FROM TWELVE TO THIRTY-SIX SOMITES 



167 



forms an epithelial sac, which at first practically fills the cavity 

 of the optic cup. However, it very soon becomes detached from 

 the posterior wall of the optic cup, which expands with great 

 rapidity, and the lens is left at the mouth of the cup. The walls 



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of the lens sac are at first of practically even thickness (28s), 

 but by the 35 s stage a great difference has arisen by the elonga- 

 tion of the cells of the inner wall, which are destined to form 

 lens fibers: the cells of the anterior (outer) wall elongate much 



