272 CHARLES R. STOCKARD 



ever of eye parts within the brain. Could any one ask, whethei 

 this be due to the failure of non-ophthalmic parts of the medul- 

 lar}^ plate to arise or, on the other hand, to a failure of the future 

 eye forming cells to arise, or to differentiate after they have 

 arisen? 



It is not meant to convey the idea that all eyeless brains 

 are related to the cyclopean series, as this is not the case. The 

 future eye forming cells may in some cases have been absent 

 from the start. In other specimens the future optic vesicles might 

 have been in positions to arise normally and laterally and for 

 some reason were incapable of outpushing or of differentiation. 



Certain eyeless brains, however, such as those in individuals 

 having the proboscis-shaped mouth, do belong to the series and 

 must be caused in the same way as are the various cyclopean 

 conditions. They have merely responded to a more exaggerated 

 degree. 



The most extreme cases of cyclopia with actual eye structures 

 are those in which a small pigmented vesicle arises from a ventro- 

 median part of the brain, as is shown in my figure 52, 1909. 

 This pigment la^^er of the retina seems to be its most persistent 

 portion, as it may appear when all other recognizable retinal 

 parts fail to arise. There is a possibility that the small tapetum 

 nigrum groups of cells which in some cases formed from Spemann's 

 transplanted portions of eye anlagen may not be due to the fact 

 that only the anlagen qf such cells were transplanted, but that 

 all other retinal cells except these were incapable of differentia- 

 tion when so small a piece of future eye tissue was isolated by 

 the operation. 



The next degree of cyclopia is exhibited by an individual hav- 

 ing a median eye that is much smaller than one normal lateral 

 eye. We then have a median cyclopean eye of about the same 

 size as one normal lateral eye. The latter case has been termed 

 the 'perfect' cyclopean condition. 



All these abnormalities are best explained as follows: The 

 future eye forming cells occupy a median position in the medul- 

 lary plate and the cells destined to form the two eyes are ar- 

 ranged in one group. This median group of future eye cells 



