RECONSTITUTIONAL PATTERNS IN EXPERIMENT 397 



gradient in vertebrates is dorsal, and the developing eye may be expected 

 to possess a dorsiventral differential representing a part of this gradient. 

 According to this suggestion, lens regeneration occurs normally from the 

 dorsal iris margin, and lens potency decreases ventrally from this region, 

 because the dorsal region represents the highest gradient-level present in 

 the eye and reacts most rapidly to removal of the lens, and probably also 

 exercises some degree of dominance over lower levels. In earlier stages 

 the dorsiventral differential is reversible, but perhaps becomes specific 

 later, though it is possible that even in the adult the difference is one of 

 susceptibility or reactivity. Tests of sectors of the iris of reversed eyes 

 for lens potency show that the potency of the dorsal region (originally 

 ventral) is not very high and not widely different from that of other 

 parts. This suggests that the original dorsiventral gradient and that im- 

 posed secondarily by reversal partially neutralize or obliterate each other, 

 and Sato's experiments in general indicate that either may become the 

 effective one. That a dorsiventral differential is involved in lens regenera- 

 tion is further suggested by the relation of orientation of the lens fibers 

 to the dorsiventrality of the optic cup (Dragomirow, 1930). 



Interpreted in these terms, lens regeneration does not appear to differ 

 fundamentally from the decreasing capacity for head regeneration from 

 higher to lower levels of the polar gradient in various planarians and an- 

 nelids and the disappearance of the capacity at certain levels in some 

 forms. In all these cases it still remains to be determined whether the 

 apparent decrease and disappearance of potency at lower gradient-levels 

 results from specific differentiation or from a difference in susceptibility 

 or intensity of reaction to the experimental conditions. The reversal of 

 dorsiventrality of the eye implanted in dorsiventrally reversed position 

 is paralleled by the reversal of dorsiventrality in similarly reversed, trans- 

 planted limb buds. 



DOMINANCE AS AN INHIBITING FACTOR 



In both plants and animals a dominant region tends to inhibit develop- 

 ment of another similar region within a certain distance of itself. Various 

 instances of this effect of dominance have already been discussed in other 

 connections. Here a few further cases are considered in which dominance 

 either prevents a certain kind of reconstitution, inhibits it to a greater 

 or less extent, or brings about regression or destruction of parts already 

 morphologically distinguishable. 



