GRADIENTS, FIELDS, AND DETERMINATION 283 



rise to two eyes. Also, removal of the mesoderm underlying the anterior 

 part of the neural plate soon after it attains that position results in a high 

 frequency of cyclopia or median approximation of eyes (Mangold, 1931a, 

 p. 365). Adelmann concludes that eye potency is higher in the median 

 region than laterally and that the underlying inductor determines bilateral 

 eye development. The following suggestion perhaps serves to bring verte- 

 brate cyclopia and related modilications more nearly into line with similar 

 modifications in other forms. The inducing action of the underlying mes- 

 entoderm unquestionably brings about a change in physiological condition 

 in the ectoderm involving activation, whatever its other effects may be. 

 The mediolateral decrease in inducing capacity of the inductor tissue 

 (p. 459) makes it probable that the change in condition of ectoderm re- 

 sulting from induction is more rapid and greater in the median region 

 than laterally. In the course of this change the median region passes 

 through a stage of physiological level which represents capacity for eye 

 formation; and since it attains this condition earlier than lateral regions, 

 it may, when physiologically isolated by transplantation at certain stages, 

 give a higher frequency of eye development than lateral ectoderm. Under 

 natural conditions, however, the median region is prevented, by its rela- 

 tions with other parts, from developing an eye or eyes at the stage when 

 capacity for such development is present; instead it undergoes further 

 change in condition, probably with further activation, and finally becomes 

 part of the brain floor, while lateral regions finally attain the condition 

 representing full capacity for eye development. In other words, what de- 

 velops under natural conditions represents, in general, the full or final 

 effect of induction; the eye potency of the median region represents 

 merely a temporary condition intermediate between the condition pre- 

 ceding induction and the final condition. Eyes are normally bilateral be- 

 cause the final conchtion constitutes the action system initiating eye de- 

 velopment in lateral regions at a certain distance from the median plane 

 and at a certain level of the anteroposterior axis. When the underlying 

 tissue is present in the transplant, the median region may be activated 

 above the level determining eye development; consequently, two eyes, 

 bilaterally localized, develop, the distance from the median plane varying 

 with conditions in the individual transplant. In similar transplants with- 

 out underlying inductor tissue the level determining eye development is 

 usually present only in the median region, and a single eye is formed. 

 With differential inhibition by toxic agents optic primordia are locahzed 

 nearer or in the median plane because the more lateral regions never at- 



