Section VII 



CHAPTER 2 



Eye 



VICTOR TWITTY 



The eye has long served as a classic object 

 for the analysis of causal relations in de- 

 velopment and regeneration. Composed of 

 structurally discrete parts that are mobilized 

 with precision from diverse soixrces, the eye 

 immediately challenges the attention of the 

 experimental morphologist and at the same 

 time lends itself almost uniquely to his 

 purposes. By excision, transplantation, and 

 recombination of the eye components, he 

 has exposed many of the influences that in- 

 tegrate the development of these parts and 

 accordingly shape the final form and size 

 of the organ. In the account that follows, 

 sections will be devoted to experiments on 



(1) the determination of the retinal cup; 



(2) induction of the lens and cornea; (3) 

 growth of the eye and its associated struc- 

 tures; and (4) regeneration of the eye. 

 Limitations of space preclude extensive cover- 

 age of the literature on these subjects, and 

 certain aspects of eye development must be 

 omitted entirely from consideration. For 

 an exhaustive survey of the literature before 

 1931 the reader is referred to Mangold's 

 monumental review published in that year; 

 other invaluable sources are the books of 

 Spemann ('38) and Needham ('42). In 

 several instances, to shorten the bibliography 

 on the earlier literature, the names of investi- 

 gators will be cited without specifying 

 publication dates. In all such cases the litera- 

 ture references are available in the reviews 

 of both Mangold and Spemann. 



DETERMINATION OF THE RETINA 



The first step in the initiation of retinal 

 development, the establishment of paired 

 primordia in the anterior part of the neural 

 plate, has already been treated in the chapter 

 of this book devoted to the organization of 

 the central nervous system. Suffice it to re- 

 peat here that this localization proceeds un- 

 der the influence of the subjacent archenteric 

 roof and, once effected, enables the anlagen 



to continue their development into relatively 

 well formed cups when grafted to heterotopic 

 positions. From this it does not follow, how- 

 ever, that the eye begins its development as 

 a mosaic of rigidly prescribed parts. The 

 examples which follow show that the dis- 

 tribution of specific assignments to the con- 

 stituent cells of the rudiment is referable 

 to morphogenetic influences that continue 

 to operate throughout the period of retina 

 formation. 



The ability of retinal and pigmented 

 layers of the optic cup to interchange pro- 

 spective fates is strongly indicated by the 

 formation of complete eyes by partial rudi- 

 ments (e.g., following incomplete excision 

 in medullary plate stages), and by the fusion 

 of two optic vesicles to form essentially 

 normal single eyes (Pasquini; Detwiler). 

 Direct and conclusive evidence of this inter- 

 changeability has been provided by Drago- 

 mirow ('32, '33, '34, '36). Small pieces of 

 prospective pigment epithelium, grafted 

 between lens and cornea, or even to hetero- 

 topic positions elsewhere on the head, regu- 

 lated to form small cups comprised of both 

 sensory and pigmented layers. Even cells 

 already beginning to show pigmentary dif- 

 ferentiation were capable of reorganization 

 into sensory tissue. In fact, the ability of 

 pigment epithelium to undergo this trans- 

 formation persists into adult stages, as will 

 be seen when we come to experiments on the 

 regeneration of the eye. Dragomirow also 

 found that in young optic vesicle stages the 

 prospective sensory layer can give rise to 

 pigment epithelium. The lability of this 

 portion of the vesicle is short lived, however, 

 and by Harrison stage 29 (in Triturus taenia- 

 tus) it can deliver only retinal tissue. 



In the chick, each of the two layers of the 

 optic cup will form both pigmented and ner- 

 vous tissue when transplanted separately as 

 late as the 36-somite stage (Alexander, '37), 

 and Dorris ('38) has provided confirmation 

 of the regulatory capacity of the two compo- 



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