POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 273 



normally widens or spreads laterally while two centers of active 

 growth become established which gradually assume more lateral 

 positions until they push out as the two optic vesicles. In the 

 degrees of cyclopia mentioned in the preceding paragraphs the 

 median eye anlage does not widen or spread laterally but is 

 arrested in its primary condition; thus the two growth centers 

 are not sufficiently separated and only a single center exists, 

 and even more than this, the arrest is to such an extent that the 

 entire or normal amount of optic material does not differentiate. 

 Hence, one finds a median cyclopean eye consisting of an amount 

 of eye material far below that normally present. 



Other individuals are found in which greater masses of eye 

 material have succeeded in differentiating, and development has 

 been vigorous enough to allow the early eye anlagen to spread 

 to a greater or less degree and establish the two eye forming 

 centers. Such specimens finally present a cyclopean eye show- 

 ing distinctly its double composition, the two retinae are more 

 or less distinct and the eye large consisting of a greater amount 

 of material than a normal lateral eye, yet less bulky as a rule 

 than the sum of the two normal eyes of the species. 



The hour-glass eye or incomplete cyclopiq, is commonly ob- 

 served in the experiments. This case is due to a later or less 

 complete arrest in development than those mentioned above. 

 Both eyes have differentiated out of the medullary tissue but 

 the embryo was not vigorous enough to permit their normal sep- 

 aration and outpushing. Thus the eyes come off from their ori- 

 ginal ventro-median position and remain in close contact or actual 

 union. 



Finally one observes individuals in which the eyes are sepa- 

 rate and distinct yet unusually close together. Such embryos 

 are able to differentiate their eye material and this material is 

 capable of pushing out from the brain but a slight weakness or 

 arrest has occurred on account of which the optic stalks are 

 short and the optic cups are unable to take a normal position- 

 so that they remain unusually near together and look in an 

 .abnormal direction. 



