282 CHARLES E. STOCKARD 



cently found in studying nerve muscle preparations that Mg 

 salts seemed to prevent activity by affecting the muscle directly 

 without apparently affecting the nerve. There is, of course, no 

 direct connection between these facts and cyclopia; the mention 

 was made merely in a general way, and most decidedly did not 

 intend to convey the notion that muscle contractibility and the 

 outpushing of the optic vesicles were phenomena of similar nature. 

 They are similar only in that both are dynamic processes and 

 require energy for their accomplishment. 



There is no necessity for further discussing the fact that a 

 number of eggs when subjected to the same solution do not all 

 respond in a like manner (Spemann '12 b, p. 37)'. This is a typi- 

 cal case of differences in individual resistance and vigor which is 

 observed among any one hundred individuals of any living spe- 

 cies. It is equally true that the two sides of a so-called bilateral 

 individual are rarely, if ever, identical. 



Spemann is no doubt correct in stating that the relationship 

 between cause and effect in my chemical experiments on cyclo- 

 pia is not clear. Yet it seems to me that it is not entirely dark, 

 the entire relationship between cause and effect in biological ex- 

 periments is rarely if ever clear step for step. 



I should like, however, to point out that the chemical experi- 

 ments did one thing in proving that cyclopia could be caused 

 from normal embryos through the action of the environment. 

 This fact did away with all theories of germinal origin of the 

 defect, one of which was strongly presented by Wilder about the 

 same time. The experiments also make clear the stage in de- 

 velopment at which cylopia may occur, and they further supply 

 the richest amount of material yet available for the study of this 

 defect. Finally, they prove to my mind that cyclopia is a de- 

 velopmental arrest and may be due to any cause which lowers 

 developmental vigor at certain critical stages in the formation 

 of the eye anlagen. These important points in the study of this 

 defect have certainly not been made clear by the mechanical 

 experiments though I do not deny that they might possibly 

 have been. 



To quote again from Spemann: 



