238 THE MOSAIC STAGE OF DIFFERENTIATION 



tion. The fact itself is attested by numerous experiments, in some 

 of which the newt is made to he on its back during the period of 

 regeneration;^ in others, the whole eye-cup is rotated in situ 

 through 180°, so that the choroid fissure which is normally ventral 

 comes to lie dorsally f in others again, the lens-forming potencies 

 of all parts of the iris margin are tested by grafting definite sectors 

 representing one-sixth of the circumference of the iris into the 

 cavity of the eye of another newt from which the lens has been 

 extirpated.^ In all cases there is found to be a gradient of lens- 

 forming potencies extending dorso-ventrally through the eye-cup 

 and resulting, in the intact eye-cup, in the regeneration of the lens 

 invariably from its dorsal margin. Once lens-formation is initiated 

 here, it inhibits the formation of lenses at other points. In this 

 connexion, it may be mentioned that the presence of the normal 

 lens inhibits such grafted fragments of iris from regenerating a new 

 lens. The presence of the normal iris, however, does not act in this 

 way, and it may sometimes regenerate a second lens from its own 

 upper border.'^ 



If, as already noted (p. 187), there is in the early neurula stage 

 a labile preliminary determination of a lens-area prior to the 

 definitive determination of a lens, we may suggest that some of this 

 area overlaps the eye-area, and that a portion of it becomes in- 

 corporated in the eye-cup. Further, topographical considerations 

 make it clear that if this were so, the region of maximum lens- 

 forming potency in the eye-cup would be the dorsal part, since, on 

 the analogy of the limb, lens-forming potencies must be assumed 

 to decrease along a radial gradient from some central spot in the 

 presumptive lens-area, and this spot lies nearer the dorsal than the 

 ventral part of the future eye-cup. If this is so, then, in the absence 

 of a lens, this dorsally situated tissue in the eye-cup may well be 

 stimulated to exhibit its original lens-forming capacity.^ 



1 G.Wolff, 1901. - Wachs, 1920. ^ Sato, 1930. 



4 Spemann, 1905; Wachs, 1914; Sato, 1930. 



^ Recent experiments have shown that the restriction of the site of lens-re- 

 generation to the dorsal margin of the iris is also due to the fact that this is the 

 region of the eye-cup which is farthest away from the choroid fissure, which is 

 always formed ventrally, and appears to exert an inhibitory effect on lens- 

 regeneration. If at the early tail-bud stage the optic vesicle is rotated in situ 

 about its stalk through 180°, the choroid fissure will be formed ventrally, in 

 tissue which was the presumptive dorsal part of the eye-cup. If, then, in later stages 



