anteroposterior and dorsiventral pattern of the amphibian limb arises 

 in definite relation to anteroposterior and dorsiventral pattern of the body. 



The question of the relation of metabolism to developmental pattern, 

 and particularly to the developmental field as this is conceived by some. 



- See, e.g., Dembowska, 1925, 1926, and citations. 



