284 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tain the eye-level; with increasing inhibition this level is localized pro- 

 gressively nearer the median plane or becomes median because all lateral 

 regions fail to attain the eye-level. Apparently, however, approximation 

 of eyes and cyclopia cannot be regarded as resulting entirely from differ- 

 ential inhibition of the inductor; inhibition of eqtoderm seems also to be 

 concerned. Even the maximum inducing or activating action may not 

 bring any but the median part of the inhibited ectoderm up to the eye- 

 level. The occurrence in fishes of cyclopia with normal brain indicates 

 inhibition largely or wholly ectodermal. 



This suggested interpretation not only avoids the assumption of specifi- 

 cally different actions of closely adjoining median and lateral regions of 

 the mesentoderm but also accounts for the change in localization of eye 

 capacity or potency in the course of development and in differential inhi- 

 bition. According to it, eye potency and optic primordia are not the 

 same: potency may be present in an extensive area, though not neces- 

 sarily at the same developmental stage in all parts of the area, but optic 

 primordia originate where eyes actually develop. The eye field appears 

 to be primarily a gradient system resulting in large part or wholly from 

 induction in the ectoderm and subject to experimental alteration with 

 altered localization of optic primordia. 



The eye field of the chick embryo at certain stages, as indicated by 

 chorio-allantoic grafts of different regions of the blastoderm, is also a bi- 

 lateral field including median and lateral regions.^ At these stages eye 

 development or differentiation of eye tissue occurs in both median and 

 lateral pieces from a certain blastodermal level, but more frequently and 

 with more advanced differentiation in median pieces.'' A slight asym- 

 metry of the field is indicated by a larger amount and more advanced dif- 

 ferentiation of eye tissue from left than from right lateral grafts. The 

 presence of a mediolateral gradient of susceptibility and rate of dye re- 

 duction in the chick blastoderm has been shown (pp 159-62). Except as 

 regards the asymmetry, the interpretation suggested above applies here 

 also. If it is correct, the mediolateral extent of the eye field in the am- 

 phibian and bird, the higher eye potency in transplants of median regions 

 at certain stages, and the approximation of eyes and cyclopia under in- 

 hibiting conditions are all expressions of a mediolateral pattern, primarily 

 a quantitative gradient pattern rather than regionally specific; and ap- 

 proximation of eyes and cyclopia in vertebrates do not differ in principle 



5 Clarke, 1936; Rawles, 1936. See also Figs. 167, 170, pp. 531, 534. 



