GRADIENTS, FIELDS, AND DETERMINATION 285 



from approximation to the median plane of other bilateral organs in ver- 

 tebrates and invertebrates under inhibiting conditions. 



One other point requires notice in connection with these transplanta- 

 tion experiments on parts of the eye field. It is by no means certain that 

 isolation and transplantation of a piece of the embryo is possible without 

 any alteration of physiological condition of the piece. In the light of ex- 

 periments with hydroids and planarians it appears possible that a tem- 

 porary stimulation may follow isolation and perhaps be followed in turn 

 by a depression. If the pattern is a gradient pattern, such changes will 

 certainly play a part in determining what develops from the transplant. 

 There may be differences in susceptibility to the altered environment in 

 pieces from different regions of the field, and these may influence the de- 

 velopmental result. In short, potency for eye development or for any 

 other development may depend as much on the environment of an isolated 

 or transplanted piece and the operative eft'ects on it as on the condition 

 of the region concerned before operative procedure. 



THE AMPHIBIAN LIMB FIELD 



In consequence of the great amount and variety of experiment more is 

 perhaps known concerning fields of organ systems and organs in amphib- 

 ian development than in other forms. Among these the amphibian limb 

 field has received much attention and serves as an excellent example. 

 The limb arises laterally on the body; and the potency field, the region 

 in which limb development can occur, is in earlier stages more extensive 

 than the region of actual development in any particular case. Its physio- 

 logical center, where the potency is highest, and where the limb normally 

 develops, is apparently nearer the anterior than the posterior border, that 

 is, in the higher levels of the portion of the anteroposterior gradient in- 

 cluded within the field. In the primitive limb disk the anterodorsal quad- 

 rant is apparently to be regarded as the dominant region (Swett, 1923), 

 suggesting that perhaps the dorsiventral gradient of the body may also 

 be concerned in its localization. The frequent duplication or triplication 

 of limbs in connection with transplantation of limb buds and regeneration 

 of limbs, evidently a result of the appearance of more than one physiologi- 

 cal center, is discussed in another connection (pp. 390-95). As others have 

 pointed out, the limb field is not established all at once; but certain of its 

 characteristics appear at one stage, others at another. It is not a static 

 pattern but a product of continuous progressive change. It is at least a 

 pertinent question whether anything other than a metabolic pattern with 



