Eye 



411 



iris, involving an increase in height of the 

 cells, especially those of the inner layer; 

 separation of the inner and outer layers; 

 depigmentation of the inner layer and pupil- 

 lary border of the iris; cell proliferation at 

 the pupillary border and downgrowth of 

 an unpigmented epithelial vesicle; forma- 

 tion of a lens nucleus through elongation 

 and differentiation into lens fibers of the 

 cells of the inner wall of the vesicle; forma- 

 tion of secondary lens fibers from the equa- 

 torial zone; and finally, detachment of the 

 lens from the dorsal iris. Van Deth ('40), 

 who has described lens regeneration from 

 explanted eye cups of the chick, finds that 

 the steps are substantially different in this 

 organism. A plate of tissue grows across the 

 pupillary space and then curls inward to 

 close into a lens vesicle. Equally odd, in 

 comparison with the Amphibia, is the fact 

 that the lens arises from the ventral instead 

 of from the dorsal margin of the cup. 



In the normal eye, the regenerative ca- 

 pacity of the iris epithelium is held in check 

 by an inhibitory influence, apparently chem- 

 ical in nature, exerted by the resident lens. 

 This was first demonstrated by Wachs ('14), 

 who found that an isolated piece of dorsal 

 iris introduced into the vitreous chamber 

 fails to regenerate when situated in an in- 

 tact eye, but forms a lens readily in a lens- 

 less eye. If the regenerate comes in contact 

 with the lens forming simultaneously from 

 the host iris, the two sometimes fuse into a 

 single harmonious lens (Sato, '30, '33b). If 

 implantation is delayed until the host iris 

 has begun to regenerate the lens, the latter 

 will suppress lens formation by the im- 

 planted iris. According to Sato ('35) the 

 host lens begins to exert this inhibiting ef- 

 fect during the early stages of its regenera- 

 tion. 



The second condition essential to WolfBan 

 regeneration was revealed by Wachs in 

 transplantations of dorsal iris to an extra- 

 ocular site, the cavity of the ear capsule 

 (see also Ikeda, '36). Pieces thus isolated 

 were incapable of regenerating lenses unless 

 accompanied by transplanted retinal tissue. 

 On the basis of these results it was postu- 

 lated that the behavior of the dorsal iris is 

 controlled by a "secretory balance" existing 

 between lens and retina. According to this 

 concept a retinal secretion conducive to lens 

 formation is normally counteracted by an- 

 other originating from the resident lens; 

 with the removal of the lens the retinal 

 secretion is then free to stimulate regenera- 

 tive transformation of the iris epithelium. 



However, the actual nature of this retinal 

 stimulus, and its relationship to that exerted 

 by embryonic retina in eliciting lens for- 

 mation from epidermis, are completely un- 

 known. 



Lens regeneration from the intact iris al- 

 ways proceeds from the middle of its upper 

 pupillary margin, and it is likewise this part 

 of the iris that manifests the greatest re- 

 generative potency following isolation of iris 

 pieces in the vitreous chamber (Sato, '30). 

 Adjoining portions of the dorsal iris are also 

 capable of delivering lenses under these 

 conditions, but regeneration is often less 

 complete, and pieces of ventral iris give 

 negative results or at best undergo changes 

 suggestive of early stages in lens regenera- 

 tion. On the basis of the regenerative be- 

 havior of eyes that had been rotated 180 

 degrees in embryonic stages, Sato ('33b) at- 

 tributes this graded distribution of lens- 

 forming capacity in the iris to influences 

 associated with choroid fissure formation at 

 the ventral margin of the developing optic 

 cup. Correlated with the stage at which the 

 rotations were performed, the choroid fis- 

 sures appeared ventrally (host-wise) in some 

 cases, dorsally (donor-wise) in others; occa- 

 sionally two fissures, one better developed 

 than the other, arose in a single eye. When 

 these eyes were lentectomized in late larval 

 stages, the new lenses arose from the por- 

 tion of the iris directly opposite the point 

 where fissure formation had occurred; or, 

 when two fissures were present, opposite 

 the one that was most strongly developed. It 

 thus appears that the lens-forming capacity 

 of the Triturus iris, although eventually 

 restricted to its dorsal border, is originally 

 widespread, and becomes inhibited secon- 

 darily at the ventral margin by influences 

 involved in the determination or formation 

 of the choroid fissure. On its face, the finding 

 of van Deth ('40) that in the chick lens 

 regeneration proceeds principally from the 

 ventral instead of from the dorsal margin 

 of the cup would appear to call Sato's inter- 

 pretation into serious question, but it remains 

 to be seen whether localization of regenera- 

 tive capacity within the iris is referable to 

 the same factors in the two groups of 

 animals. 



In a recent study Sato ('51) has extended 

 his survey of the distribution of lens-regen- 

 erative potency in the eye, and finds that 

 it extends into the dorsal portion of the 

 pigment layer of the retina above the iris. 

 Pieces of pigment epithelium taken at in- 

 creasing distances from the iris along the 



