GRADIENTS, FIELDS, AND DETERMINATION 289 



of Amhly stoma in the normal differentiation field of the ear (orthotopic 

 implantation) with various degrees of rotation may result, at certain 

 stages, in reversal of both anteroposterior and dorsiventral axes, or one 

 may become the other. It seems evident that these changes in pattern 

 are imposed on the implant by the pattern about it. 



The field of the larval urodele balancer in the ventral head ectoderm 

 is apparently not axiate, and the balancer itself appears to be radially 

 symmetrical. A radial potency gradient decreasing from a physiological 

 center has been demonstrated; ectoderm from this region, but not other 

 ectoderm, transplanted to other head regions at certain stages, will de- 

 termine development of a balancer with mesoderm from the region of im- 

 plantation. In later stages just preceding appearance of the balancer bud 

 transplanted ectoderm of this region will determine balancer develop- 

 ment in trunk as well as in head regions. Harrison (19256) regards this 

 difference in inducing capacity as indicating increase in specificity, but it 

 seems entirely possible that it may indicate merely a more intense activa- 

 tion of the balancer ectoderm at the stage when outgrowth is about to 

 begin. Development of supernumerary balancers in the field can be in- 

 duced by implants of various tissues, neural plate cells, foregut cells, even 

 tissue from urodeles which have no balancer (Mangold, 193 16) and cells 

 of the anuran neural crest (Raven, 193 1, 1933&). That the inducing agent 

 is anything more than a nonspecific activator here, as in limb induction, 

 seems improbable. 



These examples are sufficient to indicate the character of field phenom- 

 ena in amphibian development where they have been most studied. That 

 the potency field is primarily a gradient system of a certain kind and that 

 the normal differentiation field represents its high region is indicated by 

 various lines of experimental evidence. The limb field, the eye field, the 

 ear field, appear in the course of development, but their patterns may 

 not be established all at once. The hmb field is a region from which limb 

 develops before the pattern of a particular limb is finally fixed, that is, 

 the pattern of the limb may be altered by transplantation to an altered 

 physiological environment. According to data at hand, amphibian po- 

 tency fields, as areas more extensive than the fields of actual differentia- 

 tion, undergo progressive restriction in most cases with progress of de- 

 velopment and finally disappear completely. The possibility remains, 

 however, that under other experimental conditions potencies might ap- 

 pear which now seem to be absent. The disappearance of a potency field 

 is evidently associated with the progressive differentiation of its different 



