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THE AMERICAN NATURALIST [Vol. XL 



been observed. After each division into two, one nuclens is left 

 behind to contribnte one additional row to the thickness of the 

 retina while the other grows to its original size and divides again. 

 Thns the external limiting membrane is contiiuially movijig 

 outward the width of a nucleus with each successive generation 

 of germinal nuclei. The fate of the nuclei thus left behind will 

 be considered in connection with the various retinal layers. 

 Differentiation begins at the time the majority of germinal 



