POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 257 



CONSIDERATION OF THE EXPERIMENTS 



1. Sticking the future brain portion of the medullary plate with 



needles 



The anterior portion of the medullary plate was stuck with 

 steel needles in such a manner as to disturb the cells over an 

 area extending from the anterior border of the medullary fold 

 back to the constricted portion of the plate and laterally from 

 fold to fold. The needle was inserted below the outer layer of 

 cells and raised so as to push the cells apart; this was done a 

 number of times with each specimen. The needles were also 

 swung to the right and left in the medullary tissue until the 

 cells were considered to be disturbed to quite an extent. The 

 object in such an experiment was to determine how severe an 

 injury to the cells was necessary in order to prevent the develop- 

 ment of the optic vesicles. 



Twenty-three embryos were treated in this manner, and all 

 were killed four days after the operation. Under a high power 

 binocular microscope most of them distinctly showed that the 

 optic cups were well pushed out laterally and in contact with 

 the ectoderm at the sides of the head. Both eyes were clearly 

 seen in seventeen of the individuals, while six seemed to have 

 eyes yet not so well developed. These six doubtful specimens 

 were sectioned and studied microscopically. 



Both eyes were present and apparently normal in structure 

 in five of the six embryos. The sixth individual showed eyes 

 which were slightly irregular in form and poorly developed, yet 

 both eyes were distinctly present. 



The experiment would indicate that a disturbance, of the type 

 employed, of the cells constituting the eye anlage in the medul- 

 lary plate was not sufficient to prevent the normal development 

 of the eyes in these embryos. 



Another point of interest might have been attacked, by such 

 an experiment provided the embryos had been allowed to develop 

 sufficiently long after the operation. That is, whether or not 

 the cells destined to form the tapetum nigrum layer might be 



