Location of the Optic Anal age in Amblystoma. 163 



of the cases, showed absence of the eye. In one case the presence 

 of the eye was questionable, in five cases one eye and in one case 

 both eyes were absent. The absence of the eyes in the latter 

 cases was possibly due to the cut having been made in a more 

 median position than was intended. 



Nine embryos studied after having been operated upon so as 

 to remove a narrow median strip of cells from the anterior portion 

 of the medullary plate showed in four cases, or about forty-five per 

 cent, of the specimens, entire absence of eyes. In four other 

 individuals the eyes were highly defective, one specimen having 

 one poorly formed eye while the other was questionably present. 

 In only one of the nine embryos did the eyes approach the normal 

 condition, from this specimen an extremely narrow median piece 

 had been cut out of the medullary plate. The optic anlage might 

 have been sufficiently wide at the time of the 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 investigators the removal of this median 

 material should have caused the cyclopean defect, yet it did not. 

 In a more extended report of these experiments I shall show that 

 cyclopia is not due to a coming together of lateral materials in the 

 median plane, but to a failure of median material to spread 

 laterally. 



Contrasting the results obtained after the lateral and median 

 cuts mentioned above 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 not occupy 

 lateral positions during this stage of development. 



Based upon these experiments and a study of a large number of 

 eye abnormalities it is concluded that the cyclopean defect is a 

 developmental arrest. The eye anlage fails to widen laterally 

 so that only a single median growth center arises from which 

 develops the ventro-median cyclopean eye. In normal cases 

 the anlage widens and two more or less lateral growth centers 

 become established and give rise to the ventro-lateral optic 



