POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 287 



further comparison between the glass needle operations on earl}^ 

 stages with open medullary plates and later stages where the ecto- 

 derm was raised and the optic vesicles cut out from beneath it 

 show a difference in the response of the ectoderm in producing 

 free lenses in favor of the less injured or less disturbed ectoderm. 



A few of the early lenses figured by Spemann are at least ques- 

 tionable. For example, figure 44 ('12 a) shows on the operated 

 side an irregular thickening within a single cell layer. Such a 

 thickening might readily have resulted from the disturbance to 

 which the portion of ectoderm had been subjected. This speci- 

 men is cited as evidence that the accessory or supernumerary 

 lenses in my experiments may have arisen as buds from a com- 

 mon origin. The possibility of early lenses constricting or bud- 

 ding into two or more is freely admitted. Figures 14, 17, 18 and 

 20 ('10 b) show constricted or double lenses. If the constriction 

 had been carried further two lenses might have resulted. Yet the 

 relative positions occupied by several other of the lenses figured 

 are difficult to account for on the above basis. 



Finally, there is no question of the fact that in numbers of 

 the fish monsters which I have figured and described small and 

 ill-formed eye vesicles or fragments are associated with large 

 well-formed lenses. The fact of the constant association of such 

 optic structures with lenses whenever the optic parts chance to 

 lie near the ectoderm makes it practically certain that the defect- 

 ive e3'"es have stimulated the lenses to arise in these positions. 

 The size of the lens in Fundulus is not regulated by the size of 

 the optic cup. This is further proven* by the small lenses in 

 large eyes and by large protruding lenses in rather well-formed 

 but small eyes. The error in logic which Spemann ('12 a, p. 81) 

 claims to exist on page 405 of my 1910 b paper in discussing these 

 eye fragments I am unable to detect and several other embry- 

 ologists have been unsuccessful in pointing it out. 



