THE EMBRYOLOGY OF THE EYE. 57 



nerve-fibre layer, with the long axis of the 

 nucleus in the direction of the nerve-fibres, 

 and are called glia cells. 



At a very late period the rods and cones are 

 developed by the outer layer of columnar cells 

 sending processes through the limitans ex- 

 terna. In my specimens from the foetus and 

 from new-born animals this layer is not suffi- 

 ciently preserved to justify a description. 



b. The pigment-epithelium is formed from 

 the outer epiblastic layer of the secondary optic 

 vesicle, which becomes reduced to a single 

 stratum of cells hexagonal In flat section. In 

 the chick these cells are for a time all cuboidal 

 in vertical section and of nearly equal size 

 (Figs. 3 and 6). In the pig the posterior cells 

 are cuboidal, while the anterior ones soon be- 

 come long columnar in vertical section (Fig. 7). 

 In the pig, when the lens-sac is forming, rod- 

 shaped bits of pigment are deposited in the 

 cells along the inner margin of the cells, the 

 nucleus lying In the outer portion (Figs. 4 

 and 7). This deposition of pigment begins 



