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THE AMERICAN NATURALIST [Vol. XL 



an extent that the nuclei here have fallen nito hne so that the 

 layer consists of a smgle row of nuclei. The condition at the 

 margin of the optic cup remains the same as long as the eye con- 

 tinues to increase in size. Hence the row of actively chviding 

 nuclei is alwavs relatively short compared with tliat of the retina 

 of which it is a direct continuation. After division the resulting 



