36 THE EMBRYOLOGY OF THE EYE. 



sides and cross the space between the external 

 epiblast and the lens-sac. A layer of meso- 

 blastic cells is thus formed, which may rest on 

 the posterior surface of the external epiblastic 

 layer, or which may at first lie slightly pos- 

 terior to the external epiblast. After the first 

 continuous layer has been formed from the 

 mesoblast at the sides, it gradually increases 

 in thickness, the new cells having their origin 

 probably in the division both of the cells of 

 the existing layer and of the cells at the sides. 

 These cells become long spindle-cells, and the 

 anterior cells, if at first they have not rested 

 on the external epiblast, soon do so, and there 

 is then no trace of any intervening material 

 between corneal stroma and epithelium. For 

 a time the rudimentary cornea is much thicker 

 at its periphery (Fig. 7). At length the cen- 

 tral portion fills up until the cornea is of a 

 uniform thickness. Then a single stratum of 

 endothelial cells, the origin of which is uncer- 

 tain, passes out over its posterior surface 

 (Fig. 8). 



