254 CHARLES R. STOCKARD 



even though this ectoderm is not exactly that from which the 

 normal lens would have arisen. Lenses were induced to arise 

 only from head ectoderm and independent lenses invariably 

 arose in the head region, it was thus concluded that the head 

 ectoderm principally possessed the independent lens forming 

 power. 



A consideration of numerous eye abnormalities which occurred 

 in these experiments seemed to throw light on the earlier condi- 

 tions of origin of the optic anlage from the medullary tissues. 

 A number of the abnormal eyes appeared to lend themselves to 

 a common interpretation and for this reason I advanced in an 

 entirely hypothetical manner the view that these conditions might 

 be considered as arrests in eye development. 



Spemann in two more recent contributions to the development 

 of the eye has confirmed all of my observations that were touched 

 upon by his experiments. In an equally general manner he has 

 disagreed with most of the deductions which were drawn from 

 my experimental results. ' An attempt to satisfy these disagree- 

 ments will be made in the body of the present paper. 



It is now concluded from a study of abnormal eyes and the 

 experiments below that the eye anlage in the medullary plate 

 is primarily median and single and normally separates or spreads 

 into two almost equal growth regions which develop in lateral 

 directions reaching further and further out until finally the optic 

 vesicles come in contact with the ectoderm at the sides of the 

 head. Provided this view is correct cyclopia is then an arrest 

 in eye formation. 



Spemann., on the contrary, holds that the eye anlagen origin- 

 ally arise lateral in position along the borders of the medullary 

 plate. The cyclopean defect according to him is due to a failure 

 of central medullary tissue to develop so that the lateral eye 

 anlagen slump towards the median plane, fuse and form a single 

 cyclopean eye. Spemann, however, presents no experimental evi- 

 dence to show that the eye anlagen do occupy lateral positions 

 since all of his operations included the median medullary tissue 

 as well as the lateral. 



