Eye 



j-J invagination into vesicular form is probably 



7 the consequence of tensions created by local 

 contraction of the continuous "surface coat" 

 Q which binds together the free ends of the 

 epidermal cells. The effect of mechanical 

 pull by the invaginating retinal cup, once 

 proposed as a causal or contributory factor, 

 can be discounted in view of the formation 

 of lenses independently of the cup (as in 

 R. escidenta), and their induction by frag- 

 mentary or abnormally folded optic vesicles 

 and dead tissues. 



The development of lens fibers ensvies by 

 elongation of the cells forming the proximal 

 (medial) wall of the lens vesicle. This 

 polarization is apparently acquired during 

 early lens stages, since Woerdeman ('34) 

 has shown that vesicles developing from 

 placodes isolated at closure of the neural 

 folds by insertion beneath the flank epi- 

 dermis preserve their original polarity even 

 when the implants have been introduced 

 with mediolateral faces reversed. Normally 

 the polarity of the lens is probably imparted 

 by the retinal rudiment, but judging from 

 the results of Dragomirow ('29) the influ- 

 ence is not highly specific in nature. When 

 an eye cup is grafted to the auditory re- 

 gion, and the lens induced from host epi- 

 dermis lies partly interposed between retina 

 and labyrinth, a secondary fiber-forming 

 center develops at the point where the lens 

 touches the ear epithelium. Another fea- 

 ture of lens asymmetry, the presence of ver- 

 tical and horizontal sutures on its distal 

 and medial surfaces, respectively, also ap- 

 pears to reflect the impact of localizing 

 factors in the lens environment. By rotat- 

 ing the lens epidermis or the retinal rudi- 

 ment through 90 degrees in Rana, Bom- 

 binator and the axolotl, Woerdeman ('34) 

 has shown that the position of the sutures 

 is still a labile feature of lens organiza- 

 tion during early neurula stages, but be- 

 comes ingrained shortly thereafter; rotation 

 at the time of fold formation does not 

 disturb the normal topographical relation- 

 ships, but rotation of either component dur- 

 ing or after closure of the folds results in 

 the formation of suture lines at atypical 

 angles with reference to the cup. 



The development recently of serological 

 methods by Ten Gate and Woerdeman (see 

 Woerdeman, '50) for detecting the first ap- 

 pearance and subsequent development of 

 specific lens proteins in the lens rudiment 

 promises to provide a valuable tool for more 

 penetrating studies of lens induction and 

 differentiation. 



407 



INDUCTION OF THE CORNEA 



The epidermis covering the eye after sepa- 

 ration of the lens undergoes depigmentation 

 and fails to develop the gland cells and 

 chromatophores characteristic of other epi- 

 dermis. Meanwhile an unvascularized and 

 pigment-free stroma of mesenchymal origin 

 begins to form, and together with the epi- 

 thelium constitutes the transparent cornea. 

 For the attainment, and also the mainte- 

 nance, of its histological differentiation the 

 cornea is clearly dependent on association 

 with the rest of the eye. No cornea forms, 

 when the eye is lacking, as after extirpa- 

 tion of the retinal rudiment in neurula or 

 tail-bud stages, and the corneal epithelium 

 transforms into ordinary epidermis after 

 removal of the lens and eye cup in young 

 larval stages. Moreover, when the cornea is 

 replaced by skin from other body regions, 

 even in advanced larval stages, the graft will 

 undergo histological modification into typi- 

 cal cornea. Not only the eye as a whole, but 

 component portions of it acting alone (lens, 

 cup, retinal fragments, iris), are capable 

 of eliciting corneal differentiation. Inert 

 objects such as glass pellets introduced be- 

 neath foreign skin may cause thinning of 

 the overlying epidermis through mechanical 

 tension, but true histological transformation 

 into cornea, including disappearance of the 

 gland cells, apparently requires a specific 

 inductive stimulus. (For literature on 

 cornea, see Mangold, '31.) 



GROWTH OF THE EYE AND ITS 

 ASSOCIATED STRUCTURES 



The determination of the initial scale of 

 the retinal and lens rudiments, the main- 

 tenance of harmonious size relationships 

 between these and other components of the 

 eye, and the growth of the entire organ 

 relative to the rest of the animal, have been 

 studied primarily by transplantation of the 

 eye or its parts between species of different 

 size or hereditary growth rates. Most of 

 these experiments have already been sum- 

 marized in an earlier review by me ('40), 

 and in the present section I shall draw from 

 portions of that account. 



Rotmann ('39), employing two species of 

 European Tritvirus, cristatus and taeniatus, 

 has studied the factors that determine the 

 size of the retinal and lens anlagen at the 

 time of their first demarcation in the em- 

 bryo. Of the two embryos, T. cristatus is the 

 larger, and its eye vesicle is correspondingly 



