524 E. I. WERBER 



until finally the optic vesicles come into contact with the ectoderm at 

 the sides of the head. 



On the basis of this hypothesis he now ('13, p. 273) offers the 

 following explanation for the morphogenesis of synophthalmic 

 and one-eyed monsters: 



the median eye anlage does not widen or spread later- 

 ally but is arrested in its primary condition; thus the 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. 



This for cyclopia. In the various degrees of synophthalmia 

 he assumes that the ''developmental \'igor' is less suppressed, less 

 'weakened.' Here the separation of the single anlage into two 

 'growth centers' is inhibited only to a certain (varying) degree 

 and depending on the variation in the degree of inhibition various 

 synophthalmic conditions, such as the "cyclopean eye showing 

 distinctly its double composition," the "hour-glass eye or incom- 

 plete cyclopia," approximation of two separate eyes, etc., result. 



For the genesis of lateral monophthalmia, finally, Stockard 

 ('13) makes the following suggestion: 



The growth centers representing the two future eyes of an individual 



are rarely equally vigorous It might be that at some 



critical point in development one of the future eye centers is affected 

 after the growth centers had begun to localize in more or less lateral 

 positions. 



It is very difficult to understand why (in the same experi- 

 ment!) in some embryos the inhibition of one of the potential 

 eyes should begin after the division of the single anlage (mon- 

 ophthalmia asymmetrica), while in other embryos this single 

 eye anlage should be inhibited before its division into two parts 

 takes place ('cyclopia'). 



That the latter is primarily single and median in position in 

 the medullary plate Stockard now regards as a fact, which he 

 thinks he. has established by experiments described in his 1913 

 paper. However, it seems more than probable that the method 



