POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 267 



operation to allow its median portion to be removed and yet 

 enough material remain on either side of the cut to give origin 

 to the two eyes. According to the views of several investiga- 

 tors the removal of this median material should have caused 

 cyclopia, yet it did not. I shall presently attempt to show that 

 cyclopia is not due to the coming together of lateral material 

 in the median line but to a failure of median material to spread 

 laterally. 



When the results of these two classes of experiments are con- 

 trasted one must conclude that: The eye anlage in the medullary 

 plate occupies an antero-median position as shown by the various 

 abnormalities incurred when this region is cut away. The failure 

 to injure the development of the eyes in the great majority of cases 

 when the lateral portions of the medullary plate are removed by 

 operation indicates further that the eye anlagen do 7iot occupy lat- 

 eral positions during this stage of development. 



A DISCUSSION OF PREVIOUS IDEAS REGARDING THE GENESIS OF 

 CERTAIN EYE ABNORMALITIES IN THE LIGHT OF THE ABOVE 



RESULTS 



There has lately been considerable discussion regarding the 

 way in which the cyclopean defect occurs. The experiments 

 described above may serve to concentrate the case on a definite 

 developmental period and, to my mind, settle the question as 

 far as the medullary plate stage is concerned. Whether it is 

 possible to carry any question of vertebrate eye abnormalities, 

 as such, further back in development than this stage seems doubt- 

 ful, since here it is that the localization of the eye anlage is first 

 knOwn to exist. It is certain, of course, that this anlage does 

 come from cells which are present before the medullary plate 

 has formed. Whether these cells are localized and are entirely 

 future eye anlagen cells, and not indifferent ectoderm cells which 

 might have the power of forming any portion of the brain or 

 central nervous system, one is at present unable to state. 



As the case stands, it seems possible to explain the entire 

 genesis of the cyclopean defect from the earliest time at which 

 the optic anlage is definitely known to be localized. This as- 



