410 



part by part replacement; that is, cornea, 

 sclera, choroidea and retina are renewed 

 from the corresponding components of the 

 eye fragment (Colucci). Of particular inter- 

 est is the regeneration of the sensory retina 

 after surgical removal of this layer alone, 

 or following its degeneration as a result oi 

 section of the optic nerve. Since it separates 

 readily from the pigment epithelium, the 

 sensory retina may be forced out by slitting 

 the eyeball and exerting pressure, or by 

 flooding the vitreous chamber with a gentle 

 stream of physiological salt solution. Wachs 

 ('20) described the formation of new ner- 

 vous retina through proliferation jointly 

 from the iris and the pigment layer of the 

 retina; but according to the recent studies 

 of Stone ('50a and b) on the adult Triturus 

 eye, replacement appears to proceed ex- 

 clusively from the pigmented epitheliiun of 

 the retina. 



Following removal of the sensory retina 

 the cells of the pigment layer become 

 rounded and enlarged, undergo depigmen- 

 tation, and increase in number through ac- 

 tive mitotic division. Some of the daughter 

 cells then differentiate into a normal sensory 

 retina, which establishes functional connec- 

 tion with the brain through ingrowth of 

 new optic nerve fibers, while the others de- 

 velop pigment again and reform a typical 

 pigment epitheliimi. The same sequence of 

 events occurs following degeneration of the 

 sensory retina resulting from replantation or 

 transplantation of the eyeball. Even minor 

 defects in the retina, produced by sucking 

 out small areas with a micropipette intro- 

 duced through a slit in the cornea, are re- 

 paired by proliferation from the underlying 

 pigmented retina, instead of from the cells 

 of the sensory layer bordering the wound. 

 Mere detachment of a limited area of the 

 sensory retina from the imderlying pigment 

 epithelium is sufficient to provoke formation 

 of secondary retina by the pigment cells. 

 Stone also found that pieces of pigment epi- 

 thelium grafted into the pupillary space 

 following removal of the lens consistently 

 give rise to sensory retina. Ikeda ('35) de- 

 scribes the regeneration of retinal cups 

 from the pars iridica retinae of iris pieces 

 grafted to the eye cavity, or even to foreign 

 sites such as the fourth brain ventricle. But 

 according to Stone, pure iris tissue never 

 forms retina when implanted in the cham- 

 bers of the eye. Stone believes that earlier 

 workers, in claiming regeneration of retina 

 from the iris, were misled as a result of 

 incomplete removal of the peripheral portion 



Special Vertebrate Organogenesis 



(ciliary border) of the retina. Since this re- 

 gion remains proliferative as the source of 

 new retinal cells throughout normal eye 

 growth, its continued activity when left as 

 a remnant following removal of the retina 

 might well account for erroneous conclu- 

 sions concerning the importance of periph- 

 eral regions as a zone of retinal regenera- 

 tion. 



Wolffian Regeneration of the Lens. Re- 

 generation of the lens, normally an epi- 

 dermal derivative, from the dorsal margin 

 of the iris has intrigued embryologists since 

 it was first described by Colucci and by 

 Wolff. "Wolffian regeneration" usually con- 

 notes replacement from this source after re- 

 moval of the lens from differentiated eyes, 

 but it has frequently been reported that the 

 faculty of lens formation is also possessed 

 by the rim of the optic cup in embryonic 

 stages of development (e.g., Beckwith; 

 Ikeda, '37b; Woerdeman, '39, '50). When 

 the normal channel of lens formation was 

 blocked, as after replacement of the lens 

 placode by foreign, nonreactive epidermis, 

 the dorsal rim of the embryonic retinal cup 

 was sometimes found to enlarge and pinch 

 off as a vesicle which differentiated into a 

 lens of normal structiire. Also in the chick, 

 van Deth ('40) has obtained lens regenera- 

 tion from young optic cups developing in 

 vitro. However, Stone and Dinnean ('40) 

 believe this phenomenon to be of much rarer 

 occurrence in Amphibia than has been re- 

 ported; and Reyer ('50) finds that in 

 Triturus viridescens the margin of the cup 

 is imable to begin regeneration of a lens 

 until young larval stages, after it has al- 

 ready attained a high degree of functional 

 differentiation. 



Renewal from the iris epithelium follow- 

 ing removal of the lens in larval and adult 

 stages was earlier believed to be of wide- 

 spread occurrence among Amphibia, but re- 

 investigation by Stone and his students sug- 

 gests the possibility that true Wolffian re- 

 generation is characteristic principally of 

 the species of Triturus. The positive results 

 reported for other forms are attributed by 

 Stone to reconstitution from lens fragments 

 remaining after incomplete extirpation. In 

 Triturus, however, the phenomenon is 

 clearly established, and has been described 

 in detail by Wachs ('14), Sato ('30), Din- 

 nean ('42), Zalokar ('44), and Reyer ('48). 

 As siunmarized from the accounts of Reyer 

 and Dinnean, the principal histological 

 changes following removal of the original 

 i:ens are as follows: swelling of the dorsal 



