POSITION OP OPTIC ANLAGE IN AMBLYSTOMA 263 



part some of the material constituting the lateral wall of the 

 medullar}^ groove (fig. 3). 



Fom- embryos were operated upon in this manner. Three days 

 after the operation they were killed and prepared for study. 

 A careful examination showed that three of the four specimens 

 possessed both eyes, while one had no eye, or a questionable 

 formation, on the operated side although a perfect eye was pres- 

 ent on the unoperated side. Thus in three-fourths of the few 

 cases used, neither of the future eyes had been injured by the 

 operation. 



Six other embryos were operated upon in a similar fashion. 

 These were killed two days after the operation. Studying the six 

 embryos in cross section it was found that three possessed both 

 eyes in perfect condition, one individual had two eyes, yet one 

 eye was small and defective on the operated side, and two of 

 the specimens had no eye on the operated side. Thus four of the 

 six specimens had two eyes each, and two of the six had only one 

 eye each. 



Combining the results of both experiments it is shown that, 

 of the ten operated embryos seven possessed both eyes and three 

 lacked an eye on the operated side. One of the seven which had 

 both eyes present showed a defective eye on the operated side. 



A comparison of these experiments with those recorded in the 

 preceding section in which the operation was performed on ear- 

 lier embryos would indicate that the eye anlage has possibly 

 widened out so as to extend into more lateral regions in these 

 later stages. The weight of evidence, however, would indicate 

 that the position is not directly lateral along the edge or border 

 ol the medullary plate, as Spemann has assumed. 



It must be remembered that in the folding process which con- 

 verts the medullary groove into a tube the original borders or 

 edges of the medullary plate come to meet in a middorsal line. 

 After the medullary tube is thus formed the optic vesicles push 

 out from a lateral or really ventro-lateral region and certainly 

 do not in any sense come from the original borders of the medul- 

 lary plate which are now dorso-median in position. 



