POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 283 



Gegen diese Defekthypothese erhebt Stockard (1909b, p. 172) die 

 Fragen: warum sollte bei den ]\Iagnesiumembryonen gerade das Gewebe 

 zwischen den Augen ausf alien, und keine anderen Gewebe?; warum sind 

 die Riechgruben bei Cyclopie manchmal verschmolzen und manchmal 

 getrennt?; . . . . ist bei den asymmetrisch-einaugigen Missbil- 

 dungen der einseitige Augendefekt etwa auf die Abwesenheit der einen 

 ersten Augenanlage zuriickzufiihren? — ^Darauf mochte ich mit der Gegen- 

 frage antworten : sind denn all diese Tatsachen verstandlicher bei Zugrun- 

 delegung der Stockard 'schen Lahmungshypothese? 



I should answer now, as it was inferred above, that they most 

 decidedly are, and an attempt to show this fact in some detail 

 has been made in the preceding pages. 



The title of the recent paper by Spemann ''Zur Entwicklung 

 des Wirbeltierauges" might better have been '^The development 

 of the vertebrate lens," as little or almost no attention is given to 

 other parts of the eye. The problem of the lens formation is 

 very fully considered. 



"Cyclopia of the lens" is discussed from the same standpoint 

 as cyclopia of the optic cups. In the case of the lens median 

 ectodermal cells may be missing so that the two normally lateral 

 lenses fuse in the median line. It seems to me that this is the 

 one straw too much for the 'Defekthypothese.' The imagina- 

 tion which pictures the falling out of just the exact amount of 

 median ectodermal tissue which would allow the primary lens 

 forming cells of the ectoderm to fuse towards the center and 

 keep proper pace with the movements and final position of the 

 various cyclopean eyes which myFundulus material presents must 

 be most vividly active. An even more plausible possibility out 

 of this imaginary dilemma is to consider the lens anlage as a 

 single median group of cells that divides into two parts which come 

 later to lie in lateral positions. This view would at least have 

 the advantage that in cyclopia of the eye ''cyclopia of the lens" 

 would maintain the lens forming cells in a fairly median region, 

 and in normal development the lens cells would have a more or 

 less definite path to follow and place to reach. The pineal eye 

 in many forms possesses a fairly definite lens which must have 

 arisen medially and the present lens may have been somewhat 

 more anterior yet also median. These suggestions are of the 



