280 CHARLES R. STOCKARD 



as early as the first cleavage are much more readily affected in 

 such a manner as to cause the cyclopean defect. The optic ves- 

 icles appear at about thirty hours after fertilization, but the 

 stimulus must be applied at a time sufficiently long before this 

 process occurs, since a number of important steps in eye forma- 

 tion are doubtless taking place before the visible signs of optic 

 vesicles are present. 



The fact that cyclopia may be produced after the beginning 

 of the germ ring and embryonic shield in the teleost embryo in- 

 dicates directly that the defect may occur in what would be the 

 medullary plate stage of amphibian embryos. Thus explana- 

 tions of the cause of cyclopia must consider it as occurring in 

 the medullary plate stage. 



Spemann ('12 b, pp. 38-39) has taken exception to my state- 

 ment regarding the non-occurrence of cyclopia when eggs are 

 treated later than fifteen hours after fertilization (although the 

 optic vesicles do not arise until about the thirtieth hour) "since 

 insufficient time exists for the substances to act on the eye anlagen.^' 

 The paragraph following this statement in my paper (p. 388, 

 '10a) is : ''The solutions are effective up to a stage in development 

 preceding the formation of the germ ring and embryonic shield, 

 and the action of the Mg on the eye anlagen probably takes place 

 while the embryonic shield and outline of the embryo are forming J^^ 



This statement seems to me perfectly direct and clear; Yet 

 Spemann intimates in one sentence that I mean to infer that fif- 

 teen hours are necessary for the substances used to penetrate the 

 egg membrane! He then states that I showed in 1907 that KCl 

 would penetrate within a few minutes and stop the embryonic 

 heart beat. The Mg and other solutions used may pass through 

 the membrane equally as rapidly, this I have not fully tested. 

 The fact that it does has no bearing on the fact that stimuli do 

 not have sufficient time to act upon the optic anlagen to induce 

 cyclopia when applied to the eggs at later periods in normal de- 

 velopment than fifteen hours after fertilization since they must 

 act on the anlagen in the embryonic shield, and to act later is 



1 The last clause was not originally in italics. 



