496 S. R. DETWILER AND HENRY LAURENS ‘* 
period, the nuclei of the external nuclear layer show numerous 
mitoses. 
The first step in the formation of the visual cells is the simul- 
taneous appearance of a protoplasmic bud and a small achromatic 
PED Ve 
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= 2 K) 
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se STA ay } oo 
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Fig. 1 Transverse section of optic cup and lens in an embryo of 11 days after 
the tail-bud stage of development. ON, optic nerve; PE, pigment epithelium; 
R, retina; V.C, developing visual cells. XX 275. 
globule on the pole of the nucleus applied to the external limiting 
membrane (fig. 2, Aand B). In sections stained with iron haema- 
toxylin, groups of deeply stained granules are usually seen just 
distal to the globule. The protoplasmic buds appear to be pushed 
through the external limiting membrane. 
